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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.


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