[Fozzie]

I have a wall wart that you can set to run at different voltages.

It says 600ma.

Assuming it is set to 9v (not that I think that matters for the question I'm about to ask), how many LEDs can I safely run off of this?

Am I correct in saying thinking that we're only talking about 25-30...?

 

[Model Man]

If your leds are 25mA each, then 24 is the ballpark answer. However, you may squeeze more out of it. Some transformers will die if overloaded, some are built of hardier stock, some may be listed low.

 

[jwrjr]

That depends on how much current you are going to push through each led. Most max out at 25ma. As MM said, that gives you 24 leds. But you don't have to max out leds. So 600ma will power 40 leds running at 15ma each (for example). With high-output leds running them at lower current may be acceptable.

 

[Model Man]

Good to know, JW. I've had situations where I should've maxed at x led's, but got x+y worth without frying the transformer.

I really gotta get this math down.

A sage was telling me the other day that because leds are semi-conductors, regular math, such as ohm's law, goes out the window -to some degree, and that it comes down to gettinga feel for what works and going with it.

I'm talking with him more tomorrow, so will absorb as much as I can and relate all here.

 

[Fozzie]

I bought the LEDs bulk with no real documentation on their specs so I've just been ballparking it with their draw. So far, so good.

That's good feedback, guys. Thank you.

 

[jwrjr]

So figure 25ma max and calculate the resistors for the current to be a little below that. You may not get max output, but you will be unlikely to be burning out leds, either. "Bulk" leds are sometimes not known for reliability, which is essential in a model.

 

[Fozzie]

I've done pretty well with the resistance and wiring them up. Just don't know when to stop! LOL I'm using 9v batteries as my power source and I thought I could get away with 4 circuits in my 350 refit, but now it looks like I'll need 5 simply because of the number of LEDs I'll need (one circuit being a board to control the flashing lights which runs off of 6v). So now I'm running into the issue of getting 10 wires through the hollow rod that the model is mounted on.

 

[MrJuice]

Contrary to what was said earlier, use ohms law.... V=IR where V is measured in volts, I is current measured in amps (so convert values of mA, 600mA=0.6A, 25mA=0.025A, etc.) and R is resistance measured in ohms.

You want to make sure that you aren't driving your leds too hard, they will easily take more than they can handle, but will significantly reduce their longevity. Protect them by placing a resistor in series to limit the current before the leds.

 

[Fozzie]

I've been using one of those LED calculator apps to make sure that I'm putting adequately sized resistors on the LEDs...but thanks for the advice!

 

[Model Man]

Contrary to what was said earlier, use ohms law....

The other week I finally decided to commit that formula to memory. What i didn't know was whether the volts and amps applied to the power source, the led or the difference between the two.

In talking with an old timer, he was saying that as leds are semi-conductors, ohm's law was not completely out the window, but mostly out the window, and that the only way to really be accurate was to measure the load under current and test various resistors against various power sources and leds. His final bit of guidance was that once you get a feel for what works, that's good enough. But I really wanted to know the numbers for maximum value. In that case, his advice was to just use an led calculator.

Part of my prob in learning was not having real world examples for study. Now that I've got several models lit w/ various resistors, leds and supplies, playing with iterations of those values in an led calculator (such as this one, http://led.linear1.org/led.wiz) has given me some interesting insights (which I will share when I have some free time to type a while).