Led powered from AC tracks diode not necessarily needed

I run a 7 car set of Lionel 10" streamliners lighted simply with 12 volt LED strips.  I wouldn't recommend doing this with cars in a command control environment.

Bruce

It takes a while, and I suspect some LED's aren't as tolerant of reverse voltage as others.  I know that before I started diode protecting LED's running on track power that I lost several for no apparent reason.  Since I have wired them with diodes, I've never lost another one.

Bruce, I tried an experiment with a segment of the LED strips some time back when this topic came up.  They run BRIGHT and HOT at 18V track power.  After a couple of hours, one of the three LED segments was dead, one of the LED's failed.  I'd use proper current limiting...

The LED strips typically have 3 LEDs in series.  Since each LED has a reverse voltage rating of at least 5 volts, the string of 3 would have a nominal reverse voltage of more that 15 volts.  Although this isn't quite up to the 25V peak from an 18V sinewave, at least you are in the right direction.

Kingbright's data sheet says that continuous reverse bias can result in ion migration.  That is usually a rather slow process, and our intermittent operation may give us quite a few operating hours before any problems pop up.

But for my money, four pennies for a 1N4148 diode buys peace of mind.

Since I've had defective LED's in the strips out of the box, I probably had a marginal one.   However, I used to lose an LED occasionally when I ran them with no diodes, one resistor and the LED on track power.

Another factor, at least for my taste, the LED strips on full power are WAY too bright for passenger cars!  I typically run on 20-30 ma for an entire 18" car, running them at full power would look like sunshine in the cars.

 But for my money, four pennies for a 1N4148 diode buys peace of mind.

 It's on my lower list...or the diode spirits will get me.willygee

I suppose you could wire 2 equal strips back to back and not use a diode.

Dale H

Many a manufacture I have seen don't use the diodes on series wired markers in tenders when powering from Track AC.  G

And manufacturers never do something that exceeds a component's ratings...

Dale H, back-to-back strips won't help the situation because there isn't just one limiting resistor for both strings of LEDs.  If just the LEDs are back-to-back, that would indeed limit the reverse voltage to be the same as the forward voltage of the ON LEDs, but since the series resistor on the forward-bias string sops up a bunch of extra voltage, the total reverse voltage is higher.  If you ran two strings off 18V and added a series resistor feeding both strings to keep the effective voltage down near 12V, that would be better.  One of the problems is still the peak of the sinewave, which would be higher than 12V even with the series resistor.

The moral of this story is use the proper design practices.

Thanks Dale

Just seems like people want to take a short cut and save on a 5 cent diode. I run into this a lot with a lot of circuits. I experimented wiring 2 equal strips in series with a bridge rectifier and capacitor. That splits the 24VDC to 12 volts to each strip. If they are still too bright a series resistor can still be added. Without a capacitor,passenger car lighting would tend to flicker. Without a diode or bridge you can't use a capacitor.   This seems to work well shown here

LINK

Dale H

I don't know do you  I just have found those markers in the K-line tenders pretty robust, and wired directly off the Track where all those Voltage spikes are on unprotected tracks and that little 1k resistor seems to work.  Nice and simple.

Not saying a more complex, rectified setup doesn't needed it.  Or that it isn't more protective model.  But is it absolutely necessary?

The mechanical guys will have you balance every motor and flywheel to improve efficiency and reduce motor failure.    G