# What resistor to use for a single LED

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I have one of those general purpose 12V auto testers. It came with a tubular incandescent bulb, about the size of a 3AG fuse. The bulb burned out, and I am not going to waste a lot of time trying to find a replacement. The whole tester cost about \$15.

But before I toss it, I got to thinking that all I really need to do is open it up and wire a LED inside of it. Pretty basic. I think the LED should outlast those bulbs too. I have a bunch of LEDs, but none have the built in resistor.

These are just generic LEDs. I have several colors. None of them have any specifications. All I know is that they are the indicator type, not the super brights. I am aware there are all kinds of math formulas to determine the needed resistor, but since I dont know the LED specs, I can only take a guess at best.

Knowing this, what value resistor should I use? I'd rather go on the high side, so the LED dont burn out. If it seems dim, I can lower the resistor value some. What would be a safe resistance to use? This will ONLY be used on 12V DC automotive batteries.

I'll clip it together before soldering it on place, to make sure it's bright enough and working.

Thanks

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10/.015 = ohms. .015 squared times ohm = watts

Greg

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Run it at 10mA, they're normally specced 20mA max. LEDs drop 2-4v apx. So your R will see 14.4-4=10.4v at 10mA, so 1k is fine.

NT

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LED's have a typical forward Voltage drop of about 1.8V Red, 2V yellow, green, 3.2V blue. So assuming you are using a green one then it's just a quick bit of Ohms law.. Car battery charged is nominally 13.8V, take 2V off for the led = 11.8. Run the led at say 10mA then it's 11.8/0.010 = 1180 Ohms. An easy to find 1k resistor will do nicely.

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All Electronics in Van Nuys, California (actually part of Los Angeles) has them in 6V and 12V versions for less than US\$1 each (the 8V versions are a little more expensive, probably because they were used in Marantz receivers).

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At the risk of pointing out the obvious, an incandescent bulb doesn't care which end is positive. An LED on the other hand..................

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That can easily be addressed by using two LEDs in parallel, in opposite polarity.

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One of the important features of those testers is that they provide a load to the circuit under test, something the LED won't be able to provide.

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That too can be addressed with an appropriate resistor across the entire circuit.

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Yes, but now it's getting absurd. We're up to two LEDs (to counteract polarity, a current limiting resistor, and now a load resistor.

In the meantime, Oldfart could run to Harbor Freight and buy this for U.S. \$3.99:

If he pays attention to the Sunday circulars and can work a scissors (or tear paper reasonably well), he can get an LED flashlight, moving blanket, tarp, or DMM *free* with *any* purchase if he brings in a coupon.

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Agreed. I was just pointing out solutions.

Oldfart is going to do whatever he wants anyways. Probably something else absurd.

As long as the solution doesn't involve China. Unless that's convenient.

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I guess that depends on distance traveled.

Greg

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Thanks, 1K sounds like a winner. And I will likely use a green one too.

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Although this tester is shaped like a pen with a needle point (made to puncture wire insulation) and a wire with a clip on the end. I normally use the wire as the ground (to any metal part of the car body). But I do like the idea of putting two LEDs reversed polarity in there. Easy enough to do...

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Why would I need a load? This is not to test the car battery, it's just used to check for live wires under the dash or on fuses. I mostly use it when there is something like the heater blower wont run, or the radio is dead, or head or tail lights not working. Trailer light wiring... Stuff like that. My multimeter works for that too, but when I'm under the dash it's a lot easier to see the tester light up, than to have to look at a meter. Plus the wire puncture point on them testers is handy.

I should mention that the bulbs in those testers never seem to last very long. I bet changing to a LED will last me the rest of my life.

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On Friday, April 13, 2018 at 4:43:18 AM UTC-4, snipped-for-privacy@tubes.com wrote: But I do

Now I'll ask what is certain to be a dumb question. But maybe I'll learn something.

If we have two LEDs in parallel in reversed polarity, and we put 12 volts across them, aren't we exceeding both Vf and Vr? will they share the current, at some calculable ratio?

Sorry for my ignorance, that electrical circuits class was in the 80s.

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They both see 12v. Manufacturers only spec them to 5v but IRL they can survive far more.

NT

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rn something.

ts across them, aren't we exceeding both Vf and Vr? will they share the cu rrent, at some calculable ratio?

rvive far more.

Apologies, this is way too elementary for you all, but I'm a manager now, t hey haven't let me do anything technical for years.

Well of course they can both survive, you have a 1k current limiting resist or in the circuit. If the forward biased LED were alone, it would drop 1.6 V, the resistor would drop 10.4, the current would be 10 mA. (Assuming au tomotive 12 V) If the reverse biased LED were alone, it would drop 5 V, th e resistor would drop 7, the current would be 7 mA. When both LEDs are in the circuit, does some current flow through each? or does the 1.6V of whic hever one is forward biased limit the other to 1.6V, in which case it will never conduct, so there's no problem.

If Radio Shack still existed I wouldn't even ask. I'd just go buy two diod es and see what happened.

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If decide to use back-to-back diodes, I would use one of those bipolar red/green lamps instead of sticking with a single color.

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As usual,your lack of information with your first post, resulted in a shit load of wild goose chasing by others.

If you had mentioned it was just a cheap Chinese piece of shit pencil tester it would have been "1K resistor" and that would have been the end of it.

```--
"I am a river to my people."
Jeff-1.0 ```

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