Model Railroad Power Conversion Circuit Desired - Page 1
Forum section: electronics / beginners

author: jwchenard
author_rank: Newbie
timestamp: May 26, 2025, 10:41:34 pm
content: I need the pink rectangle part shown in the attached diagram. Ideally, it would be small and not generate too much heat, so it can be inside a plastic locomotive, but that part's negotiable. I didn't show it, but the power supply for the larger motor would also be the battery shown. I suspect maybe this is pathetically easy? I'll be eternally thankful!---Adding information, to reply to a couple of folks who replied with questions. My device is a Railpro RC receiver, and yes, it's intended for HO/S scale, and I want to use it for G Scale. (Railpro does make G scale receivers, but they are too big to fit in smaller G scale locomotives.)I want to take the 0-18V coming out of the Railpro receiver, which normally would be used to drive a small motor up to 1A, and use that as the signal to drive the larger motor.This is the receiver:https://www.ringengineering.com/RailPro/Documents/LM-3Instructions.pdfTo add a little more info, the Railpro can take power anywhere from 12 to 20 volts, I think. And that voltage is the maximum voltage that it can then supply to a motor. Generally, I don't think it would ever be more than 18 volts on normal G scale equipment.Finally, I failed to state that the polarity would need to reverse when the "input" from the Railpro receiver reverses polarity. So the "Input Signal" is really -18 to +18 VDC. Thanks!

author: Analog Kid
author_rank: Super Contributor
timestamp: May 26, 2025, 10:58:11 pm
content: I assume the "manufactured model railroad locomotive control device" is a standard DCC decoder?So what exactly is its control output (voltage range)? That would be very helpful info for anyone designing the pink module.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 26, 2025, 11:57:16 pm
content: Ah, I think I get it (but not sure):The DCC decoder outputs the variable voltage for a motor, but only up to 1A or so (say, HO or O scale). But you're running something bigger, like say G scale, that draws more current (up to 5A peak), so you need something to provide more current, at whatever voltage the DCC decoder outputs.I don't have the answer, but it seems simple enough: basically a current amplifier. Someone should be along shortly to help you with that.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 12:26:57 am
content: OK, here's a first pass:Simple emitter-follower amplifier. Since its gain is < 1, the output voltage will be a little bit less than the input, but not much less. You'll need to pick a transistor that can handle the current (probably needs a heat sink as well).Problem:this will only work in the forward direction. Loco won't go in reverse at all. That'll take some modification.Others will step in shortly and tell us what else needs to be changed here ...

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 12:29:32 am
content: Analog Kid. Nice. I'll take that as a "Signal" that you're a fan.

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 12:30:18 am
content: I have clarified that by modifying my original post. Thanks!

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 12:31:47 am
content: Do you need it to run in reverse?

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 12:41:19 am
content: Yes, the polarity would need to reverse when the "input" from the Railpro receiver reverses. Sorry, I should have said that. So the "Input Signal" is -18 to +18.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 12:57:54 am
content: Now I'm guessing a push-pull emitter follower might work here.Anybody?

author: ledtester
author_rank: Super Contributor
timestamp: May 27, 2025, 01:25:39 am
content: How much space is there to work with? Can you provide a pic of what it looks like in your locomotive?QuoteHow does the G-scale receiver compare with the LM-3 in size?The LM-3 in all likelihood has some sort of H-bridge. One approach would be to see what can be done to increase the current capacity of the bridge. Possibilities include cooling, doubling up and replacement.The first step in the approach would be to open up the LM-3 to see what components it's using. It might even be the case that the bridge they are using can handle your current.Update: After looking at the LM-3 instruction PDF again it seems that it does measure current through the motor so there might be some software enforced limits.

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 01:36:51 am
content: Are you talking about cutting open a $120 device? And modifying it, like with a soldering iron? Thanks, but that's about as likely as me building a spacecraft in my back yard. 😃 If there's a reasonably inexpensive and straightforward way to do it, that would be cool, but otherwise I'd just abandon the project. The small steam engines I'm working with aren't worth enough to warrant cutting up control boards that cost practically as much as they do.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 01:44:04 am
content: So is your module in fact a LM-3? (Dunno how the previous poster figured that out.)(Duh; it was right there in your first post)Ifound the manual for itonline (made by Ring Engineering). What is it with these hobby manufacturers? They're apparently unwilling to reveal any of their "secret sauce": I was unable to find any real specs for this with an extensive online search. However, the manual does say that the maximum motor stall current is 8 amps, so it sounds like that module should be beefy enough to drive your larger loco motors. if their max. current is 5A.

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 01:51:06 am
content: That manual is for the LM4S which is intended for G-scale. I use those in larger locomotives and they work great. The device that I'm trying to work with here is the LM3, intended for smaller scale drains, which is only good for about 1 amp.

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 01:59:33 am
content: For the record, ring engineering is notorious for refusing to release any technical details at all. It's a sole proprietor business, and when the guy passes away, that will leave all of the gear basically undocumented. Works great, though.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 03:09:28 am
content: OK, so someone please come up with a circuit of a push-pull amp capable of supplying ~+-18V @ 5A, compact enough to fit in a small G-scale steam locomotive.(What is it, a 4-4-2?)

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 03:47:46 am
content: I "built" this li'l circuit in LTspice and ran it: it copied the input (12V p-p sinewave) directly to the output, so it should work with DC in both directions.You'll notice it has split supplies, which means you'd need two batteries.The TIP31/32s can handle a continuous current of 3A and a peak current of 5A.If that's not quite enough you can find beefier transistors, or use MOSFETs.If you want to try this, you should breadboard it first, of course.Either use the actual motor as a load or a big enough power resistor for a dummy load.See how hot the transistors get. You don't want to melt the body of your loco!

author: ledtester
author_rank: Super Contributor
timestamp: May 27, 2025, 04:01:20 am
content: Quote from: jwchenard on May 27, 2025, 01:59:33 amMaybe the guy can make a one-off device for you.Quote from: Analog Kid on May 27, 2025, 03:09:28 amAs I mentioned above, it would be nice to know how much space there is for the circuit.Suppose we try using a simple bridge driver like the L6203...https://www.st.com/en/motor-drivers/l6203.htmlWe need to figure out how to drive the EN, IN1 and IN2 inputs.Call the motor outputs of the L3 device M1 and M2. It stands to reason that these outputs will be 18V PWM signals -- for instance, in one direction we could have M1 be at ground and M2 be a 18V PWM signal and in the other direction M2 would be at ground and M1 a 18V PWM signal. There are other possibilities and looking at the signals with a would be helpful.From M1 and M2 it might be possible to derive signals for EN, IN1 and IN2. Could it be as simple as wiring M1 to IN1 and M2 to IN2?Quote from: Analog Kid on May 27, 2025, 03:47:46 amI don't think you'll have +/- supplies - but I could be wrong.

author: Ian.M
author_rank: Super Contributor
timestamp: May 27, 2025, 06:55:11 am
content: The odds are close to 100% that the Ring Engineering RailPro modules have on-board MOSFET H-bridges, either discrete or integrated. A bipolar complimentary emitter follower board increasing the load current for both outputs would have about 5W dissipation @3A load current, more if the module's outputs are *NOT* unfiltered rail-to-rail PWM, so with enough heatsinking not to cook itself, there is no way it + a LM3 is going to be smaller than a LM4S. A MOSFET H-Bridge + driver is more promising - you can use MOSFETS with low enough Rds(on) not to need heatsinking, but it + the LM3 still likely to be larger than a LM4S, simply because you are duplicating circuits already on the module. OTOH, the separate buffer board may offer more flexibility fitting available cavities in the loco, especially if you customise the layout for particular locos.However the show-stopper is likely to be current feedback. None of the buffer suggestions so far will proportionately load the original LM3 outputs with a fraction of the motor current, so the LM3 is going to 'see' a no motor situation. What it does in that situation is anybody's guess and may even change if you do a firmware update. If its control strategy is dumb enough, it may not matter - if the current sensing is only to set a current limit for protection. However if it senses actual motor current to do closed loop torque control for smoother acceleration and better low-speed running, expect problems . . .

author: jwchenard
author_rank: Newbie
timestamp: May 27, 2025, 09:08:42 am
content: A lot of that was a bit too technical for me, but I got the gist. Your final point however is extremely relevant. The Railpro devices do indeed monitor motor current and do all kinds of stuff with it. Controls speed when using multiple locomotives at a time. Adjusts the rate and volume of sound effects. Etc. I had not even thought of that, and as a practical matter it's probably a deal-breaker. I guess I will have to consider the classic fallback of pulling the battery and electronics behind the engine in a dedicated car. Crude but effective. Thanks for all your input.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 06:50:14 pm
content: Might be worthwhile asking Mr. Ring Engineering if he can provide a hybrid module that would fit and supply enough current (for less than an $arm plus a leg). Maybe he'd see it as an interesting challenge; who knows?You're probably not the only person in the world who needs a smaller module with higher current.