Large Scale Central

LED and Diode Reverse Current Question

Electronics
#1

My grandson likes to disassemble computers and salvage parts. He had a couple of LEDs and wanted to know if we could use them on my railroad. We got together and put them on a Hartland Mighty Mack as marker lights on the roof. We wired them so forward was the front headlight (which we had converted to an LED) and green LED; reverse is the rear white light (LED) and red LED. Test:

The problem is that whichever LED is supposed to be off blinks. In the video below, note that the green LED in on and the red LED is flashing (look carefully):

When I noticed this problem, I put a Radio Shack diode (below) in series with each LED in order to block the reverse current but the ‘off’ LED still blinks. How do I solve the 'flashing/blinking problem?

Diode

Thanks,
David

#2

What is your power supply. Is it possible there may be some stray AC on the rails? The flashing looks like it may happen when there is a slight loss of track power.

#3

Jon Radder said:

What is your power supply. Is it possible there may be some stray AC on the rails? The flashing looks like it may happen when there is a slight loss of track power.

Thanks for the comment. My power supply is an MRC Tech II Railmaster 2400, originally used on my HO layout, now in service on my indoor test track. Not current high tech, but has done a good job with my low amp engines. I never have the ‘Pulse’ switch on when running. I don’t know how clean the output is. 120v input, 14 Volts DC variable output for the trains and 18.5 Volts AC output for accessories and another 15 Volt DC fixed output.

#4

I have no data, but I suspect it’s a half-wave rectifier. You could easily add a robust bridge rectifier to the output. You could prove that the supply is the issue if you have a drill battery you could hook up to the loco with clip leads. I suspect on pure DC with a solid connection, you would not get the flashes. It may also be back EMF from the motor as it momentarily looses connection with the track. You could hook up directly to your supply (not with track) and see if the problem goes away.

#5

Perhaps a capacitor wired in parallel with the LED will smooth out these “peaks” and solve the problem. Use the series diode to be sure it only gets dc and is rated for more voltage than it will ever receive. I do this for the LEDs in my streamliners to prevent them from flickering over dirty track.

#6

I disconnected the MRC unit and connected an 11.1 volt Li Ion battery pack directly to the track:

The flicker appeared to be exactly the same (I tried slo-mo but YouTube speeded it up so I ended up with a grainy, fast video):

I did find more specs on the diodes I added and it shows ‘Reverse Current at Peak Inverse Voltage’ of 10 Micro Amps. I assume that the diode is feeding that amperage back to the off side of the LED when at PIV, which is 200V for this diode. Is there enough amperage at the 4 or 5 volts of the power feed to flicker the LED?

I have used capacitors to keep passenger car lights on but not sure how to keep an LED off.

Still a lot to learn, I’ll keep trying. Appreciate your suggestions.

Thanks,
David

#7

OK - Can you take that pack and connect it directly to the loco - no track? I still suspect intermittent loss of track power may be the cause.

#8

Jon Radder said:

OK - Can you take that pack and connect it directly to the loco - no track? I still suspect intermittent loss of track power may be the cause.

Well, Gentlemen, your assumptions are correct. I hooked the battery pack to a Kadee wheel cleaner and ran the motor in both directions. If I held the cleaner firmly to the wheels, flicker was eliminated. If I applied intermittent pressure, the ‘off’ LED flickered, as shown in the video. I guess the next attempt will be the parallel capacitor. Since that will require the third disassembly, I will take time to do some other up grades. Will report back when it is finished. Thanks for your insights!

Regards,
David

#9

Just a thought,

It appears the flickering LED lights when the loco looses connection with the track.

I’m wondering if the motor is acting as a generator as it free spins, especially if it has a flywheel, when it no longer has input voltage applied and back feeds voltage to the LED? A capacitor in the circuit may even intensify the problem. It doesn’t take much voltage or amperage to momentarily light an LED.

Adam

#10

Adam Dziuk said:

Just a thought,

It appears the flickering LED lights when the loco looses connection with the track.

I’m wondering if the motor is acting as a generator as it free spins, especially if it has a flywheel, when it no longer has input voltage applied and back feeds voltage to the LED? A capacitor in the circuit may even intensify the problem. It doesn’t take much voltage or amperage to momentarily light an LED.

Adam

Interesting idea. There is no flywheel and, of course, it has worm gears but there is some momentum when it loses power. Maybe I just need to make it into a test sled when I take the cab off and try some ideas.

Or maybe make this the next battery RC conversion on my list and go with direct lighting from the RC receiver. Hmmm…