# LED calculations for amatuer

#### Zellarman

Nov 9, 2011
8
Please excuse this long post. I'm starting to play around with some led circuits, different voltage power supplies driving single or multiple LED's in series with resistors as needed. I've seen the calculations for determining the necessary resistors, and can follow them, but ultimately just use the on-line calculators available for this. BUT I'm having a hard time understanding and determining the best, or most efficient power supply (transformer) and its necessary amperage rating. I'm capable of using Ohm's law, but I'm missing (or at leat think I am) some necesary info, and when I test the circuit's current my multimeter isn't reading what I'd expect, except when powering a single LED w/o any resistors. Being a newbie, my grasp on the workings of an LED is very lacking, I know that current only flows in one direction, and they operate most efficiently at a specific current. I also played around a little just to actually see thermal runaway, and I did, powering the led w/o a resistor and an unregulated power source, I saw the current through it increase the longer I powered it, I also dipped it in ice water and then watched the current drop as it cooled. This is all good, but I only confirmed for myself what most of you already know, however trying to determine a resistance was futile, though it was again interesting to witness a very high reading slowly decrease, I guess from the little heat genereated by the current running through it from the multimeter. So, are there equations that a simpleton could use to determine the necessary transformer current? The circuits I'm dealing with will change depending on the required number of LED's, the LED's may change, the resistors will change, I think it's more the voltage that I'm looking to determine to keep the required amperage as low as possible. Today while testing circuits a single +- 3v led powered by an unregulated ps 3.4 v no load read a little higher than the rated 24 ma as expected, but when I hooked up 4 in series and powered them w/ 12 v regulated ps the tested current was just around 5 ma. Now changing things further, 3 of the same LED's wired in series with a 150 ohm resister draw 15 ma when powered by a 12v regulated ps. Going forward with this circuit, I have 9 panels wired in parallel comprised of two of these circuits in parallel, essentially 18 similiar circuits in parallel and the draw is about 4amps when powered by an unregulated 12 v ps. Am I reading my multimeter, or doing something else, wrong, these numbers just don't seem to make sense to me.

Last edited:

Replies
3
Views
1K
Replies
1
Views
73
Replies
1
Views
43
Replies
13
Views
144
Replies
27
Views
259