CDUs: The Power Behind Model Railway Point Control

Photo of a model railway CDU from Gaugemaster

If you’re a model railway enthusiast, you know how important it is to keep your points (or turnouts) working properly. A Capacitor Discharge Unit (CDU) can help ensure that your points operate reliably and minimize the risk of derailments. Here’s what you need to know before buying one.

What is a CDU?

CDU stands for “Capacitor Discharge Unit,” and it is a device used in model railway layouts to ensure that the solenoid and SEEP point motors operate reliably and consistently.

When a point motor is switched, it requires a surge of electricity to move the rails into the correct position. A CDU is designed to provide this pulse of power by building a reserve of power up and then releasing it rapidly to the point motor. The reserve is held in a capacitor, hence the name Capacity Discharge Unit. This ensures that the point moves quickly and reliably, even if there is some resistance or other factors that might normally slow it down or cause it to stall or hinder operations elsewhere around the layout.

Why Use a CDU?

Adding CDUs to your layout and point control helps in several ways.

Firstly, the surge of power overcomes stickiness in points or there isn’t sufficient power to operate them correctly. This is especially the case if you are operating multiple points at the same time. The stored jolt of power from a CDU gives solid point throws for far more consistency and reliable and reduces derailments. I hadn’t appreciated how often this was a problem and how much more enjoyable running the trains is without derailments around points until I used my first CDU. It was a game-changer for the enjoyment I got from my old DC layouts!

Secondly, operating solenoids draws a lot of power which can deprive other aspects of the layout of their power when the points are activated. If when operating your points, your trains slow down, a CDU will help – again, improving the operation of the layout.

Third and lastly, and although I’ve not encountered this personally – my layouts aren’t that large, CDUs can help with the loss of power where there are long runs of cable between the power supply and point motor. This reduces the power available to the solenoids, causing similar problems to the first issue whereas with a CDU they have the power they need. As said, my layouts aren’t that large and I’ve not had this problem but it would be worth exploring if you have very large layouts and occasional point problems.

Wiring a Capacitor Discharge Unit 

Wiring a CDU is fairly straightforward but there are some important considerations.

Wiring a CDU for analogue

The normal way for modellers to use a CDU is to connect it to a 16v to 24v AC supply (such as the auxiliary port on the back of many controllers), attach the negative wire from the CDU to the point motor and positive wire to the middle prong of the switch, and the two outer connections of the switch to the inputs on the point motor.

The following video explains and illustrates very simply.

Wiring a CDU for DCC

If you have a DCC layout, it makes sense to also control your points via DCC. And here Gaugemaster, DCC Concepts, Rails and others have helpfully produced CDUs with DCC decoders built-in. Wiring these is the same as for analogue except the power feed for the CDU, in the case of DCC decoded fitted CDU this comes from the rails instead of the controller. This makes it easier to wire (less long cable runs) and gives automatic integration with the DCC controller.


Hopefully, this has helped you understand CDUs and why they are useful for a model railway.

  1. Can CDUs be used for controlling other aspects of my model railway layout?
    Yes, CDUs can be used for controlling other aspects of your model railway layout, such as signals, lighting, and accessories. However, you’ll need to ensure that the CDU has the necessary power rating and connections for the specific equipment you want to control.
  2. How many CDUs do I need for my model railway layout?
    The number of CDUs you need will depend on the size of your layout, the number of points and other equipment you have, and how many points you want to change at the same time. A larger layout with multiple points being operated simultaneously will require more or large rated CDUs than a smaller layout with fewer points. The Gaugemaster CDU above will control up to 6 points, assuming the wiring, points and solenoid are in good order.
  3. What do the uF numbers on CDU specifications mean?
    CDUs are rated in terms of their maximum capacitance, which is usually given in microfarads (µF) or farads (F). The higher the capacitance, the more energy the CDU can store and release, and the more points it can control simultaneously. For example, a CDU with a capacitance of 1000 µF would be able to control more points than a CDU with a capacitance of 500 µF
  4. How do I troubleshoot issues with my CDU?
    If you experience issues with your CDU, such as points not responding or unreliable operation, you should check the wiring and connections to ensure they are correct and secure. You should also check the power supply and make sure it is sufficient for the CDU and other equipment connected to it. If the issue persists, you may need to replace the CDU or seek professional assistance.
  5. Can CDUs be used with both DC and DCC layouts?
    Yes, CDUs can be used with both DC and DCC layouts. However, the wiring and connections may vary depending on the type of layout, so you should ensure that the CDU you choose is suitable for your specific layout.

Recommended CDUs

I have used the following capacity discharge units and recommend them.

Gaugemaster CDU: Well thought of and my go-to unit for OO and N gauge small layouts. -operates up to 6 solenoids.

Gaugemaster CDU + DCC Decoder: Ideal combination of a DCC decoder and CDU, allowing operation of point motor from a DCC controller. Wired from the track for easy connection.


If you’re experiencing issues with your points, consider using a CDU to improve their reliability. Check out some of the popular CDU brands such as Gaugemaster and make sure to properly wire your CDU for your layout’s power supply. By taking these steps, you can help prevent derailments and ensure the smooth operation of your model railway layout

If you have some electronics experience you could also make your own CDU. There’s a guide on how to do this here:


In conclusion, a CDU is s useful and recommended accessory for a model railway with solenoid-powered points. It ensures that points operate quickly and reliably without affecting the rest of the layout. Wiring up a CDU is a straightforward process, and it’s a worthwhile investment if you want your layout to run smoothly.


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Founder of ModelRailwayEngineer, Andy Leaning

Andy is a lifelong modeler, writer, and founder of He has been building model railways, dioramas, and miniatures for over 20 years. His passion for model making and railways began when he was a child, building his first layout at the age of seven.
Andy’s particular passion is making scenery and structures in 4mm scale, which he sells commercially. He is particularly interested in modelling the railways of South West England during the late Victorian/early Edwardian era, although he also enjoys making sci-fi and fantasy figures and dioramas. His website has won several awards, and he is a member of MERG (Model Railway Electronics Group) and the 009 Society.
When not making models, Andy lives in Surrey with his wife and teenage son. Other interests include history, science fiction, photography, and programming. Read more about Andy.

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