Colin Mitchell
Colin Mitchell
July 09, 2019
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Capacitor Discharge Unit with SPDT Switch SLIM

Capacitor Discharge Unit with SPDT Switch SLIM

Kits are available for $9.50 plus $6.50 postage.
Click [HERE](mailto:colin@elechelp.com?Subject=Buying capacitor Discharge Unit using SPDT switch - SLIM kit $9.50 plus $6.50 postage &Body=Please%20e-mail%20the%20cost%20of Buying capacitor Discharge Unit using SPDT switch - SLIM kit $9.50 plus $6.50 postage by%20air%20mail%20to%20my%20country:**
___****%20%20and%20send%20details%20of%20how%20I%20can%20pay%20for%20it.%20My%20name%20is:____) to buy kit.**

Fully built and tested for $10.50 plus $6.50 postage.

The name: Capacitor Discharge Unit with SPDT Switch - SLIM has been chosen to identify it from all the other CDU modules made by Talking Electronics. We do not used code numbers such as ZL-36-A as they mean NOTHING, so a full name is used.
Talking Electronics has produced a number of Capacitor Discharge Units with different features and different costs. But nothing comes close to the low-cost and the number of features of this model.

It combines a number of features with a clever circuit to allow a single 1,000u to be charged to 35v and produce sufficient energy to operate a point. It also has an on-board mini toggle switch and two indicator LEDs - this reduces your wiring.
The first electrolytic on the PC board is called a RESERVOIR CAPACITOR as it does not play any part in the delivery of the energy to the point, but it sits between the positive and 0v rail to reduce the impedance of these two rails.
When the toggle switch is in the UP position, the other electrolytic gets charged and it puts a heavy load on the supply when this is happening.
This is when the peak current is drawn at the beginning of the charging cycle and rather than taking a lot of current from the supply, it draws much of the energy from the RESERVOIR CAPACITOR - about 80% to 90%. This energy is taken from the Reservoir Capacitor during the initial part of the cycle and the power supply assists during this time as well as the remainder of the time when the DELIVERY CAPACITOR is charging to its final value. This results in a very short charge-time.
When the switch is in the DOWN position, all the energy in the DELIVERY ELECTROLYTIC is passed to the point and the rest of the circuit is not involved.
Normally, the DELIVERY ELECTROLYTICS take a period of time to recharge, but with this circuit the Reservoir capacitor contains a lot of the recharge energy and the supply only has to supply a small additional amount during the activation and the remaining amount can be delivered after the point has changed position.
This means the point can be changed from one position to the other repeatedly, in less than a second.
The circuit also includes a voltage doubling arrangement for 12vAC to 15v AC - (from a train power supply) and two indicator LEDs to show the position of the point.
The mini toggle switch can be easily fitted to your console and small holes on each side of the switch used for the LEDs to shine through the console.
All these features are in this narrow PC board so that a row of these modules can be aligned on a panel to show the position of the points.
This module reduces your wiring considerably as only 3 wires are needed for each point and a common “bus” of positive and negative to the modules.


The circuit for the CDU point controller

The circuit looks simple but the action of the toggle switch produces two different paths for the DELIVERY ELECTROLYTIC and the two power diodes connected to the ends of the solenoid determine which coil will be activated.

The voltage-doubler, made up of a 100u electrolytic and two power diodes is very effective in charging the 1,000u Reservoir Capacitor very quickly to make the “recovery time” very short.
12v AC can have a peak as high as 18v and the voltage doubler will increase this to nearly 36v (less a volt drop across the diode). The voltage across the Reservoir Capacitor only drops about 10v during an activation and it quickly recovers. It is only used every alternate activation.
1,000u at 35v provides twice the energy as a 1,000u at 25v. That’s why this module is equal to our previous module using 2 x 1,000u @ 25v.

These 3 photos below clearly show how the components are fitted and especially the switch. It is soldered with the middle tag to the top of the board and one of the wires cut from the 1,000u electro is pushed into the hole next to the switch and bent over. It is soldered to the top tag. The same is done under the board.
This makes the switch very firmly connected to the board.

CDU1 000uSw slim side sm

A close-up of the switch, showing the leads soldered to the tabs on the switch

CDU1 000uSw slim Sw sm

CDU1 000uSw slim input sm
All the components fit on the PC board and the module comes with 2 x 1,000u electrolytics. The input can be 12vAC to 15vAC or 20vDC to 35vDC.

This module requires a supply of 35-36v to get the maximum output.
But you may find the output will be sufficient at 12v or 24v. This is the first thing you will experiment with when fitting the module to your layout.
You can make a 12v 24v or 36v supply very cheaply by using 24v and 12v plug packs. These are available on eBay for less than $10.00 (combined) including postage.
If you are making 36v, simply connect the leads as shown in the following diagram.
Some of these plug packs have a current detecting overload circuit and “shut-down” if the current is more than 1 amp (even for a millisecond).
To prevent this we add a 47 ohm resistor.
This only applies when you are using the plug packs for our CDU modules (like this project) as the high current is only required for less than a second and then falls to a very small value. The resistor will not get warm when used in this particular application.


All the parts fit neatly on the PC board.
The diodes, electrolytics and LEDs are identified on the board.
The screw terminals allow easy connection to your power supply and point motor without the need for any additional soldering.

The cost of the module is less than buying the components from your local electronics shop. That’s because we produce the kits 20 at a time and build them 10 at a time and pass the savings to you. We have sold thousands of CDU modules during the past 35 years and they will all be working perfectly.


$9.50 plus $6.50 postage.
Click [HERE](mailto:colin@elechelp.com?Subject=Buying capacitor Discharge Unit using SPDT switch - SLIM kit $9.50 plus $6.50 postage &Body=Please%20e-mail%20the%20cost%20of Buying capacitor Discharge Unit using SPDT switch - SLIM kit $9.50 plus $6.50 postage by%20air%20mail%20to%20my%20country:**
___****%20%20and%20send%20details%20of%20how%20I%20can%20pay%20for%20it.%20My%20name%20is:____) to buy kit Fully built and tested: $10.50**

  • 2 - 22k

  • 1 - 100u 25v electrolytic

  • 2 - 1,000u 35v

  • 2 - 1N 4004 diodes

  • 2 - 1N 5404 diodes

  • 1 - 3mm blue LED

  • 1 - 3mm green LED

  • 2 - 3-screw terminal blocks

  • 1 - mini toggle switch

  • 1 - 20cm fine solder

  • 1 - CDU with Sw - slim PC Board

Colin Mitchell

Colin Mitchell



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