Last April I bought 8 LEDs from Town & Country Hobbies and installed them in 4 passenger cars. They were OK at first, but now I wonder if I am doing something wrong. There is not much info at that website, but they imply that they are direct replacements for lionel lamps, such as number 432. They look just like the screw base incandescent 432s, and are rated at 18V. The trains were not used at all over the past few months, and I am just setting them up again. One bulb is just burned out, and 2 of them won't show any light unless the voltage is over 11 volts. I initially thought they were burned out also. I took the burned out bulb apart, and I find there is a 470 ohm resistor in series with the LED. No diode is present, unless it is inside the LED.
If I put a diode in series wit the lamp assembly, the bulb lights in only one polarity, which is what I would expect.
Could these LEDs be getting damaged by the reverse voltage?
I went to another website that sells LEDs for model trains, and they mention that they put a diode in the base of the lamp, and that their bulbs light up with either polarity of DC, or with AC.
A simple LED in series with a resistor will be damaged by reverse DC voltage over about 5 volts, or AC voltage over about 3.5 volts RMS. There are several ways to design an incandescent replacement to prevent this. One, which you mentioned, is a diode inside the LED. This can be in series or parallel. In either case, the LED will light with only one polarity of DC. Another possibility is two LEDs in anti-parallel, so that one lights on either polarity, and the LEDs protect each other. Yet another possibility is a bridge rectifier upstream of the LED, which will light with either polarity.
But I know no way to get a simple LED to light with both polarities using only a single diode and the ballast resistor.
Bob Nelson
I did some testing of a few LEDs I had in my workshop. All of them would withstand 30 volts DC, which is pretty close to the peak voltage from an 18 v sine wave. (25.46v). This includes one over 20 years old which has a PIV (Peak Inverse Voltage) rating of only 3 vdc. I used a dc power supply with a 1 megohm resistor in series with the LED.
A "good" Town&Country 423 LED withstands 25.6 vdc, reverse, but starts drawing a little current.
The 3 bad LEDs are in effect shorted in the reverse direction.
I emailed Town & Country to find if they expected these to be used on ac, and they promptly replied:
"Yes the LEDs work on AC". Next I will try to find what the PIV ratings on the LEDs is.
I bought 25 red & 25 green, also from Town & Country Hobbies. The reds seem to be fine. 23 of the greens are no good... burned out quickly. All of them, green & red, were/are being used in K-Line switches, all from the same power source, all on the same layout.
Rob
Bob.MNo diode is present, unless it is inside the LED.
It's true that a light-emitting diode is itself a diode. But the concern here is that they typically are specified for a maximum reverse voltage of only 5 volts. So, for an AC circuit, it is usual to supplement the LED with an ordinary diode, either in series to block the reverse voltage, or in antiparallel, to shunt the reverse current around the LED and limit the voltage to the forward voltage of the added diode.
It seems to me that the manufacturer was betting on the LEDs' standing much more reverse voltage than they were meant to. I think he lost his bet.
ADCX Rob 23 of the greens are no good... burned out quickly. Rob
23 of the greens are no good... burned out quickly.
Here is what the people at Town& country said:
"Not sure of the PIV. If they where Green LEDs we did have some defective ones. You can return them and we will send you new ones.
I have had no problems with the clear (blue tint) LED-1445 bayonet bulbs which fit in the O-22 switches and their controllers. I think that is because they run on a constant 16vac. The track voltage is a little higher.
Maybe what we need is some transient protection. There have been several discussions about the need for that with the newer electronics in the engines.
lionelsoni It seems to me that the manufacturer was betting on the LEDs' standing much more reverse voltage than they were meant to. I think he lost his bet.
He probably did the same tests I did, and assumed they all would accept ~30v. The way I fixed the problem, at least for the remaining "good" bulbs is to add one diode in series with the 2 lamps inside the car. It made for a neater and more reliable connection to the lamps which previously had wires under the car which flexed on each curve. Now there is only 1 wire which has to flex, the one supplying the diode. This fix won't work on the Lionel passenger cars, since they have a more reliable connection to start with.
I wonder if any LED manufacturers make an AC led? i.e., put 2 LED chips inside the package, back to back, or in antiparallel as you mentioned. They would be brighter, and would protect each other.
Guys,
The thing you have to watch is not only the peak reverse voltage rating of the diode, but also how much current is going through the diode when it is reverse biased. All diodes will break down and act like zener diodes when the reverse voltage is high enough. Then, unless the current is limited by a resister, the current will become very high and destroy the diode. This happens with all diodes. If you put a high voltage diode (1N459, for example) in series with the LED, the 1N459 will protect the LED as its reverse breakdown voltage is about 250 volts. A 1 meg resistor in series with an LED should limit the current to a low enough value that the LED is not damaged. The LED cannot stand as much reverse voltage as forward voltage because the power dissipated in the LED is much higher in the reverse direction than the forward direction. The forward voltage drop of most diodes is around half a volt and the reverse voltage of the LEDs is ~3 volts. Using a current limiting resistor (470 ohms) the current is about the same in both directions, so the power dissipation in the reverse direction is probably enough to destroy the junction in the LED. I suggest you point this out to the folks at Town & Country.
Bruce Baker
"LEDs DO NOT GET HOT AT ALL!"
This statement on their web site suggested to me that they don't really understand the electrical principles involved.
lionelsoni "LEDs DO NOT GET HOT AT ALL!"
I took one of their bulbs apart, and noticed a very small 470 ohm resistor. The 1/4 watt I was using for testing ran very hot. I would guess they used a 1/8 watt. Lets see; 16 volts squared is 256. Divide by 470 is 0.54 watts, but that is assuming full sine wave. I guess actual would be 1/2 of that for half wave. Still too much power and heat. Their bulb would feel quite hot.
I gave up on bulk LEDs and buy LEDs from Evans Designs. He builds a bridge rectifier into the circuit so that it hooks up quickly and doesn't fail. Dave's always very helpful, has reasonable prices, ships quickly, and stands behind his products 100%. See for yourself...
http://www.modeltrainsoftware.com/bl-212.html
Cheers!
I was about to buy some LED's to replace my old GE Mazda lights, but from what I am reading here, I would need to buy some extra kind of current protection?
I have 2 LED's so far and would like to know if I need to protect these somehow. Bought them at a train show in Lake Worth FL and they are screw-in LED's. Have not examined the LED's I bought so I don't know what I have, but was told they will work with Lionel passenger cars # 2460 from the 1940's.
Lee F.
The 2460 is the 12-wheel Bucyrus Erie crane car. However, Lionel did make many small streamliners in the 2400 series. But I believe that all the early ones used number 51 lamps, which are 7.5-volt 220-milliampere G-3.5 bayonet lamps, two of which were wired in series.
Whether you need to protect your LED assemblies depends on whether they already have protective diodes inside the assembly. Any protection needed is from voltage, not current. Whether or not they are already protected, you can do the job by putting a diode, like a 1N4001, in series. If they don't light up, reverse the diode.
The correct number for the cars is 2640 from about 1941. I am using an older style 275 watt ZW with 10 amp circuit breakers on the A to D outputs.
What would be a Radio Shack equivalent part? Or at least ratings for the diode.
Radio Shack has 1N4001, 1N4003, 1N4004, and 1N4005, any of which will work.
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