LED Headlight replacement for MTH locomotives.

White Molex WM18873

They're available in other colors as well.

seems this needs a bump for someone looking for this.

Hey everyone! I came across this thread while trouble shooting some MTH light issues and the thoughts of moving my locos to LED. I have all MTH, PS2 locos. For quite some time now, I've been making my own replacement bulbs using 6v grain of wheat bulbs and crimping my own molex connectors. All has worked fine so far. I got curious about swapping things to 3mm LEDs so I went to confirm voltage outputs on my locos. So, here's where I got confused and I'm hoping you all can help. So, you guys know that on MTH locos with ditch lights, there's a contact board with the +/- plates that contacts the springs on the loco's body shell. When I put my voltmeter across that to check ditch light voltage, I'm getting 22v DC!? Is that right?? I checked other DC sources such as LED strip lights and power adapters where I KNOW the voltage and all checks out. The 6v GOW lights work but how could they on 22v DC without burning out instantly?? Something doesn't seem right. Am I doing something wrong? Is the voltmeter not reading something correctly??

Any help on this would be greatly appreciated.

Thank you!

The drivers are PWM pulse width modulated hence the erratic reads...

Don't worry about the voltage readings.  The LED with a 220 ohm or larger resistor will do the trick, the average voltage ends up being six volts DC.

TrainmanJohnson posted:

... I'm getting 22v DC!? Is that right?? I checked other DC sources such as LED strip lights and power adapters where I KNOW the voltage and all checks out. The 6v GOW lights work but how could they on 22v DC without burning out instantly?? Something doesn't seem right. Am I doing something wrong? Is the voltmeter not reading something correctly??

Thank you WILLYGEE, GRJ & STAN2004!! All that makes sense. I never considered it being PWM signal. When I saw 22v I was like, what?? At least now I know I'm not "completely" crazy ;-)  And I will definitely mind the polarity... Now I know I can trust the meter's polarity reading in this situation. 

Thank you again!

Oh, I forgot one thing to clarify... .5 watt or .25 watt resistor?

1/4W is fine, it's only carrying 20ma and dropping an average of around 3 volts.

Thank you GRJ!

Depending what you connected to and what other lights driven from that contact pad (Markers) you could have measured Positive Voltage against PCB Ground.  That would have given you 22VDC if track was at 18VAC.  Measurement for actual light out put should have been lower, unless you have a shorted fet.  In which case that 220 ohm resistor will not work as you will have 22VDC on the light output.  G

George, a cheap meter will measure the peaks of the PWM lamp signal and give you an excessive reading.  A simple means of telling if the FET is shorted is if the existing lights still work.

Thank you guys for your replies! Are you referring to shorted MOSFETs?

Yes, George was speaking of a shorted light FET.

John, 22VDC is the PV against ground.  I do not think even a cheap meter will show that voltage if on the PWM Return.  Why guess.    I am just warning to make sure he measured PV (purple) against the return of the ditch light.  Not some other random ground.  G

Trainmanjohnson, was that 22V DC measurement made when there was a bulb attached?  And was this a PS2 5V board (with 9V type battery)? 

As I recall, those board have capacitors on the lighting outputs so if the measurements were made with NO bulb attached the DC voltage might show the peak voltage on a typical digital meter.

Hey GGG! When putting the probes to the ditch light board, the positive is the single plate and, yes, it was indeed the purple wire. The negative for the Dutch lights are the two independent plates, on the same board of course. I was only probing the solder plates that come in contact with the springs for the ditch light molecule connectors.

DITCH lights... Not Dutch lights ;-)

Darn spell correct... MOLEX CONNECTORS...

Hey Stan2004... The measurement was made with no bulbs attached. Just sitting on the track in neutral with ditch lights set to ON. And it's the 3v system...2 NiCads in the blue pack

Should I attach a 6v gow bulb while probing the voltage?

Oh, and my other locos all "measured" the same 22 vdc on my standard meter

If the bulbs are working, you can just use the 220 ohm resistor and the LED, no further measurements required.  Just make sure to get the polarity right on the LED, job done.

John, from the start of the post I have had many people wanting to buy some done up.  The directions are easy to follow.

Awesome.. Thank you guys!! I did want to tell you that I wrapped a gow bulb around my + & - meter probes and tested. With the bulb on the probes and lit, the meter is showing about 1.97vdc. Without the bulbs, 22vdc. Thanks again for all your input! The led conversion shouldn't be a problem. I got even more curious about figuring voltages on these locos.

Yup, that's about what I'd expect if a bulb is attached.  My prediction for when the bulb is in place is 22V x (6*6) / (22*22) = 1.64V DC which is close enough to your 1.97V DC.  

The eyes glaze over when discussing exactly what's going on with pulse modulated DC, true-RMS voltages, and how the typical hobby-grade meter responds so I'll just leave it at that.   

Marty Fitzhenry posted:

John, from the start of the post I have had many people wanting to buy some done up.  The directions are easy to follow.

I'm not really getting the difficulty either, a resistor and an LED, it doesn't get a lot easier than that.  All of this measuring and guessing is pointless if the existing 6V bulbs work!  Obviously, the headlight circuit is working...

I've done a bunch of them, and I have a few sitting behind me that get the treatment.  Whenever I run my PS/2 stuff at the club with the LED's, I get a bunch of requests to fix more of them.

Ya learn something new everyday and I like knowing how things work, inside and out!  :-D Especially our trains!

Hey GRJ! It doesn't really have anything to do with the difficulty... It's a very simple thing. I personally like to know the "why" behind things. If I'm told to us a certain resistor, I like to know why. So measuring voltages and such it's just figuring out the whole picture. So, it's not really pointless. It's just learning more of how it all works....A better understanding.

Thank you for all your advice!

Today, many of the diesels we buy already run LED headlights.  For steam, I am not a fan of the bright whites.  I like the yellowish/amber LED.  Sadly, I am old enough to remember steam and always remember the big Boston and Maine freight engines in the distance with the yellow looking headlights. 

I use the warm white in steam, and if it's old enough, I actually use amber or yellow for stuff like the 4-4-0 steamers.  I even use the warm white in many early diesels, the later stuff I ask what the customer would like to see.

I tried yellow strips in some heavyweight passenger cars, and they look cool as beans, so I'm going to do that from now on for my heavyweights.  The different colors really do make a difference in the look.

GRJ, I never considered beans as "cool."

You've just never had the beans I get.

LOL! I thought it was great when I saw GRJ refer to beans being cool...I'm all the time saying "cool beans" ;-)

I am a day late and a dollar short for the conversation.  So am I correct in assuming that I can just wire a LED directly to the appropriate wires on a PS 3 board without a resistor?

Correct, the PS/3 board drives LED's directly.

You are a mass of knowlege and wisom as always GR

Well, in this case, fairly simple knowledge, but thanks.

Note that this is the "real" PS/3 board, not the PS32 board that is in the steam upgrade kit or the similar board used to replaced older PS/2 boards.  Those have a special daughter board that still outputs 6V for the lights, and so you'll need the 220 or larger resistor for the LED.

This is a real PS 3 board so LED's should not be a problem.  I need to power 4 LED's off the same line. One for the headlight, one for the cab, and two for the number boards. Can I hook them all to the same wires?  The LED's are rated at 1.5 volts.