So, thanks to Wayneh and Bernard for answers to other questions on this circuit.

Here's the final one:
What should I worry about when removing R5, R27 & R31? Here're my observations with experimentation:

without R31, the Green LED draws a maximum (recorded over several hours of the local "Oldies" station driving the circuit) is 805ma
With R31, max draw is about 450ma. I like the way the colors look at the higher current, and according to the datasheet, the green LED in this module has Max rated drive current of 1A. It says that the recommended operating current is 700ma with a footnote that higher currents are possible, but requires adequate cooling. (One other thing, the 800ma was a peak, not constant, since the purpose of the circuit is to 'flicker' the LEDs. I would think average may be down around 300ma or so.)

I've built what seem to be pretty good heat sinks for the TIP120s and the LED module, as all run mildly warm.

So, my question is if nothing if overheating, should'nt it be okay to run these LEDs without the current limiting resistor as long as my measurement assures that the 1A max limit isn't exceeded?

Thanks!

 

You may be fine, if you think your data is a good test of real-world usage. It comes down to risk management. If you can live with a modest risk of blowing the LED or wearing it out a little faster, then you have little to lose by losing the resistor. If you were putting this on the Mars Rover, you'd take a different view.

What sort of 12V supply are you using? Can you tell if it's sagging voltage a little under load? You don't want the whole thing to work with one supply - because it sags - but then blow up if/when you happen to switch to a beefier one that doesn't.

I think I'd still want a resistor in there, maybe even 5Ω, just to prevent runaway.

 

You may be fine, if you think your data is a good test of real-world usage. It comes down to risk management. If you can live with a modest risk of blowing the LED or wearing it out a little faster, then you have little to lose by losing the resistor. If you were putting this on the Mars Rover, you'd take a different view.

What sort of 12V supply are you using? Can you tell if it's sagging voltage a little under load? You don't want the whole thing to work with one supply - because it sags - but then blow up if/when you happen to switch to a beefier one that doesn't.

I think I'd still want a resistor in there, maybe even 5Ω, just to prevent runaway.

Thanks Wayne, very good points. I think I have a way to go before I put in to design circuits for NASA, so I'm probably going to be okay. Part of my thinking is that with the wide variation of current draw into the LEDs bumping over the recommended max may not be a problem; as long as I don't blow past the 1A limit - that gives me around a 200ma cushion at this point.

I checked the supply - 12v 1A wallwart - and it measures 12.23v without load and 12.22 when I flip the switch. When I add audio signal to the input, the supply bounces around a bit and drops to a minimum of 11.44; I set my meter to record minimum voltage and over an hour or so, that's the lowest it went. Watching the live reading, it bounces all over a range of 11.5 to 12.0. So, I'd guess that part may be okay.

I tried a 5ohm 5watt power resistor in parallel and had two issues with it, one is getting a mount in my case worked out and the other is that it gets screaming hot. I figured I'd have to put a heat sink on all three. Getting three 10W resistors into the case was even more challenging.
So, once I tried the circuit without the resistor - on the Green leg - I saw that max current was acceptable and none of the components were getting hot.

I think next step is to take them out, make one last measurement just to be sure and then let it run for a while and see what - if anything - breaks.

Thanks for the input!

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I think next step is to take them out, make one last measurement just to be sure and then let it run for a while and see what - if anything - breaks.

Sounds reasonable. And yeah, 5W into a 5W-rated resistor will get crazy hot. Those ratings aren't really a guideline until you divide by 2 for real operation.

 

Sounds reasonable. And yeah, 5W into a 5W-rated resistor will get crazy hot. Those ratings aren't really a guideline until you divide by 2 for real operation.

Thanks for the input. Been running it for several hours and went back through each color for maximum over the time period. The Red channel bumped up to 920ma, but the others were around 100ma less than that.
The transistors got warm to hot, but never got too hot to touch, so I think I'm good to go.

I really appreciate your help with this!


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