Nub Questions

SdWashb1 · Nov 2, 2013

So I have been researching, researching, and researching some more. I am getting a little confused on some LED powering devices. I have been making simple little projects any where from 8x5mm LEDS and 240x5mm LEDS. Thus far I have only encountered one problem. That problem was I had two strips of lights, Blue and a white. Both with the same voltage req's and ma reqs. Both strings powered by their OWN 12V 5A Power supply. The blue LEDS are working great, the white ones are all blowing. The LEDS all were in sets of 3 with a 150 OHM resistors. And by all of my calculations, even at 13v that should be drawing less than 20 ma per LED. So now I am trying to move to a bigger project using 3W LEDS. I have these 12V 5A power supplies. And I have read that you don't want to just toss a resistor in there and call it good. Ive seen these meanwell Power supplies that are constant current. 48V 0-1.3A. They say they can handle 8-14 LEDS. Does that mean I could toss about the same amount on any of these Power supplies that I already have and toss a resistor in to limit the current?

DIWdiver · Nov 2, 2013

Yes it does. You just have to be a bit careful about how you do it.

The reason lots of people don't like simple resistors is that they are running either in vehicles or off batteries. Both of those experience a significant range of voltages. Coupled with a resistor and some LEDs, that equals a HUGE change in current, which is the last thing you want in an LED.

Since you have a fixed voltage, you have much less of a problem. You do still have to deal with the fact that from one LED to the next, they are a little unpredictable. Say you are going to buy some Cree XR-E emitters, and plan to run them at 1A. You don't know if the forward voltage will be 3.4V, 3.3V, or 3.1V. So you can't determine in advance the exact resistor to get you exactly 1A. Also, the forward voltage drops a bit as they heat up. In marginal designs, this can lead to a situation called thermal runaway, where the LEDs heat up, the forward voltage drops, which causes the current to increase, which causes more heat, which causes more voltage drop, and around and around until something fails (usually the LED). The way to cause thermal runaway is to have the supply voltage close to the LED voltage, and use a small resistor, and have poor heatsinking on your LEDs. The way to avoid it is to use a larger resistor value, and a higher supply voltage (or lower LED voltage). Unfortunately, this reduces the overall efficiency of the system. Good heatsinking helps too, but is also a tradeoff.

But you can plan on starting at a safe point, and gradually decrease the resistance until you get the current you want. Or, if your power supplies have a voltage adjustment (many 'fixed' voltage supplies have a small trim range), start at a low voltage and turn it up to where you want.

Say your supply can adjust from 11 to 13V, and your LEDs are somewhere between 3.0 and 3.4V at 1A, at operating temperature. You MIGHT be able to run 4 LEDs in a string, but you'd have to be careful of thermal runaway. Much safer to run 3 in a string, starting at 11V and 2 ohms. That would give you 1A if the LEDs are at 3.0V, but you probably will have less than 1A. Now you can measure the LED voltages, and get a better idea of the right resistor value.

By the way, measure current by measuring the voltage across the resistor, not by measuring current directly with the meter. The meter has enough resistance to change the current in the circuit.

But say you wanted to run 9 LEDs. Your supply can put out 5A, and each string is only 1A, so you can run 5 strings. I would use separate resistors on each string. If you buy all the LEDs in the same batch, it's safe to use the same resistor value on each string. If they are different LEDs or are from different batches, I would test each string individually.

SdWashb1 · Nov 2, 2013

Thank you very much for your reply. It provided a lot of information for me. Reading through this forum I found out that the leds have different voltage drops and that's something I can't change. I thought I could choose. So one more question real quick. If I have my power supply of 12v 5a, and I only hook up 3 leds with a resistor. , is it going to try and shove all 5 amps through those 3 leds? And right now I only have 1/4 watt resistors. So will I want to run more than one in series or parallel? If if done my research right (which I may have not) then a 1/4 watt resistor is only 250 ma?

DIWdiver · Nov 2, 2013

You have a 12V supply, which can provide UP TO 5 amps. If you connect something that only wants 1, it will only take 1. But it will always be 12V. LED drivers are usually the opposite. You might buy a 350mA driver, it will always want to put out 350mA (0.35A), and the voltage will vary. Connect one LED and you'll get 350mA at around 3.3V. Connect 2 and you'll get 350mA at around 6.6V.

The power in a resistor is the voltage times the current, or the resistance times the square of the current. So a 2 ohm resistor with 1.2A will dissipate 2*1.2*1.2 = 2.88 Watts. In general, 1/4W does not equate to 250mA.

SdWashb1 · Nov 2, 2013

Again very helpful thank you. So assuming I wire 3x cree LEDs 3.2-3.6 volts. Requiring 700ma each making 2.1A. Is it the resistor at the end that limits the current ? Again I appreciate all your help. I'm diving in to this head first doing all the calculations and double checking everything so I don't make an expensive paper weight haha.

SdWashb1 · Nov 2, 2013

So I went to walmart today and got myself a different multimeter. turns out my math was wrong. on my 12v 5 a power supply. with my blue LEDS power supply @9V I was pushing 25 ma, and at 13V I was pushing 45ma. I will hook my white ones up to see what they were running. but if they were running 45 ma I can see why they would be blowing so bad. I am getting a little more of the hang of things. TY again so far for your help DIWdiver.