Driving MRServo from DCC Accessory Decoders

We’ve received a number of questions now about hooking MRServo switch machines up to Digital Command Control (DCC) stationary (aka accessory) decoders. It’s not as simple as one might think.

Most stationary decoders are designed with Circuitron TortoiseTM machines in mind. Tortoises and similar stall-motor machines have a steady draw of about 20mA, and as such, the stationary decoders are designed to source about that much current without damage. This is less the current needed while MRServo-1 units are throwing, and can be below what MRServo-2 and -3 units need even just sitting there if the relays are energized. So, we can’t power the MRServo unit directly from the accessory decoder.

We also can’t power MRServo directly from the track. It needs a clean 8-15 volt DC supply. DCC is a form of AC (alternating current – the polarity is changing back and forth rapidly) and can go as high as 22 volts – far too high for the MRServo. So we need a power supply for MRServo, and a power supply for DCC.

The problem lies in the stationary decoders, where the MRServo power supply meets the the DCC power supply. If wired just exactly right, it might work in some setups. However, if wired slightly wrong, it creates pathways for high current shorts of the MRServo or DCC power supply through expensive electronics – the stationary decoders and the boosters. It’s not worth the risk of even trying this solution.

The solution is something called an optical isolator (optoisolator for short). It uses the output of the stationary decoder to turn on a tiny LED when the output polarity is one way (meaning a TortoiseTM or similar would throw one direction) and off when the polarity is swapped. That tiny light triggers a nearby switch called a phototransistor that will conduct current (think of it as “closing a switch” to complete the circuit) when it sees light. The stationary decoder now turns on the small light, and the phototransistor acts like the switch between MRServo’s control input and MRServo ground. Yet there’s no electrical circuit between the two, and thus no chance of blowing anything up due to a wiring mistake.


Figure 1: Diagram showing how to connect MRServo to a DCC stationary decoder using an optoisolator board

Figure 1 shows how to accomplish this. The DCC Stationary Decoder (on the left) should be one of the type that are designed to drive a slow motion switch machine, such as an NCE Switch-ItTM. (I have no connection with NCE, other than I think they make a quality product, and the Switch-ItTM seems to be quite popular with our customers. Other stationary DCC decoders capable of driving slow motion switch machines should also work just fine.) The decoder is wired and programmed as normal – DCC inputs (brown) are wired to a DCC booster, any front panel switches and such wired in, and the outputs (purple) for a given switch motor (such as the two SWA wires on the Switch-ItTM) go into the inputs of the isolation module. The purple can be reversed without risking damage – in fact, if the MRServo moves opposite the direction you want, you’ll need to reverse.

The outputs of the isolation module are sensitive to how they’re connected. The output marked “GND” or “EMITTER” must be connected to the same ground as the MRServo control board. Likewise, the output marked “CNTL” or “COLLECTOR” must be wired to the control input on the MRServo board. Obviously the MRServo will also need some source of clean 8-15 volt DC power.

Shown below is a single channel optical isolation module – the CKT-MRISO1 (no longer available). However, a 2 channel board (CKT-MRISO2) and a 4 channel deluxe board (CKT-MRISO4) are also available.


Figure 2: A complete example wired on my bench, showing how the various components are interconnected.

NCE Switch-ItTM is a trademark of NCE Corporation.
TortoiseTM is a trademark of Circuitron.

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