Technical Background Information

What is PWM

What exactly is  Pulse Width Modulation? how does PWM control work, and what does a PWM circuit look like? The principles To control the speed of a d.c. motor we need a variable voltage d.c. Read More

Full Bridge

This page describes the switching scheme used in our 4QD series full bridge [aka H bridge] controllers. The circuit shows a full bridge of four MOSFETs. In forward drive, current must flow in the direction 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


Our name comes from “4 quadrant” 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


Heat All components or wires which are carrying any current get hot. How hot it eventually gets depends on the rate that heat is generated in the component 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

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]

Half Bridge All of our controllers use state-of-the art MOSFET circuitry switching at around 20kHz – that is 20,000 times per second. In the half bridge two sets of MOSFETS are connected across the battery, 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]

Regenerative braking is a way of diverting the kinetic energy of a moving motor / vehicle back into the battery so that it re-charges the battery and provides a braking effect. 1, 2 and 4 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 [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 is a precision generator used to sense mechanical speed of rotation of a motor. Varying loads If such a ‘tacho’ is used and fed to an appropriate controller, then the controller can 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]

Both motor and controller should be chosen by determining the mechanical task that is to be done. So – you first of all need to understand the mechanics: What mass do you need to move? 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

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

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

48V and Above

Because of 4QD’s designs, most of our controllers can be made in 48v versions. This generally involves upgrading several components to higher voltage devices and the use or higher voltage MOSFETs and capacitors, as well 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

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

MOSFET Failure Mechanisms

MOSFETs are well known to be prone to inexplicable failures  [MOSFET is sometimes thought to mean ‘Magically Obliterated, Smoke and Fire Emitting Transistor’]. The truth is that MOSFETs are incredibly robust – but that they 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? When voltage is applied to a motor, it rotates. This “applied Read More