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Which resistor to use?

S

shocksyde

2[H]4U
Joined
Jan 24, 2001
Messages
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i know of resistor calculators on the web, but i have a specific question.

when calculating what resistor you need, do you use the max mA or the peak mA?
i'm going to hook (4) 3.4V LEDs together that have a "max" amp of 20mA but a "peak" amp of 75mA. The power source is from a car so use 14.4V as the source, just to be safe (even though my car generally stays in the 13.7 - 13.8 range.

what resistor should i use?
 
Ive never realy seen a "peak" ma rating on an led but yes typically you calculate leds for 20ma draw. The resistance will change depending on how you wire them.
 
thanks for the reply

another question if anyone is still paying attention to this thread...

i'm using the LEDs for my car's dash lighting.

do i need to use a rheostat to be able to control the LED brightness? or will the stock dimming knob in my car be able to handle that just fine?
 
Go with the max mA - 20mA or whatever it is.

But keep in mind that LEDs at max current tend to be ridiculously bright... far more than you might ever want in a car at night. You might want to pick up a few extra resistor values and try a few lower values, in the car, until you get something that's half reasonable.

Assuming you can tag onto the dash illumination voltage on your car, that'll be just fine for dimming the LEDs with everything else.
 
yeah i've already taken the dash out and inspected everything

there's a printed circuit that's behind the gauges with everything labeled, including "ILLUM +", so that made it easy. however, when i was about to test the exact voltage it puts out to the lights in the gauges, my digital multimeter ran out of batteries....life's a bitch
 
Leds have a minimum energising voltage, so the "dimmer" will probably work, but maybe not below 1/2 turn or so. Probably no biggie though.

They also shine light along a narrow beam, unlike the unidirectional bulbs the dash was most likely designed for, so Id suggest try some sanded plexi to disperse it a bit and maybe mount the leds sideways to illuminate more than a dot.

Anyway im just babbling, you could have factored for this stuff already, what do I know :)
 
Just a thought here, take it or leave it...

You may want to connect a rheostat instead of the car's dimmer because then you'd have two things: 1. A built-in backup system in case either your dash lights or led system goes down. 2. A really easy time reversing the process if you ever need to.

It would be as simple as one more mount and two more connections. Like I said, it's your project-take it or leave it.
 
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