What resistor to use for a single LED

Okay. 12 volt car battery, 1k resistor, the most current you can get is 12 mA, any LED should handle that. But that's not the fault condition I was wondering idly about.

My first thought is if he used one LED and connected it backwards, he could think a circuit was dead that was really hot, and burn up his new red spor ts car. Or he'd always have to fuss with getting the right lead on. I did n't realize he had a ground clamp and probe, so I was probably just wrong o n this one. I was thinking two probes.

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

12 mA, any LED should handle that. But that's not the fault condition I wa s wondering idly about. n

ld think a circuit was dead that was really hot, and burn up his new red sp orts car. Or he'd always have to fuss with getting the right lead on. I d idn't realize he had a ground clamp and probe, so I was probably just wrong on this one. I was thinking two probes.

ve 5 volts, then they're both conducting. How much current goes through th e forward biased one? enough to give a bright light, or maybe dim enough h e thinks the circuit is dead and he burns up his new red sports car.

someone is not familiar with LEDs

You put them so one diodes anode is connected to the cathode of the other. Only one will lite at a time on DC.

Best little handy dandy checker I ever had is one made by Fluke. I think it is a T2. Has about 10 or 12 LEDs in it and 2 leads. You just hook the leads to anything you want to check. If open, nothing hapens. if low resistance it beeps and a led lights up. Anything over about 3 volts DC and it gives an indication of power and direction. Good to about 200 or 250 volts dc. When used on ac, it starts somewhat higher and goes to atleast 600 volts. The leds inbetween nothing and mix give a rough indication of more common voltges such as 120,220 480 and 600 volts AC. It starts at 6, 12, 24 and I forget the others for DC.

One variant is to get a bicolor LED; these are usually red forward, green reverse biased. It's better not to tease the R/G colorblind population, though, and a white LED pair, behind + and - shaped windows, would be a better indicator.