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LED Reverse Polarity Protection

His point is that at 6V, the resistor is dissipating more power (power is voltage times current) than the LED.

There are special power supplies for driving big LEDs which use a technology called "switch mode power supply". It uses a trick with inductors to minimize this power loss in the resistor. However, for 20mA, it is probably overkill. If you have a big lantern battery, the thing will last for days pulling 20mA. The incandescent light bulb is probably pulling 5x or more of that current. If you used a switch mode power supply, it might double the time before the battery fails to 10x the endurance of the lamp with an incandescent bulb. However, it is somewhat complicated, and will require special chips or a purchased power supply to do it.

Regards, Bob Monsen

Reply to
Bob Monsen
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Dave, since you have the LED's datasheet on hand, what is the Vr rating? Many LEDs can handle 6V to 10V across them in reverse-polarity, although most are about 5V. If you're lucky...

... Steve

Reply to
Steve

It is 5v.

Reply to
Dave.H

On Feb 10, 6:57=A0am, "Dave.H" wrote: \\> [red LED]

The forward voltage of a red LED is compatible with two cell flashlights (3V fresh, 2V depleted) and it's a waste to drive it from a lantern battery (6V). Also, light from an LED is only shed into a hemisphere, not spherically as from a filament, so most of the flashlight reflector is not needed.

At 20 mA, two D cells will last about 500 hours; I'd go with that and not bother with more expensive lantern batteries.

Reply to
whit3rd

Blue, white, non-yellowish-green, violet, UV, purple and pink LEDs are extremely intolerant of reverse breakdown. The amount of reverse current through a 1N400x diode is enough to cause actual damage. To make your LED of such color immune to reverse polarity, add a second diode in parallel with the LED.

Then again, I never blew one of these LEDs at 9 volts, let alone 6. (I know, the datasheets only say they're good for up to 5 volts reverse reverse voltage, and some of the datasheets say avoid reverse bias.)

A reverse parallel diode is good if the LED will have any exposure to static. I have blown these LEDs with imperceptible amounts of static.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

I noticed elsewhere in the thread that this LED is a red one. I would just put a diode in series with it, probably a 1N4148 although I would not worry too much about a 1N400x one.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

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