Re: LED lighting



Randall wrote:

Actually, running an LED on an AC system will shorten it's life. 

Perhaps, but I've not had one fail yet.

An LED is essentially a diode (Light Emitting Diode) which prevents current flow in the opposite direction. When in the proper current flow, it emits light. If you apply reverse polarity, it will be trying to prevent the flow of current in the reverse current and hence will probably burn it out during the reverse current phase.

There is no reverse current - it's a diode, and won't burn out if it has an appropriate reverse _voltage_ rating.

If you remember the older sleds that used diode bridges to change AC to DC current for charging batteries, they had to be mounted onto a metal surface to dissipate heat. Those diodes are handling quite a bit of current. Those itty bitty diodes don't stand a chance.

Sure, if you try to charge batteries with a LED rectifier, it will blow - they aren't rated for the forward current.

Plus, even if it didn't burn out, it would be constantly switching between off/on and thus much reduced light output. It actually gets worse the faster the phase.

I use 6 LEDS in parrallel running on 12v AC to calibrate my turntable after the neon tube died - works perfectly :-)

Even if you were to add a mini diode bridge to rectify the AC to DC before it got to the LED, it would still not be 100% since it would be a "faked" DC current. Voltage would still go +12V 0V +12V 0V instead of +12V 0V -12V 0V +12 0V -12V but it would prevent it from burning out since the reverse phase is eliminated. You're simply changing the reverse phase back to a positive phase. 

That's why you put a filter capacitor after the bridge rectifier.

I recently built a GPS power supply for my sled. I used a bridge rectifier and filter capacitor, followed by a DC-DC converter - one power transistor, zener diode, and a couple of resistors. Output is nice clean 5v DC for the GPS unit.

Sorry if it got a bit technical and complicated, just a bit of my old college electronics course coming back to me. 

;-)

You're right, it doesn't make sense to run LEDS on AC when you can build a sled DC power supply for $10 - less if you get the parts from a dead TV :-) (Actually it was easier to raid the TV than drive to the stores)

Sunny
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