I'm trying to make a circuit with ~100 leds in parallel from an automotive source. each led has 1.5 forward voltage and assuming the alternator is pushing 14.4 volts, how will i determine the resistor to choose? i ordered the leds online in bulk and the shipper sent free resistors (470 ohm) as a 'gift' and im not sure how they chose that.

Can i just use R=V/I (finding current with a multimeter)?

Also, will there need to be a resistor at each led or is there a way to just place a few at the beginning of the circuit prior to the leds?

thanks for any advice!

 

Welcome to AAC!

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

It covers the basics and more. Be very sure of the Vf, it could bite you if you have it wrong. They sound like older LEDs.

Another thing to be aware of at this site,

Automotive Guidelines

 

You mention 1.5v forward voltage. What are they, infrared or ultraviolet LEDs? Why would you need that in a car? Typical LEDs for visible light start at around 2v and go up from there.

You're saying 100 LEDs. Why so many? What are you trying to do?

 

Wook, the 2V limit is a function of the newer high brightness LEDs, older red models ran around 1.5V. At the moment my assumption is the OP has a bag of these antiques. We'll find out.

 

It seems sketchy but I assure you its all off road, nothing illegal or dangerous. I have a backup camera with no light or built in IR source, so im going to be using one of the 3 lights that light up my plate as a socket source for a chain of ir leds, and then ill use a relay to control it from inside.

theyre infrared (850 nm). ive read over the notes on this site for automotive, and im assuming as well that theyre the older ones because its claimed to have a 1.6v forward voltage. is there a way i can test the leds myself for the correct value?

im no expert but i do have experience with automotive wiring, relays, basic circuitry and im always trying to learn a little more

 

IR LEDs naturally use less voltage, so the 1.5V Vf is a reasonable number. Are you going to try running them off the car battery? With the engine off the voltage drops to around 12VDC.

The article I referred you to has several regulator schemes that would work, both current and voltage. Which you select doesn't really matter, the end effect is the same.

I'd use 14 chains of 7 LEDs each, and regulate the current through the chains, using a modern low dropout voltage regulator and individual resistors per chain. It should be pretty simple.

 

agreed on the 12v but they would only ever be on with the ignition running, so the 14.4 volts can be assumed constant and i wont need a regulator for this application. they'll be run off an existing source that i'll fuse, i just dont understand the chain idea. are you suggesting to have 14 chains in parallel, with the 7 leds in each chain in series using just one resistor? (as in a total of 14 resistors in the entire set up?)
again, how can i test the forward voltage myself?
i guess my main question is would it be reasonable to somehow do this all in one parallel circuit instead of chains?


edit: ignore the question about single resistor per chain, just realised its been answered!

 

The math is pretty straightforward, and spelled out in the link.

I don't know the current you are aiming for, so I'll assume 10ma (I believe IR LEDs aren't as robust, but I'm not sure about this).

7 LEDs in a chain mean they will drop a total of 10.5V. If your power supply is 14.4VDC then the resistor drops 3.9V at 10ma. Ohm's law works this out to 390Ω. So you will need 14 390Ω ¼W resistors and no regulator.