Hi all, on a 12v circuit if I was to design a simple tester using an LED what would be the best resistor as a safe measure.

I was thinking perhaps 1kohm?

 

A 1k Ohm 1/4 Watt resistor would allow 10.3mA of current through a standard 1.7v red LED.
It would allow 8.1mA of current through a standard 3.9v white LED.

Yes, it would be safe. However, you may wish to use a 510 Ohm 1/2 Watt resistor instead.
A 510 Ohm resistor would allow 20.2mA of current through a standard 1.7v red LED.
It would allow 15.9mA of current through a standard 3.9v white LED.

 

Thanks SGT thats great.

 

Well, you can increase the voltage a bit, if yo have the possibility. I think a 470Ohm resistor would be a good bet for 12V. Most old LEDs hardly stand a 20mA current, and you have to take into account the resistor tolerance. Thus a 510Ohm resistor with 5% tolerance can have around 484Ohm, and that is a bit unsafe.

If you have the possibility to use a higher voltage, you can use 24V to test your LEDs. Thus, the intensity varies less with the type of LED being tested. Of course you should add a safety switch as this might be a bit dangerous. If you use 24V DC you should use a 1K resistor supporting 2W (safety factor is 2, and the actual wattage is 0.49W, gives 0.98W but 1W is the next rating available). To calculate the resistor value, use this formula:
Rres = (Vtot - Vled) / Iled

To calculate the wattage (without safety factor):
Pres = (Vtot - Vled) ^ 2 / Rres

The wattage is best calculated using voltage rather than current, since it will give a more precise result (current varies much more than forward voltage, according to the I/V curves of and diode, LEDs included).

 

I like current regulators, but when I built one I used a variable resistor in series with a 100Ω, the 100Ω is good for current sensing, I measure the voltage across it and adjust for a specific value.

I have been seeing comercial units from BG Micro and other sources with sockets for 5ma, 10ma, 20ma, and 50ma. They use a 9V battery, and are quite reasonable in price (if memory serves I think it was $10, maybe $5). I've bought several of those at the various electronic flea markets in Dallas.

Thinking about it I would use a LM317 in current regulator mode and a 100Ω resistor if I were to do it now.

 

Indeed. A current regulator would be a good idea. I thought on it while sleeping. If you use a LM317 as a current regulator, you don't need to use high voltages anymore. Even with 9V it should work. Don't forget that the LM317 needs about 3V in excess.