Loco Wiring: Auto Reversing Lights

Some models have separate forward and reverse lights that need to change automatically when reverse is selected. The 4 relay version of the HRB can be configured to do this by following the diagram below. Read More

Pro-160 / 360 Dual Control

We were asked if it was possible to have dual joystick controls on the Pro-160 / 360 to allow interior and exterior helm positions on an electric launch installation. This diagram shows how we did Read More

Noise Suppression for Multiple Motors

One question that often comes up is how to best arrange the motor noise suppression components on locos that have multiple motors driven from a single controller and spread across two or more bogies. There Read More

Pro-160 / 360 Radio Control Failsafe

When using the Pro-160 / 360 with radio control it is necessary to consider the various failsafe scenarios that can occur. The Pro-160 / 360 will detect the following A break in the connection between Read More

RBT Pot Support

The pots used in the RBT have a rated life of 20 000 operations. Although we’d like this to be greater we haven’t found a manufacturer that can supply long life pots with the 50′ Read More

Double Heading Pro & 4QD Series

A lot of customers are now seeking to double head [or MU] locos with different models of 4QD controller, and a common request is how to link a new Pro-160 / 360 to an older Read More

Testing an Electric Motor

Over the years we’ve seen a number of controllers that have been damaged by faults in the motor they are driving, so we thought we’d write a short article on what can go wrong, why Read More

Pro-160 / 360 to JSK-100 wiring

This diagram shows how to hook up one of our JSK-100 joysticks to the Pro-160 / 360. The Pro-160 / 360 should be set to volt mode and joystick mode, and then use pot learn Read More

Pro-160 / 360 Failsafe Contactor

This page shows how to incorporate a failsafe contactor into the wiring for the Pro-160. The same schematics also work for the Pro-360 but the J1 link is in a different position. Method 1 – Read More

Improving Heat Dissipation

We’re finding that more of our controllers are now being used in warmer situations, both in hotter countries, and in small enclosures with restricted ventilation. So we thought we’d write this guide on improving heat Read More

Double Heading the Pro-160 / 360

Here’s a schematic of how to connect two Pro-160s or 360s together for double heading [master / slave operation. The ignition wires are kept seperate to prevent stray currents causing damage to the controllers if Read More

DNO Fault Finding Guide

This is the fault finding guide for the DNO range of controllers. The DNO is the end result of a long line of 4QD analogue controllers including the NCC and VTXs. It is therefore a Read More

A Basic Installation

We often get asked for a full list of the parts needed when installing a controller. Although that’s a bit of a “how long is a piece of string” question, here is our view of Read More

Rhino VX Internal Wiring

We recently had a Rhino VX electric outboard motor in the workshop and were intrigued to know how the internal wiring was configured to make use of the four wires to give multiple speeds on Read More

Pro-160 / 360 Software Versions

This is a list of the formally released Pro-160 / 360 software versions. The Pro-160 and 360 use the same control board with largely the same software. Each software version has a number of different Read More

Sound System Suppliers

We often get asked about sound system suppliers, so here’s a list of all those that we know about. Brian Jones Dallee Electronics Mtroniks Phoenix Sound Sense Innovations Soundtraxx South West Digital Trax Controls These Read More

Pro-160 / 360 Fault Codes

The Pro-160 and Pro-360 have a number of warning messages or error codes that can appear depending on what the controller thinks is happening. This table lists; the code any associated sound signal whether the Read More

Component Suppliers

We’ve put this page up to list handy sources of the sorts of components often needed for motor installation. Connectors PEI- Genesis    

DNO and Sound Cards

When using DNO with a sound card you will need to give the sound card a signal that relates to the speed of the model. There are a number of ways to do this described Read More

Motor Noise Suppression

Electric motors are electrically noisy, the sparks commonly seen at the brushes are a source of radio frequency interference [RFI] that can interfere with both the host controller and other systems. Good motor noise suppression Read More

Clubs with controller test boards

Here’s a list of all the clubs that have a controller test board… Ashmanhaugh Light Railway Belfast & County Down Miniature Railway Society Bromsgrove Model Engineers Budmouth College Camp Creek Railroad Cleveland Association of Model Read More

RBT Plug Alignment

Our RBT hand throttle is fitted with an 8 pin DIN plug which is inserted into a corresponding 8 pin socket. The plug should be correctly aligned with the socket before it is pushed in. Read More

Pro-150 Hall Throttle Supply

The Pro-150 can easily be programmed to work with a voltage output from a hall type throttle. What takes a little more thinking about is where the feed for that hall device comes from. It Read More

How To Wire Up a Porter 5

We were asked if we could make some videos on how to wire up our controllers. First up is Dillon demonstrating how to wire up a Porter 5 controller. Dillon was our first work experience Read More

DCC / Airwire Interface

We’ve been asked by a number of customers if our controllers can be made to work with a DCC or  Airwire system. This little circuit ought to do the trick…. With pins 2 and 3 Read More

Control Board Jumper Positions

This picture shows the factory default control board jumper positions, these allow single ended operation. For joystick [wig-wag] operation move J1 and J2 to the opposite positions.

DNO Current Limit Adjustment

As standard the DNO-5 and DNO-10 have their current limited to approx 50A and 100A respectively. These values are controlled by the value of the DNO current limit adjustment resistor R25 in the diagram below, Read More

Closed Loop Current Control

Although our Porters have current limiting which can be adjusted, they were not really designed to provide an active closed loop current control mode. So many thanks to Ian Beaver for making public a neat Read More

SST-031 Settings For Hall Throttle

This page gives the settings to allow an SST-031 to work with a JD2 Hall effect throttle pedal. We used the standard basic_PWM_control_v1_1 as a basis and modified the  user settings so that it read Read More

Pro-150 Hall Throttle Programming

This page describes how to use the “pot learn” feature of the Pro-150 to set it up for operation with a hall effect throttle. Before starting this sequence the Pro-150 should have power connected to Read More

RBT Pot Rotation

After extended use we have seen a few cases where the potentiometer fitted to our RBT series of hand control boxs have rotated slightly in their mounting brackets. The symptom is that the motor will Read More

Pro-150 with shunt wound motors

Here’s a diagram showing how to use the Pro-150 with shunt wound motors. The principle is to use the brake driver on the Pro-150 to activate a relay that feeds the field winding, and to Read More

DNO Under Voltage Protection

Although the DNO does not have an under voltage protection cut-off fitted as standard it is easy to retro-fit this feature. There are two ways to do this; Method 1; the easiest, is to cut Read More

Pro-150 Push Button Use

If you need to control a Pro-150 via forward / reverse push buttons, a neat way to do it is to use the Pro-150 in joystick mode. This diagram shows how to wire it up. Read More


We get a lot of enquiries about batteries, their performance, and what is the best type or brand of battery for an application. One very promising chemistry is the newer Lead Carbon type, it has Read More

Catching diodes

If you have horns or relays fitted to your vehicle, you should fit catching diodes, one across each device to prevent a damaging voltage spike, why? read on for the full story…. Coils, inductance and Read More

Multi speed control

We’ve been asked for a way to switch a motor between a number of pre-set speeds [ multi speed control ]. Here’s a simple little circuit that we put together for the 4QD range but Read More

Ampflow motor

We recently encountered a strange issue with a 4QD-300 driving an Ampflow A28-400-F24 motor. Forward operation worked as expected but in reverse a very slight pulsing or oscillation of the motor speed could be detected Read More

Pro-150 Fault Finding Guide

We’ve written this guide to help customers diagnose and fix problems that may occur with the Pro-150. We don’t release complete circuit diagrams for current models but we hope this guide will shed some light Read More

Customised Arc Potentiometers

We occasionally get asked about potentiometers with operational arcs that are different to the standard 270′ or our own custom 50′ version. Rob Smallwood gave us this idea which involves using a log pot such Read More

Damage From Blown Mosfets

It doesn’t happen often but when the mosfets in a 4QD series controller do blow they can take a number of other components with them. If you are attempting a DIY repair this photo shows Read More

Radio Receiver Fault

This problem deserves a place in the knowledgebase if only because we posted a lot of equipment backwards and forwards before we found the solution. The system; 2 x DNO-10, DMR-203, Ex 2.4 GHz radio Read More

DNO Version history

The DNO PCB number is 30-2D-01. The following is a list of modifications listed by serial number. The DNO was originally designed to block transition from ahead to reverse without reducing the throttle to zero. Read More

Disabling regen braking [DNO]

Although the DNO is very similar to the VTX, one feature that it does not have is the provision for disabling regen braking via on-board links. A number of people have asked if it is Read More

Double Heading Wiring Diagram [DNO]

This diagram shows how to connect two DNOs in different model locomotives together using the expansion port.     You can download a PDF version here. For the slave cable we recommend using a bulgin Read More

Battery Contactor

This optional battery contactor arrangement can be fitted as part of the 4QD-200/300 installation to provide reverse polarity protection and also an emergency power disconnection. The relay should be chosen so that it is capable Read More

Parking Brake Voltage

We have seen a number of cases recently where a 24V parking brake coil has been fitted to a 48V motor. Generally this has been where 2 motors have been fitted to a 48V golf Read More

Porter Current Limit Adjustment

It is possible to adjust the Porter current limit setting by fitting either a fixed resistor or a potentiometer across the pads circled in red below A resistance of 100k gives a motor current limit Read More

Mtroniks and regen braking

One of our customers has brought to our attention an issue that can arise when using an Mtronics sound card with a controller that has regenerative braking [all 4QD controllers]. The model in this case Read More

Motor Suppression in Robot Wars

Robot wars is a hostile place for motor controllers, and motor suppression should be given careful thought. The main areas for concern are; Shock and vibration caused by collisions and weapon strikes. Electrical noise spikes Read More

What is PWM Motor Control

What exactly is  Pulse Width Modulation? how does PWM motor control work, and what does a PWM circuit look like? On this page we’ll go into a bit  of detail to explain the theory and Read More

Full Bridge

This is a simplified motor control circuit and shows the switching scheme used in our 4QD series full bridge [aka H bridge] motor controllers. The circuit shows a full bridge of four MOSFETs. In forward Read More

Driving the DNO by Raspberry Pi

Our DNO range is ideally suited for use as an interface between a Raspberry Pi, Arduino  etc, and a motor. The Pi can provide a PWM output via its GPIO pins and this can be Read More

Pro-150 Pt Fault Code

If a Pro-150 gives the Pt [pot top] fault code, or just gives a continuous rapid beeps, this indicates a pot or wiring fault or blown F1 fuse track such that the voltage on the Read More

Adjustment for 48V

Unless specifically requested for 48V, we usually configure the 4QD series for 24V operation. When operating at 48V the acceleration ramp control may need to be adjusted as well as the gain control in order Read More

Pro-150 programmable parameters

Introduction A programmable controller is potentially very complicated. 4QD have considered in depth how to give the user access to the full programmability, but without making it confusing to the novice. At the same time Read More

Pro-150 Mechanical Information

Pro-150 mechanical information Dimensions Board only: 145mm x 102mm x 38mm With base and cover: 181mm x 121mm x 46mm Weight Board only: 320 gm Complete: 630gm Power connectors 9.5mm push-on for battery, combined with Read More

Pro-150 Basic Wiring Diagram

This diagram shows a basic wiring scheme for the Pro-150 running on 24V with ignition and reversing switches. For clarity we haven’t shown the optional display / programmer which connects via 2 cables to its Read More

DMR-203 Drop-Out Fault

This dmr-203 drop-out fault was one of the strangest ones I’ve seen for a while, as it took so long to solve it deserves a place in the knowledgebase. The system; VTX-75-24, DMR-203, Spektrum DX Read More

Uni: Disabling Regen

Disabling Regen braking. From Issue 7, two scratch-through links are present on the circuit board. If both of these are broken, regenerative braking will be disabled. However this modification is not recommended: our experience shows Read More

Overheating in Boats

If you are using the 4QD series in a boat then you should pay particular attention to airflow and cooling to prevent the controller from overheating. It’s not kind to shut your controller away in Read More

PCB numbers

Our controllers last so long that sometimes the only marking left on them is the part number etched onto the PCB. If this is the case for you, we’ve put together this table of PCB Read More

Fraser Golf Buggies, Wiring for Pro-150

Fraser golf buggies (A La Carts), e.g. the Firefly and Dragonfly originally used 4QD’s Pro-120-48. When the Pro-150 was released, we stopped making the 48v Pro-120 as the Pro-150 is just as simple and does Read More

Starting with Robots

We get a lot of of enquiries from Robot Wars contestants who don’t know where to start building their robot. I sometimes get the impression that they are starting with the robot control system – Read More

Motors in Series or Parallel

Ordinarily a motor should be driven from a voltage source that matches its specification, but a number of our customers have asked about connecting two 12V motors in series in order to run them from Read More


Towing an electric vehicle with a permanent magnet motor can cause major problems. If any E.V. is to be towed, you must disconnect the motors or else jack the machine so that the drive wheels Read More


Torque is not really a controller function, but it is directly related to the current flowing in the motor. The mechanical torque is proportional to the motor current and the relationship is defined by the Read More

Surge Currents

If a motor is connected straight to a battery with no controller, it will draw a very large surge current.This surge current will generate a large torque pulse from the motor, we have seen relay Read More

Speed Stability

A motor which is heavily loaded will always run slower than an unloaded motor. This statement seems so obvious that it is hardly worth saying – yet there are motors (stepper motors and synchronous motors) Read More

Switching for Double Heading

We were asked for a diagram for a double heading switch that could let either loco be at the front of the pair, and could be facing either way. This seems to make selection of Read More

Motor Manufacturers

We are often asked to recommend motor manufacturers, so here is a list of motor manufactures that we know about [in alphabetical order]. Ametek Ampflow AMT Bobet motors [Shenzen] Crystalyte motors Electrocraft EV drives Fracmo Read More


Our name comes from “4 quadrants” control. The word ‘quadrant’ comes from a graph. The graph below shows the blank on which you might start to draw motor voltage and motor current. At the top Read More

Motor Matching

There are two situations where the matching of multiple motors of the same make should be considered. The first is when they are rotating in the same direction, the second is when they are rotating Read More

Motor Examples

Motor examples. Certain motors are very popular and we keep getting asked about them, this section gives our findings on a few of them. We are not the manufacturers and our opinions are just that Read More

Motor Types [detail]

There are several electric motor types that use brushes for d.c. operation. Permanent magnet motors These have two wires. Reversing th polarity of the applied voltage reverses the rotation. Shunt wound motors These have 4 Read More

Mobility Scooters

Mobility scooters generally operate at 24v and have a legal top speed of 4 mph on the pavement, road vehicles can have a top speed of 8 mph. 4QD controllers can be wired with a Read More


Electrical heating occurs in any component or wire that carries an electrical current. How hot the component or wire gets depends on the rate that heat is generated and on how quickly heat can get Read More

Fuses and Circuit Breakers

Choosing fuses and circuit breakers is a difficult subject…… Difficult because motor currents in most applications are very peaky. Difficult because the controller’s battery current is not the same as the motor current. Difficult because Read More


4QD have supplied motor speed controllers to the Greenpower battery powered electric car competition for many years, and our controllers are highly efficient. The event has been won more than once using our controllers. This Read More

Golf Carts – Overview

One of 4QD’s principal markets is ride-on golf carts or buggies. Now there is a different market for buggies in UK as opposed to the USA. Most American machines use petrol engines and the buggies Read More

MOSFETs [tour 5]

MOSFET is an acronym – I’ll give you the whole phrase, so you know why they’re always called MOSFETs.. Metal Oxide Silicon Field Effect Transistor. MOSFETs are very near to perfect semiconductor switches, capable of Read More

Half bridge [tour 6]

Our controllers use either a full bridge or a half bridge PWM circuit to to control the speed of an electric motor. In the half bridge configuration two sets of MOSFETS are connected across the Read More

Switching frequency [tour 7]

All 4QDs controllers have a switching frequency of (around) 20 thousand times per second – 20kHz. This is high enough to give no whistle or whine from the motor but, more importantly, it means that Read More

Regenerative braking [tour 8]

Regen or regenerative braking is a way of diverting the kinetic energy of a moving electric motor / vehicle back into the battery so that it re-charges the battery and provides a braking effect. Most Read More

Current limit [tour 9]

Current limit – drive Protection is fitted to all controllers so that the initial starting surge (or the current that would flow if the armature were locked) cannot exceed that which the MOSFETs can handle. Read More

Thermal shutdown [tour 10]

Thermal shutdown and current trip On some controllers a thermal sensor is fitted (or can be fitted as an option) to detect the MOSFET temperature and reduce the available current if the heatsink overheats. At Read More

Heatsinks [tour 11]

All electrical items carrying current get hot. The amount of heat is actually proportional to the square of the current, so doubling the current causes four times the heat. A high current motor and controller Read More

Ignition [tour 12]

When the controller is not in use it needs to be deactivated. The ignition circuitry (fitted to all controllers) does this, so the controller draws no current and the throttle input will have no effect. Read More

Power down state [tour 13]

When the controller is powered down (ignition turned off) but with the battery still connected, different controllers behave in different ways. There are, essentially, 3 possibilities: Open circuit Short circuit Diode Open circuit All semiconductor Read More

Ramps [tour 14]

Acceleration Ramp Even if the throttle is applied sharply, the controller must accelerate the motor smoothly, without a jerk, so an ‘acceleration ramp’ is built into all controllers. This may be a an adjustable ramp Read More

Reversing [tour 15]

A permanent magnet motor is reversed simply by interchanging its two armature wires. This can of course be done with any controller by using a suitable, high current, double pole changeover switch. However reversing should Read More

Overvoltage protection [tour 18]

If a controller includes regenerative braking, then there is a potential problem if a battery connection fails during braking. This can happen on a golf buggy if the batteries are not properly secured. If it Read More

Undervoltage cutback [tour 19]

Undervoltage cutback – or Battery Discharge Protection When a battery is discharged its voltage falls. It is a widely known fact that discharging a battery too far can damage it, especially if it is left Read More

Joystick [aka wig-wag] [tour 21]

A joystick (know in USA as a Wig-Wag control) is a lever operated speed and direction control. Move the lever forward to go forward, and pull it back to go backwards. The speed is controlled Read More

Parking brake driver [tour 22]

Parking Brake Driver Regenerative braking relies on the voltage generated by the rotating armature to cause braking so it is speed dependent. If the speed is slow enough there is no effective braking, so the Read More

Brake light driver [tour 23]

On some controller we can fit, at the factory, a ‘braking’ light driver. This braking light option replaces the standard parking brake driver. On road vehicles a brake light may be required. If the vehicle Read More

Pot fault detection [tour 24]

If a wire to the pot breaks various things can happen. If the full speed wire breaks, then the pot cannot select full speed so this is generally safe. If the output (wiper|) from the Read More

High pot lockout HPLO [tour 25]

If the controller’s ignition is turned on while the throttle pot is not at zero the vehicle could accelerate to speed. High pot lockout [HPLO] prevents this by disabling the vehicle until the throttle returned Read More

Reverse polarity protection [tour 26]

If you inadvertently connect the battery up the wrong way round , then depending on which model you have, the controller may be destroyed in quite a spectacular manner! Only you can decide if this Read More

Main capacitor [tour 27]

If you have read the page entitled Half-Bridge you will remember that the battery current is being switched on and off 20,000 times per second. Now electricity in wires has the equivalent of ‘mass’ – Read More

Radio control interface [tour 28]

Most controllers are situated close to the user, e.g. in a ride-on golf buggy, so are controlled by a speed control pot wired directly to the controller. However some markets, such as Robots and some Read More

Tacho generator feedback [tour 29]

A tacho generator generates a feedback signal proportional to the speed of rotation, which can be used to provide closed loop control of a motors speed. Varying loads If such a ‘tacho’ is used and Read More

Voltage following [tour 30]

All controllers made by 4QD work with a voltage input, normally derived from a potentiometer. However it’s the voltage input that determines the speed. The controller cannot tell whether that voltage is coming from a Read More

Double heading [tour 31]

Double heading is a term used by loco builders, it is where you have an engine on each end of the locomotive, so each end is a ‘head’, hence double heading. For double heading, you Read More

Choosing a Controller [tour 1]

When choosing a controller, both it and the motor should be matched to the mechanical task that is to be done. So – you first of all need to understand the mechanics: What mass do Read More

Waveforms and Fault Finding

  Although this page refers specifically to NCC series controllers, the principles apply to several other controllers of our manufacture as most employ very similar circuitry. The principal exception is the 4QD series. Warning If Read More

Ramp Timings (NCC)

NCC series controllers. Ramps – acceleration and deceleration. Details of the equivalent modification to the Pro-120 are on another page. Introduction The ramps on the NCC are adjustable from around 300mS to around 7 second, Read More

Motor Types Overview

Permanent Magnet Motors In a Permanent Magnet motor a coil of wire (called the armature) is arranged in the magnetic field of a permanent magnet in such a way that it rotates when a current Read More

Internal Power Supply and Protection Circuitry

The Pro-120 and Scoota series controllers have some sophisticated internal power supply circuitry which includes reverse polarity protection, pot fault detection and high pedal lockout. This is a description of the features and of some Read More

Earth Track Fuse and Earth Loops

Introduction There is a fuse track designed into most of our controllers. This page explains the most common cause of the track blowing, and why it is present. This page uses the 4QD series to Read More

Use with Series Wound Motors

Our controllers are principally designed for Permanent Magnet motors, but they can be made to work with series wound motor with some modifications to the wiring. To control speed and direction of a Permanent Magnet Read More

UNI: Use with PWM Input

Introduction The Uni controllers sense the voltage on the pot input (after the gain preset) to give pot fault protection. If the voltage on the pot wiper is ever greater than about 5.3v the pot Read More

Radio Control Wiring Hints

Any radio controlled machine with motors has potential problems, and this page of radio control wiring hints hopefully helps to identify and solve them. The block diagram shows the electrical system of a typical radio Read More

Uni: POT Track Fused

The ‘pot min’ connection on the Uni controller is directly connected on the component side of the circuit board to the battery -ve. In the event of a wiring fault that causes excess current to Read More

Tacho Set-Up

Tacho generator systems can seem quite complicated: they are closed loop feedback systems where the Tacho generator (perhaps a small permanent magnet motor) is connected to the main motor shaft so that the two turn Read More

Pro-120 Mk1 History

This page refers to the very early Mark 1 Pro-120 which is now too old for factory repair. Warning: this listing was originally generated for 4QD internal use only, so is not as fully detailed Read More

Pro-120 Mk2 History

In early 2003 we released a redesigned Pro-120 controller. However – in general it’s such a popular and good controller that not a huge redesign was possible. The main change is that it uses sub-miniature Read More

Pro-120 Key Components

The Pro-120 series are card controllers, open to the elements – and users. They are similar to the VTX and older NCC series – but have several more sophisticated protection mechanisms built in. This page Read More

Earth Fuses: Why Do They Blow?

This page deals specifically with the Pro-120, but other controllers have a fuse in the earth, or can blow a pot earth connection. This page explains how. Wiring faults can cause the fuse in the Read More

Modifying the Pro-120 for other voltages

All controllers made by 4QD are generally wide voltage operating range: limits are the operating voltages on the MOSFETs and the relays, and the power dissipation in the current source transistor used for the internal Read More

Pro-120 Fault Finding Tips

Faults on the Pro-120 series are not common, but there are a few which occur, mainly because the ‘bare board’ style of the controller makes it susceptible to certain abuses. These fault finding tips may Read More

PRO-120: Multiple Slaves

This applicaton note applies to Mark 2 Pro-120 controllers only. The Pro-120 has an expansion connector that may be used to connect a master controller to a single slave. Connecting to more than one slave Read More

PRO-120 Ramp Reduction

The Pro-120 series acceleration and deceleration ramps are adjustable down to as fast as about 330mS. It is possible to reduce this timing but any modifications should be done with extreme caution. The ramps are Read More

Fitting Expansion Connector

Pro-120, VTX and NCC series controllers. Retro-fitting expansion connector Contents, this page Introduction Mark 1 / Mark 2 mixes VTX Pro-120 NCC Double Heading Tacho feedback board Solder Side All Controllers Additional controllers Older issues Read More

Pro-120 Robot Wars Version

The Pro is available in two versions, the standard version and the -RW version. This page explains the differences. Input decoupling There is, on the input to the standard controller, a noise decoupling circuit – Read More

Pro-120 Ignition Options

The Power relay drive circuit on the Pro-120 can be factory configured in three different ways. This page explains them. The power relay is switched on and off by a transistor that detects the internal Read More

48V and Above

If you need a 48V motor controller then we have a number of options, because of our designs most of our controllers can be made in 48v versions. This generally involves upgrading several components to Read More

Energy and Losses

A motor converts electrical energy into mechanical energy. However, in the conversion some of the electrical energy is wasted as heat. Some of this loss is because motors are not perfect, so if heavily loaded, Read More

Converting Parking Brake to Brake Light

The 4QD series has a Parking Brake solenoid driver. Optionally controllers can be supplied from the factory with this converted to a ‘Braking’ Light driver. This page describes the modification. Warning. The parts that need Read More

Inhibit Ramp Modification [4QD series]

The inhibit input on the 4QD controllers is designed to be used in conjunction with a microswitch sensing the foot brake on an electrical vehicle. Our features area explains this operation of the Foot-Brake inhibit. Read More

Deadband values and removal

This page specifically refers to modifying pre issue 15 boards. From issue 15 onwards there is an on-board jumper to alter the deadband. With issue 17 boards, the half-speed reverse feature was changed to make Read More

How to dismantle the 4QD series

Hopefully you’ll never have to do this, but if the need arises then here is how to do it. Top board removal. Remove the top board by loosening the four corner screws. Disconnect the 14 Read More

Fault finding on the 4QD series

4QDs policy on all controllers is one of continuous modification. If we find a mechanism whereby a user can blow a controller, we modify the design if possible, to prevent it happening. This means that Read More

Testing the 4QD Base Board and MOSFETs

Bridge testing Method All testing of faulty controllers should be done on a current-limited bench power supply: testing a faulty controller from a battery will almost certainly exacerbate the fault, causing yet more damage. The Read More

Current limit resistors

Notes on the current limit resistors for the 4QD series. The control board current limit resistors have changed frequently, as MOSFET improve the Rds generaly decreases. This increases the current, but it has to be Read More

Use with Tacho Feedback Board

The circuit of the standard tacho-generator feedback board is described on the 4QDTEC.com site and a drawing with other information is available. Mechanically, this board is designed to plug into the NCC and Pro series Read More

Control Board Modification [Pre June 2010]

Control board modification This page details two modifications The first part should be done to all controllers with serial numbers below 4028. After that, the modification was included in production. Modification is to the top Read More

Reverse Speed Modification [pre issue 17]

How to adjust the maximum reverse speed on earlier 4QD control boards. This information applies to earlier 4QD series only. With issue 17 control boards the method of incorporating half speed reverse changed. It is Read More

Mixing 4QD 150 / 200 / 300 series boards

With several ‘caveats’, the control boards and base boards of the 4QD series are interchangeable and may be mixed and matched. However – it is important that you understand the potential problems. 4QD 300: This Read More

4QD version history

This page lists the different versions of the 4QD series circuit boards, and their date of introduction. This listing was originally generated for 4QD internal use only so the descriptions may not be as full Read More

DMR-203: Use with PRO-150 Controllers

The DMR-203 may be used with Pro-150 controllers. There are two ways of doing this. One involves a slight modification if the DMR but has simpler wiring. The second does not involve modifying the DMR, Read More

Pro-150 Capacitor Modification

This modification should be applied to all Pro-150 controllers with serial numbers 0198 or higher and lower than 0550. On 0550 and later controllers the capacitor is not fitted. On serial numbers 0198 and lower Read More

PRO-150 Current Limit

The circuit of the Pro-150 is available as a pdf. Current limit amplifier is in the bottom right corner. Current is monitored by sensing the voltage drop across the lowside MOSFETs when they are conducting. Read More

PRO-150 Issue History

These notes are by serial number, rather than by date. The list is mainly for 4QD internal reference so is somewhat technical. 0000 to 0105 Are Mark 1. In these, there was no 12v connection Read More

Positional Servo Control [DNO / VTX]

Notes on how to create a positional servo control. There are two types of servo system: speed and position. These are explained in our Answers to FAQs on Battery Motors & Controllers. There is also Read More

Battery Discharge Protection: PRO-120

The Pro-120 is fitted with under voltage cutback a.k.a. battery discharge protection which turns the controller off if the battery voltage falls too low. This operates at around 20v measured inside the controller. This means Read More

POT Dead Band

With all controllers, operating from a standard potentiometer, there will be a ‘dead band’ at the bottom of the pot, where turning the speed pot does nothing at all. This page describes the many factors Read More

Why Do Mosfets Fail?

Why do MOSFETs fail? Even with the best design, the best components, and a new motor this can occur – often for seemingly inexplicable reasons.  Indeed the term MOSFET came to stand for ‘Magically Obliterated, Read More

Uni: Alternative Voltage Usage

Warning: Information on this page is given in good faith but you should be aware that any modification to a controller is done entirely at your own risk and will invalidate any guarantee. 4QD will Read More

Wiring for Push Button Use

This diagram shows how to connect a Pro or DNO (or earlier NTX and NCC) series controller for push button use, or with a centre-off switch. Note that the diode shown between the buttons must Read More

Easybike Wiring Notes

The name ‘Easybike’ has been used by more than one manufacturer: Easybike of Colchester, United Kingdom. This page only applies to one such machine. Back in the late 90s, Easybike used our controllers in their Read More

Phoenix Loco Wiring

4QD supplied the complete wiring for Phoenix locos and the wiring of the loco is shown here for the convenience of Phoenix customers. This equipment was supplied only through Phoenix and enquiries about the locos Read More

Loco Wiring: Hand Control & Horn Relay Board

4QD’s Reversing Bell throttle hand control and associated Horn Relay Board are designed to simplify miniature Loco wiring whilst maximising features available. The Reversing Bell throttle gives a dead man speed control and may be Read More

Back EMF & Internal Resistance

A speed controller varies the motor speed by varying the voltage fed to the motor. So what happens to this voltage at the motor? Back EMF. When voltage is applied to a motor, it rotates. Read More

PRO-150 Joystick Programming

This page describes how to use the “pot learn” feature of the Pro-150 to set up joystick programming, [or wig-wag operation]. Before starting this sequence the Pro-150 should have power connected to B+ and B-, Read More

VTX Modification History

VTX modification history: The VTX has changed relatively little in its life since first introduced on 2004. The best indicator of a controller’s age is the date code hand-written on the board, of the form Read More

VTX Speed Controller

The VTX was one of our most popular 24v motor controllers which was also made in 12v, 36v, and 48v variants. It has now evolved into the DNO range which is electrically interchangeable with the Read More

UNI Speed Controller

The Uni has been replaced by the Porter 5 and Porter 10 controllers. Uni speed controller. Two current ratings are available, the Uni-8 with a current limit of over 100 amps and the Uni-4 with current Read More

Scoota Speed Controller

This page is for reference only: the Scoota is no longer manufactured We ceased production at the end of 2005. The Scoota is, in essence, a non-reversing version of our popular Pro controller. It is the Read More

Pro 120 Mark 1 Speed Controller

Professional 120 Mark 1 4 Quadrant 12v, 24v, 36v & 48v Vehicle Controller This page is for reference only. This page describes the early Pro-120. It was upgraded in 2003 to become the Mark 2 Pro-120. Read More

Porter 40: Use with PWM Input

The Porter 40 controller sense the current through the pot input to switch the controller on and to give pot fault protection. If the voltage across the pot is greater than about 7v, the pot Read More

Porter 40 Speed Controller

The Porter 40 has been replaced by the Porter 5 and 10. The Porter 40 is a simple, economical, general purpose analogue controller for power handling machinery including golf caddies and wheelbarrows. It will also Read More

NCC Speed Controller

NCC series Mark 2 4 Quadrant 12v, 24v, 36v & 48v Vehicle controllers This page is for reference only. The NCC is no longer available. The latest equivalent is the DNO range. NCC Mark 2 Read More

VTX and NCC Operating Voltages

  These notes refer to the last issue of VTX controllers. So if you controller is older, the notes may be sequentially less ‘correct’. All controllers made by 4QD are generally wide voltage operating range: limits Read More

NCC Circuit Description

This description refers to the Issue 18 and later NCC Mk1 series controllers. Circuit Diagram The NCC Mk 2 and the VTX are similar: main additions on these are an adjustable Reverse Acceleration Threshold (a Read More

VTX / NCC Fault Finding

Introduction Common faults on the NCC & VTX  series are – not common.  But there are a few faults which occur, mainly because the ‘bare board’ style of the controller makes it vulnerable to damage. You will Read More

NCC Mark 2 Issue Number History

The NCC Mark 2 was issued in 2003 as an update to the earlier highly successful NCC controller. It is very similar – the main noticeable difference being that there is a reduced deadband at Read More

NCC Mark 1 Issue Number History

These Mark 1 controllers are now too old for factory service. Warning: this listing was originally generated for 4QD internal use only, so is not as fully detailed as it should be if it were Read More

NCC Key Components

This page identifies some of the key NCC components. It shows a Mk 1, issue 24 controller. Mk 2 is similar, but uses sub-miniature resistors. Mk 1 boards issue 18 and later are similar to Read More

NCC Zener Diode Failure

Symptom If the main power line zener has been damaged, the voltage on the pot will be lower than 8.5v. However, this voltage being low can also indicated that the pot fault transistor has been Read More

UNI / Egret Wiring Options

The Uni and Egret both have a 4 pin input connector, but only require 3 wires for input. This is done to allow different options to be selected by varying the input wiring. Standard 10K Read More

Egret Speed Controller

This page is for reference only: the Eagle is no longer manufactured. Suggested replacement is the Porter. The Egret is essentially the circuit board used in the Eagle series. It has all the features of Read More

Eagle Speed Controller

This page is for reference only: the Eagle is no longer manufactured (manufacture ceased September 2005). Suggested replacement is the Porter. 4QD’s Eagle series is a motor speed controller for 12v, 24v, 36v battery operation. Read More

2QD Speed Controller

The 2QD is an ‘old’ product: it was about our simplest 2 quadrant controller, but for general use we would recommend our Porter 5 or Porter 10. This page is now for reference only. The 2QD Read More

1QD Speed Controller

Introduction The 1QD is an obsolete controller: this page is for reference only. As a replacement, we would advise the Porter range of controllers. 4QD’s 1QD series are motor speed controllers for 12v or 24v battery operation. Read More

Good Wiring Practise

Our controllers get used in a wide variety of applications often switching large currents (several hundred amps perhaps) at around 20kHz. This makes the wiring used with them more critical, and this page tries to Read More


These notes apply to the inhibit function of the 4QD series controllers. If a vehicle is fitted with a hand or foot brake, it can be useful to disable the controller. This can be done Read More

Ignition Circuit

Electronic Ignition When the controller is not in use it needs to be deactivated. The electronic ignition circuitry (fitted to all controllers) does this, so the controller draws no current and the throttle input will Read More

Motor Rating Background Information

Introduction It is not always easy to understand a manufacturer’s specification for a particular motor, not least because different manufactures vary as to how conservative they are in specifying their motors. This page is intended Read More

Robot Steering

Introduction Quite a few Robots have steering problems initially. These notes are to help you set up the mechanics and electronics. In order to steer in a straight line, any machine which is driven by Read More

Multiple Controllers [Double Heading]

This page describes wiring that allows multiple Pro-150s and 4QD series controllers to be controlled from a single speed potentiometer. This is useful when double heading model locomotives. As the Pro-150 is a programmable controller, Read More

Double Heading Problems

Systems using the expansion connector to double head with a pair of controllers (Pro-150 or Pro-120 or DNO or VTX) almost always simply work when wired correctly. But there are occasional problems with installations: Wrong Read More

Double Heading Switch

We were asked for a system such that either loco could be at front and could be facing either way. This seems to make selection of reverse very complicated, but it is in fact quite Read More

Control by Microprocessor

4QD controllers are quite suitable for use with a microprocessor e.g. Pi or Arduino etc, but may require some simple interfacing, depending on the output of the digital system used. We have a schematic and Read More

Foot Pedal

4QD’s Bell throttle assembly is a finger-tip operated ‘position sensor’ which is also very suitable for use as a position sensor for a foot pedal operated throttle. The diagram shows how it might be done. Read More

Recharging Under Sail

When an electric motor is fitted to a sailing boat, it opens up the possibility of recharging under sail by using the water flow over the propeller to turn the motor to act as a Read More

Joystick Suppliers

There are a number of joystick suppliers that we know of and we’ve listed them below. In the UK: RS components Their part number 506-3602 is a good place to start, versions can be ordered Read More