I have an MRC 1820 drop in decoder that I am installing in an Athearn RS-3.The front and rear light outputs are 1 1/2 volts, to match the 1 1/2 volt bulbs in the RS-3. However, the MRC instruction leaflet says if there are two bulbs in the headlight or rear light, to wire them in series. They are originally wired in parallel in the DC unit. If I wire the two bulbs in series to the 1 1/2 volt light output terminals, they will only have 3/4 volt across each bulb, and will be too dim to see. Why would they suggest this and why can't I wire the two bulbs in parallel to the 1 1/2 volt light pads? It also says if the light is too dim to use the blue wire as headlight common wire for both front and rear light. There is a single blue wire coming off the middle of the circuit board. Thanks.
Two in parallel will probably draw too much current for the decoder. In parallel the voltage is equal but the current adds.
Best solution, ditch those horrible bulbs that, if yours is anything like mine, protrude out the front of the light castings, and replace with LEDs and appropriate resistors. They would get hooked to the full power light outputs ont he decoder instead of the reduced voltage ones. The blue wire is probably full voltage - that will blow the light bulbs if you use that, but that's the one to use for LEDs with resistors. MRC instructions are less than clear. There is only ever one blue wire, you just splice in whatever you need to hook up multiple lights.
--Randy
Modeling the Reading Railroad in the 1950's
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Typo ?
Mark.
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They would get hooked to the full power light outputs on the decoder instead of the reduced voltage ones.
The MRC 1820 is loco specific to the Athearn RS-3, and it's a drop in. There are no 12 volt full power light outputs, only the 1 1/2 volt outputs. Scouring the internet, there are questions from several posters who cannot get LEDs to work on the 1 1/2 volt outputs. I have LEDs with resistors already inline, but there does not seem to be a way to get 12 volts from the decoder light outputs. Also the Athearn RS-3 has the double bulb headlights and a single LED is all that would fit behind the lens.
RR_Mel After going over the MRC1820 Manual it appears that that decoder isn’t configured to and wasn’t intended to drive LEDs. As a drop-in decoder it was engineered to drive the original Athearn bulbs. It will drive one 1½ volt bulb headlight and one rear 1½ volt bulb “without any modifications” according to the manual. https://www.modelrectifier.com/v/vspfiles/resources/dcc/0001820%20Athearn%20RS-3.pdf
I read that link twice and don't see where it says it will only drive ONE bulb on each end. (?)
You can utilize LEDs by attaching the positive lead of the LED to either track rail. This is known as half wave power - commonly used in N-scale to eliminate one wire from the decoder using a 6-pin plug. The half wave power is just that, the voltage the LED sees is half of the full track voltage. If you have 14 volts on the rails, this method will feed 7 volts to your LED, so calculate the proper resistor to use.
Mark R. I read that link twice and don't see where it says it will only drive ONE bulb on each end. (?) Mark.
The RS-3s, at least the ones I have, have the later style headlights where the one large (steam loco like) light actually has two smaller lights in it, on RS-3s these were either horizontal or vertical. So athearn used a pair of lamps in each one, on for each individual light. On the stock DC board, they are wired in parallel. They're also coming out when I decoder mine because as I mentioned they actually stick out from the front and back of the loco. Really looks silly, no better than the old "fire in the cab" look of the old BB locos.
I "took one for the team" a few years ago and put an MRC decoder in one of mine, but I forget which one I actually bought, but it was supposed to be an Alco 244. I didn't change anything, and the lights worked fine. And the loco ran on DCC. Which is about where the good things I can say about it end.
I've done more than 30 sound installs, and I know about the MRC decoders. But I got lazy and bought the MRC because it is a drop in for the Athearn RS-3, and a speaker already mounted on the decoder board. I had thought about drilling out the double headlight and putting in a single lens and a single bulb. Might still do it. Thanks for the replies.