IR Ring

[tyler.martin]

Hey, I need some help here... Im kinda confused about a few things...

I have a IR camera that is pointing at a studio stage, and the issue is I dont have any IR source on the stage during a blackout. So I picked up some IR LED on ebay for dirt, and now im trying to figure out how to wire them...
There forward voltage is 1.5V and there forward current is 60mA.

I have a 12V wall wort that outputs 1.2A, and I would like to have 3 rings of 8 leds, what size resistor should i use? When i got them, they had some 10 Ohm resistors in the pkg too, but according to my math that wont even work for a single led on a 12v source...

any ideas?

 

[00AVD]

That 60mA figure will likely be their max current. I wouldn't drive them any more than 20mA - 30mA. At 60mA, they will not last long at all.

Given a regulated 12V source and the fact you want the LEDs in groups of 8, I'd make up strings of 4 LEDs in series with a 220R - 330R resistor. You'd use 2 of these strings in parallel on the 12V supply for each ring.

Does that make sense?

 

[zuixro]

LED series parallel array wizard

 

[00AVD]

That's quite neat, although it a bit simplistic.

I just tried 8 x 1.5V (Vf) LEDs with a 12V supply and it suggested all 8 in series with a 1R resistor.

In reality, you'd never use that combination as it makes no allowance for batch, voltage and temperature variations and the result would be very unreliable.

Strings of 4 with a resistor method is guaranteed to work over a much wider spread of values and gives the best practical wiring method.

 

[zuixro]

Did you actually meter the wall wart? Because some of them are just a transformer, a rectifier, and a capacitor. The output voltage can vary wildly (based on load, temperature, input voltage). It wouldn't be a bad idea to put a 7805 voltage regulator in there.

I've had a 12v wall wart put out 18v.

 

[00AVD]

I took a punt that the OP's 12V was regulated given the 1.2A rating, but that's not a certain thing of course.

Using a 7805 on a 12V unregulated PSU (up to 18v) will generate a not insignificant amount of heat. You also need to ensure stability of the regulator (decoupling capacitors) and not exceed its dissipation ratings.

All too hard for the OP I reckon. Just make sure the PSU is 12V regulated or measure the actual voltage and choose a suitable series resistor.

If indeed it's 15V (18V), then 8 LEDs in series with one 120R (220R) resistor would result in lower heat losses and easier wiring per ring.

Keeping it simple (without a regulator) also means the LED assembly won't be damaged by accidental reverse polarity. KISS is the key here.

 

[zuixro]

The 8 LED's with the resistor will certainly work. I'm just saying what I would do.

Actually I would go with an LM317 Current regulator: LM317 / LM338 / LM350 Voltage and Current Regulator Calculators

It's a lot more complicated, but it would drive them at the correct voltage and current regardless of input voltage and temperature. (although it's not the most efficient driver) I'll explain it if someone wants.

 

[00AVD]

I've used the LM317 a few times for LED strings and it works great. Once you have the component values worked out, it makes for a very stable system.

I agree that it's not as easy to implement as the resistor method.