A Simple AC-to-DC Converter

Sometimes folks want radio control of their locomotives but prefer to use track AC (including DCC) instead of a battery to provide DC power for the radio receiver and amplifier. This post shows how to repurpose the DCC converter PCB customarily provided with the ProMiniAir transmitter to convert track AC to DC power for the ProMiniAir receiver.

DCC Converter Modifications

The original purpose of the “DCC Converter” is to use a DCC throttle’s Track Right/Track Left output and convert it to 5V DCC and 5V power for the ProMiniAir transmitter. See the Figure below.

One of the strengths of the modular approach used for the ProMiniAir transmitter and receiver is that you can “repurpose” components. The DCC Converter can be modified to use the AC track input to provide filtered, higher-voltage DC power.

Below is the repurposing idea: add a large capacitor (in series with a 100 Ohm resistor and a 1N4001 diode) across the “+” and “-” terminals of the rectifier and route out the rectifier’s DC output. Smaller onboard capacitors (10uf and 100nf) also filter out higher-frequency noise that large capacitors sometimes do not effectively filter.

One end of a 100 Ohm resistor and the + terminal of a 1N4001 diode are connected in series (and in parallel to each other) to the capacitor’s + terminal. The other end of the 100 Ohm resistor and the – terminal of the diode are connected to the rectifier’s + terminal. The capacitor’s – terminal is directly connected to the rectifier’s – terminal to form the DC ground. My thanks to ScaleSoundSystems.com for the idea of adding a resistor and diode in series with the capacitor.

When the throttle is turned on, the 100 Ohm resistor prevents an “in-rush” short circuit that might cause the throttle to cut off. When charging, the 1N4001 diode is reverse-biased with a large resistance. If AC power is interrupted, current flows out of the capacitor through the low resistance path of a forward-biased 1N4001 diode to maintain DC power output.

In fact, it is possible to forgo the large capacitor since these onboard capacitors do a pretty good job of “cleaning up” the DC output of the rectifier. This is a good option if space is at a premium.

The filtered DC output can now provide DC power to a ProMiniAir receiver/amp, just as a battery would. If the added capacitor is large enough, it will function as a “keep-alive” capacitor many DCC decoders use to prevent track power interruptions.

An Example

The photo below shows a real-world example of the conversion using a 10000uf “keep-alive” capacitor originally used with a Zimo decoder. The size of the capacitor dominates that of the DCC converter!

The oscilloscope trace below demonstrates the ability of the modified DCC Converter to produce clean DC power for your ProMiniAir (or other) receiver.

Note how “clean” the DC power output is (13.8VDC). Square wave track inputs at 16.8V are a severe test because they produce frequencies at odd multiples of the square wave’s frequency, e.g., at 6KHz, 18KHz, 30KHz, etc. for the example above, but very little of these frequencies “bleed through” to the DC power output.

Simplifying further, we can use the DCC Converter without an added capacitor, relying on the onboard capacitors to filter the rectifier’s “+” and “-” output. This option might be useful if space is at a premium.

The DC power output is still clean, but you lose the “keep-alive” that a large capacitor provides.

Conclusion

So there you have it – the DCC Converter can be slightly modified to provide filtered DC power and “keep-alive” capability. The modified DCC amplifier with a sizeable keep-alive capacitor costs $10 + shipping. Without the capacitor, the modified DCC Converter is $7 + shipping.