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Choosing a controller

What’s the best controller for a model locomotive? Literally thousands of our controllers have been used in model locos of all shapes and sizes over the last 25+ years. The model of controller you need depends on the power / current rating of your motors, and also the sort of loads that you intend to pull, any gradients on your track, and how long you intend to run for. You may also want to take into account other things like ease of set-up and adjustability.

As a general guide the following controllers are suitable for model locos:

DNO range – These are great for small / mid size locos up to 5″ / 7¼ ” gauge – with motors up to 100 amps. These are simple controllers that require the minimum of set-up, with adjustable gain and ramp settings. We see good results for locos pulling up to around 10 passengers with these.

Pro-150 – for mid size locos up to 7¼ ” / 7¾ ” gauge. Again good for around 10 passengers, the Pro-150 has a greater number of settings which allow the controller to be fine tuned to the operators requirements via the optional display / programmer. It will still work out of the box with the standard settings though [just like the older Pro-120].

Pro-160 – for mid size plus locos up to 7¼ ” / 7¾ ” gauge. The Pro-160 also does away with relays for long term reliability, and is ideally suited to those locos that will be worked hard pulling larger numbers of passengers for extended operating periods. With a 2 line display that can be mounted remotely the Pro-160 has features designed specifically for locos, including a dead mans handle input, a slow / fast mode switch to allow new driver training, built in radio control input, and readouts of voltage, current, and temperature.

Pro-360 – for larger locos with motors up to 360 amps, these controllers can deliver serious pulling power. There are also uprated air and water cooling options available.

One question that often comes up is whether to choose a controller that uses relays or not? At 4QD we use both approaches and here we’ll take a quick look at the pros and cons.

To make an electric motor go in reverse we need to change the polarity of the voltage supplied to it, and there are two common ways to do this.

Method 1 – is to use a half bridge controller arrangement and use relays to swap over the polarity.

The basic diagram looks like this, which is the circuit we use for the lower power controllers in our range such as the DNO and Pro-150.

half bridge and relay circuit

Advantages of the half bridge and relay design

  • Fewer mosfets are needed, the PCB can be smaller, and the mosfet drive circuit can be simpler, all of which makes for a less costly controller.
  • The relays can be arranged to put a short circuit across the motor when stopped, providing a strong braking force.
  • The motor current only ever flows through a single mosfet bank which means less heat will be dissipated.

Disadvantages of the half bridge and relay design

  • The main issue with using relays is the longevity of the contacts. If the rating of the contacts is observed then the relays can last for a long time, for example our NCC series was designed back in 1993 and a lot of them are still in service 26 years later. However if the rating of the contacts is exceeded then problems can start to occur. Electric motors can draw stall currents sometimes three or more times greater than their rated value, and over time these momentary high currents can cause cumulative damage to the contacts which may eventually lead to failure.
  • If very short deceleration ramp times are used, the relay contacts can open before the motor is fully stopped, this causes arcing which will erode the contacts and can generate sufficient electrical interference to damage the controller.

Method 2 – Full H-bridge

A full H bridge uses twice as many mosfets arranged in an H configuration to do the switching, thus doing away with the need for relays.

The basic diagram for an H-bridge looks like this, which is the circuit we use in our higher power controllers like the Pro-160 and 4QD series.

Relays or not, a full H-bridge circuit

To go forward mosfets A and D are switched on, to go in reverse we switch on B and C.

Advantages of the H-bridge design

  • There are no relays in the circuit so long term reliability is enhanced, particularly in higher load situations.
  • Switching can be done very quickly which makes this design more suitable for say a robot which must change direction rapidly.
  • There is no short circuit across the motor when stopped which means that that it is easier to push a vehicle with the power off.

Disadvantages of the H-bridge design

  • Double the number of mosfets and bigger main capacitors are required for a given power level.
  • Because there are two mosfet banks in circuit the heat dissipation may be greater.
  • The circuit is more complicated. If software is used to control the switching it can get complex although modern chip design has mitigated this to a large extent.

In summary;

Half bridge controllers with relays are ok if;

  • The controller is conservatively rated with respect to the motor.
  • Gentle deceleration times are used.
  • A smaller, more cost effective design is required.

Full H-bridge controllers without relays are better for

  • Long term reliability.
  • Any application where rapid stops are called for.
  • The ability to push a vehicle is required.

The quick answer is quality and support. For instance…..

Support- We design and build our products ourselves, we know what goes into them, how they work, and have many years’ experience in sorting out all the little issues that can occur in the wide variety of projects that our customers build. You just don’t get that from Ebay or Alibaba. Our customer support team never leave a man behind – we keep going until your installation is working properly.

Capacitors – Some of our competitors don’t put in much by way of main capacitors, you can get away with this on the test bench but in the real world that often involves long, thin battery leads, this can result in the capacitors and mosfets working a lot harder than they should, getting hot, and failing early. We put in a lot of capacitance and we spread it around the board by fitting many smaller capacitors, this helps the heat dissipation and gives better ripple performance. But it takes time to fit them.

Mosfets – These are the heart of any motor controller. As I write this a single seller [Mouser] lists 2713 different TO220 mosfets. The range of parameters is large. We take the time to sift through this and carefully evaluate the mosfets we use. We’re looking at current ratings, turn on / turn off times, capacitance values, figures of merit, and many other factors to get the best we can.

Metalwork – It’s a fact – all controllers generate heat. It’s no good having the best mosfets in there if you can’t get the heat out. Our controllers have the mosfets bolted directly to substantial aluminium heatsinks. That said, if you want to run high currents for long periods then you’ll need to think about how to manage that heat, see this article on improving heat dissipation.

Repairability – It’s also a fact that controllers can fail, and there’s a whole variety of reasons why [see this article]. If you are unlucky enough to suffer one of these there is a good chance we can repair your controller, again you just don’t get that from some other suppliers. We’ve also written an article on best installation practise which will help you to stop this happening in the first place.

If you know the current rating you need you can select directly from our range page

We have a comparison table, a flowchart, and some common application guidelines that will help  here.

We also have a page in our knowledge base that covers the basic physical principles.

If all else fails pick up the phone – we’re always happy to talk about your project.

Yes, some do. See the specification table for details. But if you are still designing your system you may want to read this page on why 24V is a better option.


Yes, most do. See a look at our specification table for details. Our new Pro-160 can go all the way to 84V.


At least not yet, we are looking into it though.

It is usually possible to tell the type of motor you have from the number of wires or terminals it has.

4QD controllers are designed to drive permanent magnet brushed motors, these generally have  2 external  wires or terminals which are usually black and red to indicate positive and negative.

Series wound and shunt wound motors generally have 4 terminals, 2 for the field winding, and 2 for the armature. Our controllers are not specifically designed to drive these types of motors but a number of our customers have managed to do so successfully. There are some articles in our knowledgebase on how to do it.

Brushless motors will have either 3 wires for a sensorless version, or 3 + 6 sensor wires if they are fitted with hall effect sensors. We do not currently have any controllers for brushless motors, but we are looking into it.

You need to consider the motors rated current, and also it’s maximum stall current. We have a flowchart here that can help you decide.


Installation questions

Electric motors are electrically noisy, the sparks commonly seen at the brushes are a source of radio frequency interference [RFI] that can cause problems with both the host controller and other systems [have a look at this video of brush arcing]. Historical fact; Marconi used an arc to transmit the first radio signal across the Atlantic.

A new motor and a properly designed motor controller may go on working for years without suffering problems, but over time the arcing and RFI will get worse, and there is a statistical probability of an energy spike of just the right parameters to blow a MOSFET.

Fitting motor noise suppression components, and following these installation guidelines will significantly improve the long term system reliability by reducing the chance of MOSFET latch up and failure.

Have a look at this short video to see examples of a motor with and without noise suppression…..

YouTube video



Yes. All 4QD controllers can be controlled by a voltage input.

You can use a DAC to provide a voltage output, or use a PWM output directly. We have tested 4QD controllers with PWM inputs between 10Hz and 10kHz.

Yes but be aware that with the high currents that our controllers can switch, come some constraints on mains power supplies. All our controllers use use 20kHz chopping and a transformer fed full wave rectified mains power supply will have a 100Hz or (120Hz in USA) output so you will need to have very good smoothing. Except at relatively low currents, a suitable transformer, rectifier and reservoir capacitor can prove very expensive, so a small battery with a suitable charger is a good [and often cheaper] alternative.

If you use a switch-mode mains supply, you should consider its operating frequency and how it will perform under full motor current chopped at 20kHz.

The main capacitor on the controllers is intended to be sufficient when used from a battery – which itself acts like a large capacitance. For high current use of a power supply you are probably going to require a lot of (expensive) extra capacitance here.

One other issue to be aware of is the voltages developed by regenerative braking. Many of our controllers feed power back into the battery during braking. If there is no battery then the regenerated energy can pump up the power supply voltage to a high level if the motor is stopped too quickly. Although our controllers are protected against such over-voltage, it may damage your power supply.

Yes,it is possible to have two or more controllers driving multiple motors. In the Loco worls it is called “double heading” and we have a number of diagrams in our knowledge base that show how this can be done.


Yes, our controllers don’t care how many motors they are connected to, so long as the maximum current rating is not exceeded. The current record is eight.


Fitting an ammeter is a much debated subject, from our perspective we see the following;


  • Can be interesting to see what you are using.
  • Can indicate mechanical faults.


  • Needs a shunt [adds losses], or hall effect sensor [expensive].
  • What exactly is it measuring? [battery and motor current are usually different].
  • Most meters are not calibrated for measuring square wave PWM currents.


A battery voltmeter is more useful – we would even say essential  since, as the battery discharges, its voltage drops, so this will tell you the charge state of the battery. Also, under heavy load, the battery voltage dips. If the voltage dips too far then either the load has increased or the battery is getting old.

The built-in approach

Our new Pro-160 controller has been designed with a built in low loss hall effect current sensor. This allows us to display both the battery and motor current as well as battery voltage and controller temperature. Additionally the built-in approach lets us set accurate limits to the drive and regen currents, and also to implement more advanced battery protection measures.






We often get asked what voltage should be used to give the best performance? To answer this question we need to have a quick look at power, what it is, and where it goes.

To give a vehicle a certain performance takes a particular level of power. This power level depends on the mass of the vehicle, the top speed of the vehicle, the acceleration rate you require and the gradients it must climb.

In an electric vehicle the power comes from the battery. Electrical power is volts multiplied by amps so that 40 amps from a 12v battery is 480 watts. But 480 watts is also given from a 24v battery by a current of only 20 amps. For a particular power, the higher the voltage, the lower the current.

Now electrical current causes heating. The motor, wiring and controller will all get hot and waste power. The heat wasted is proportional to the square of the current multiplied by the resistance. Moving from 12V to 24V halves the current but halving the current reduces the heating losses to a quarter.

It is clear from this that a 24V system is always better than a 12V system – provided you can physically fit two batteries. By the same token 36V or 48V would be even better, heavy duty systems such as fork lift trucks often use 96V, electric cars are now using 600V!

The amount of energy in the batteries is amps X hours X volts. Consider a 12V 60 Ampere Hour battery. Clearly this is exactly the same as two smaller 12V 30 AH batteries in parallel. But the total amount of energy in these two will not change whether we connect them in parallel or in series. So a 12v 60 AH battery can store exactly the same energy as a 24v 30 AH battery.

There is another factor against 12V operation, except at low currents: MOSFETs need a good voltage to fully turn them on, almost all of 4QD’s controllers use an internal 9v supply rail, which is adequate to ensure proper turn-on. However, there is not much difference between 9V and 12V. It does not take much current to be drawn from the battery before it drops 2V at its terminals. A small mount of extra drop in the wiring and the internal 9V supply drops. After that, the available current from the controller drops quite quickly! Remember that the battery current is actually a chopped version of the motor current, see our circuits archive for more detail, so the inductance and resistance of the batteries and battery wiring all contribute to any voltage drop.

For this reason, we would generally not advise 12v operation if the peak motor current is likely to be more than around 30-50 amps.


Closed loop control

The Pro-160 has been designed with a speed control sensor input. Software for this will be coming soon.

The SST-031 has a spare input port that can be used with a speed sensor. Software for this can be written by the user.

The Pro-150, 4QD series, DNO, and Porter controllers can have closed loop control implemented via our tacho feedback board.

Constant torque mode is the same as constant current control. The new Pro-160 has direct motor current sensing and uses this to implement a fully adjustable current limit. The software to provide a full torque control mode is being developed.

The 4QD-200 & 300 have an adjustable current limit which can be used to limit the torque from the motor.

Remember that the standard current limit is there solely to protect the MOSFETs from transient over-currents which could be destructive. If you run on the standard current limit for too long, then the controller will simply overheat and melt – quite literally, it is indeed possible to melt the solder on the MOSFET leads before the controller fails! So you may need to reduce the current limit quite significantly. The exact allowable current will of course depend on your heatsinking, the motor, and the controller you use.

Disabling regen braking can be done on certain models

  • The following controllers can have regen braking disabled by an on-board link
    • 4QD series
    • Pro-120 Mk 2
    • VTX series


  • The following controllers can have regen braking disabled by cutting PCB tracks
    • DNO series


  • The following controllers have regen braking which cannot be disabled
    • Porter
    • Pro-150

A variety is the answer. Full details are in the comparison table on this page but all have adjustable acceleration and deceleration ramps.



Yes, our Pro-100, Pro-160, and Pro-360 can all be connected directly to a standard radio control receiver.

For all other models we have a two channel interface [DMR-203] that connects a standard RC receiver to two controllers.



If you don’t think your battery life is all that it should be then;

  • Check the battery condition. The capacity of a battery reduces as it gets older. Also do a load test on it, we’ve seen batteries that gave 12.8V off load but that dropped to 11.1V as soon as a load was applied.
  • Check the condition of the power cabling. We’ve seen numerous cases where cable joints have degraded over time, overheated, and then caused a significant volt drop at the controller. Measure the voltage directly across the controller B+ & B- terminals whilst under load.
  • Is there mechanical drag in the system?
  • If all else fails, fit bigger batteries.  The controller can only do only one thing with the current it takes from the battery – pass it on to the motors. If the controller wasted any significant power – it would simply get hot and go up in smoke, so if the batteries don’t last – it’s a battery, a motor, a wiring, or a mechanical problem.


The great majority of lithium batteries now have a built-in battery management system [BMS] which protects the battery by sensing the voltage of each individual cell and the current flowing through it. If the cell voltage gets too low or too high, or if the discharge or charging current gets too high, the BMS will protect the battery by disconnecting it from the load. This disconnection can have serious consequences for a motor controller.

Our Pro-160 / 360 have settings that can prevent this damage, in short you need to do the following….

  • Set the under-voltage cut off point of the controller to be above that of the BMS, that way the controller will ensure a controlled stop before the BMS trips. The Pro-160 / 360 both have a low battery warning level that can be set higher than the BMS cut-off level that will allow a “limp home” mode under reduced power.
  • Set the over-voltage limit of the controller to be below the maximum charge voltage of the BMS. The controller will automatically adjust the deceleration ramp rate to keep the regeneration voltage below the BMS cut-off level.
  • Set the maximum current limit of the controller to be below that of the BMS, that way the controller will make sure that the BMS does not reach its threshold for tripping.
  • Set the regen current limit of the controller to be below that of the maximum charge current of the BMS, that way the controller will make sure that the BMS does not reach its threshold for tripping.

We’ve written a more in depth article on this subject here.

On our other, older controllers it is possible to have a low voltage cut-off, but full current and over-voltage limiting is not possible.

  • The DNO can have a simple modification done to set a low voltage cut-off.
  • Our BCM-5P1 battery meter has a low battery cut-off feature.
  • The 4QD-200 / 300 series have an adjustable current limit and low voltage cut-off.


We’re often asked to talk about brushed v brushless motors so here goes…..[we’ll put some pictures in here in due course but for now it’s just words].

Brushed motors have been around for ages, they are;

  • Simple – put a voltage across them and off they go.
  • Cheaper.
  • Less efficient – typically around 75%.
  • Noiser, both physically and electrically.
  • Brushes will wear out over time – typically 1000 – 3000 hours.

Brushless motors on the other hand tend to be;

  • More efficient due to less friction – around 90%+.
  • Quieter, both physically and electrically.
  • Longer lasting – typically 10 000 hours.
  • Able to run at higher speeds.
  • But – they need a more sophisticated controller to make them move.

So why do brushless motors need a more complicated controller? In a brushed motor as the armature rotates, the brushes and commutator switch the voltage between the successive windings, making in effect a mechanical switch. In a brushless motor this mechanical switch is not there and thus the switching has to be done electronically by the controller. To do this switching the controller needs to know about the position and speed of the armature, it can get this information either from Hall sensors on the outside of the motor or by sensing the voltage induced in the stator coils [sensorless mode].

Multiple motors. A controller for a brushed motor does not care how many motors are connected to it so long as the overall current rating is not exceeded, our current record is a single controller driving eight motors. But this does not apply for brushless controllers, because the controller needs to know about the position of the armature a single controller can only drive a single brushless motor. Note, there are dual channel brushless controllers that can drive two motors, but the general principle that you cannot just add motors in parallel to an existing controller still holds true.

Motor types. A brushless controller needs to be told quite a lot about the motor it is working with e.g. inductance, resistance, pole pairs, threshold speed etc. This is not the case with brushed.

Regenerative braking. For a brushed motor regen braking is relatively easy and done by quite straightforward switching within the controller. But with brushless motors more sophisticated measurement and switching is required, and our experience is that regen is difficult to do well without significant investment in software.





Why do speed controllers sometimes fail?

Here at 4QD we go to a lot of trouble to make sure our controllers are reliable. We make sure we have enough capacitance, we use the best mosfets we can find, we fit substantial heatsinks, we minimise the susceptibility to radiated and conducted noise, and we conduct extended life tests by running controllers on an automated test rig.

And yet very occasionally a customer will get a seemingly random failure. Actually there’s a clue in that sentence, in five years we’ve not had a single mosfet failure on our test rigs, the reason for that is that we take a lot of care with electrical noise suppression. Motors are electrically noisy things, just look at the back of an old electric drill to see the sparks. We’ve written a full article in our knowledgebase about motor noise and how to prevent it, it also describes other sources of noise, and some detail on mosfet failure modes. It’s well worth 10 minutes of your time reading this and taking the necessary steps to make sure that your controller doesn’t suffer an untimely end. If you just want the short “what do I need to do?” version have a look here.

We’re now shameless about plugging our suppression components. They don’t cost a lot but since we introduced them we have seen the field failure rate of our controllers drop significantly.

Increase the voltage. If you have a single 12V battery, consider fitting 2 smaller ones and connecting them in series to give 24V.  It is possible to arrange for a 24V system to charge from a 12V source, this page shows how. Bear in mind that if you double the voltage you will get four times the power.