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Old 31st July 2005, 13:31   #1
Sidecar Bob
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Default Tested circuits for LED headlights

This was originally posted in the Technical & Electronics section on Jan. 08, 2005.

After reading the replies to my LED headlight problem and searching the internet for more information, I have come up with a couple of circuits that seem to work with the minimum of components.

Richmond Controls RECOMMENDATIONS FOR USING WHITE LEDs confirms that I was on the right track in the first place: In a normal DC motor circuit, protection should be provided in the form of a diode reverse connected across the LED. In this connection, a reverse voltage will essentially be shorted out by the reverse diode, preventing the high reverse voltage from reaching the LED. If the locomotive lighting circuit has two LEDs, for forward and reverse headlights, generally one LED will serve as the reverse protection for the other if the LEDs are directly connected together back-to-back and share a common resistor. If they don't share a single resistor, each needs a protective diode.

In other words: you don't need diodes if you are using 2 LEDs for directional lighting in the same locomotive.

The circuit posted by Kettlestack has the diodes in the wrong place. It didn't look right to me, but I tried it because back to back LEDs had failed. I think the LEDs lasted longer with Kettlestack's circuit because reverse voltage is divided between the LED and the diode, but after about 3/4 hour of running (mostly just circling at 1/2 throttle while I worked on something else) the headlight failed.

I also came to the conclusion that connecting the capacitor across the motor isn't the best thing to do if you have a pulse throttle. The throttle's pulses will be absorbed by the capacitor if it is big enough to help the LEDs.

Richmond says: When a model railroad locomotive's motor briefly loses contact with the track while it is being driven, it will emit a voltage pulse. When the polarity of this pulse reverse biases the LED, the situation described in the previous section occurs, and the protective diode (rectifier or LED) will prevent the pulse from reaching the LED. When the polarity of this pulse forward biases a headlight LED, that LED will flash unless provisions are made to prevent the pulse from reaching the LED. These flashes can be observed in a dark environment if the flashing has not been prevented. Generally, the rear light will emit brief flashes when the locomotive is moving forward. A small ceramic capacitor connected directly across the LED will short circuit these high frequency pulses.

I had noticed these flashes. With the shell removed, I temporarily soldered in a pair of new LEDs: back to back with one 560 ohm 1/2 W resistor and tried several capacitors across the LEDs. With a 0.01 uf capacitor the flashes were about the same as no capacitor. With 2.2 uf the flashes were brighter. With 0.1 uf and 0.33 uf the flashes were greatly reduced. Since I have a bag of 0.33 on hand, I have decided to standardize on these for now.

I let it run for a couple of hours. The next day my son let it circle at 1/2 throttle for another 5 hours or so while I was at work. When I came home the LEDs both still worked and the resistor was just slightly warm to the touch, but not enough to matter (the motor was warmer).

My conclusion: this is the circuit that works reliably

If the LEDs light in the wrong directions, just reverse the connections to the original circuit in the locomotive. There's nothing magical about having the resistor connected to one side of the power or the other, as long as it is in series with the LEDs.

For replacement of single headlights use this circuit:

This is also suitable for building lighting. Vary the resistor value to suit.

As noted, I don't recommend 470 ohm resistors. Even a 560 ohm (504 min.) could allow 30 mA (the maximum rated current for the LEDs I am using).

My LED supplier sent me the following specs, and from what I have been able to find out they are pretty typical:

Emitted Colour : WHITE
Size (mm) : 5mm T1 3/4
Lens Colour : Water Clear
Forward Voltage : 3.2 - 3.5V
Reverse Current : <=30uA
Luminous Intensity Typ Iv : Average in 5000 mcd
Life Rating : 100,000 Hours
Viewing Angle : 20 ~ 25 Degree

Absolute Maximum Ratings (Ta=25°C):

Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

I also found this page of White LED Tips that includes circuits for constant lighting and information on how to make cheap white LEDs (with a bluish cast to the light) look more incandescent bulbs.
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