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CDU for Higher Current Requirements

CDU – Higher Current Requirements

Quite a number of DCC switch decoders are designed and manufactured to control modern solenoid switch machines which draw little current.

Sooner or later you will come across the problem of driving older  switch machines which require a lot of current to operate and need a capacitor discharge unit to operate reliably.

The upper part of the schematic depicts one of the drivers.

The Printed circuit Board has four drivers identical to this one (one for each of the DS54 output). In order to distinguish the drivers from each other, the second, third and fourth drivers have their parts referenced with an additional digit : “1” for the second one, “2” for the third one and ‘3″ for the fourth one(ie : transistor Q4 will be referenced Q14 for the second driver, Q24 for the third one…).
Each driver is connected to the DCC switch decoder output. The driver output is connected to the switch using screw terminals. Each driver is capable of supplying a pulse of up to 2 Amps thus allowing to connect several switches in parallel (such as two switches for a crossover junction), which is not possible with the DS54, even for low current switch machines.
The drivers do not draw any current from the DCC switch decoder. They are supplied by the Capacitor Discharge Unit. The lower part of the schematic shows the Capacitor Discharge Unit (one per PCB) which supplies both the Vcc for the driver circuits and the Pulse to operate the switches. The Capacitor Discharge Unit is supplied by an external AC source. Several Boards can be supplied by the same AC source, as long as the this source is connected to the same terminals on each board (ie : pin 1 of TB1 of first board connected to pin 1 of TB1 on other boards and the same for pin 2).
The Capacitor Discharge Unit is designed so that the power input is disconnected when a switch is activated thus avoiding burning switch motors, in case the DCC switch decoder has not been properly programmed. Also, the Capacitor Discharge Unit is equipped with a constant current circuit for recharging the capacitors, allowing very fast recycle time (approx. 200 ms).

When recharging the capacitors, the Capacitor Discharge Unit draws about 600 mA from the AC source. This current can be decreased by increasing the value of resistor R9, but the recycle time will increase proportionally. When the same AC source is shared between several boards, the AC source must be sized to be able to power all boards at the same time.

When using DCS100 Command Station it is highly recommended to set the Option Switch OPSW#31 to “Closed” to meter switch commands (insert ½ s delay between switch commands) in order to allow the Capacitor Discharge Units to recycle between switch commands. For other Command Stations, check if the “meter switch command” option is available.


Bill of material

Reference Part
Q1,Q2,Q11,Q12,Q21,Q22, Q31, Q32 2N3904
Q3,Q4,Q5,Q13,Q14,Q23,Q24, Q33,Q34 TIP120
Q6 2N3906
D1,D2,D3,D8,D11,D12,D21, D22,D31,D32 1N4004
D4 BY255
D6,D7 1N4148
R1,R2,R3,R4,R8,R11,R12, R13,R14,R21,R22,R23,R24, R31,R32,R33,R34 10K – ¼ W
R5,R6,R15,R16,R25,R26, R35,R36 22K – ¼ W
R7,R9 2.2K – ¼ W
C3 470µF-50V
C4,C5 2200µF-63V
J1,J11,J21,J31 RJH Tel socket 4-4
TB1 TB 2 terminals
TB2,TB12,TB22,TB32 TB 3 terminals




Most parts have rather standard footprints. However, some specific parts may require to modify the layout and the PCB :

  • Capacitors C4 and C5 (2200µF – 63V) : I’ve used two capacitors in parallel, instead of one big 4700µF because they take less room. The Capacitors I’ve used are rather small as compared to other, so check what’s available and modify the layout and board accordingly.
  • Connectors J1, J11, J21 and J31 : These are sockets for small 4wire-4contact telephone handset plugs (RJH). Again, the footprint of these connectors vary from one manufacturer to another. In some case, the socket may even be flipped upside-down. Modify the PCB according to what is available.
  • Terminal Boards TB1, TB2, TB12, TB22 and TB32 : These are screw terminals that I found here. You may find different ones and need to modify the PCB accordingly.
  • Transistor Q5 : this transistor has to be equipped with a heat sink since it may dissipate some heat if the Capacitor Discharge Unit is used very frequently.
  • Transistors Q3, Q4, Q5, Q13, Q14, Q23, Q24, Q33 and Q34 : the collector of these transistors are connected to both the middle pin (at the bottom of the transistor) and the metal tab (at the top of the transistor). The collector connection is made through the top metal tab, so the bottom middle pin is not used and cut short of the transistor case. The metal tab is connected to the PCB using brass screws/washers/nuts.
  • Transistors Q3, Q4, Q13, Q14, Q23, Q24, Q33 and Q34 : these transistors are very closely spaced, so make sure, when mounting them, that the metal tabs do not touch each other.






Credit:    Jean-Louis Simonet