MIDIbox heat problems

The problem has been solved, but I think it's good to share it with others.

I built a MIDIbox kit with 2 DOUT units to drive the LEDs in my illuminated stop tabs. I have 41 LEDs and in total they consume about 400mA of current, well below the maximum specs of the board and the LEDs. Everything was working fine, but when I turned on all the LEDs, they all turned of after about a minute. This never happened with only half the LEDs on. First I thought my power supply was not strong enough, and I was looking for replacement. But as I barely play tutti, it was not a pressing issue. Yesterday I opened up the console to check a few things and found the source of the problem. With all the LEDs on the power transistor on the MIDIbox board was crazy hot (it burnt my finger). I remember reading something about it, but I never thought it could actually fail. I screwed a piece of aluminum on the transistor and installed a pot to control the brightness of the LEDs (something I've wanted to do anyways), and now everything works fine.

Hello Imre,

the "power transistor" on the MIDIbox is actually a voltage regulator of the type 7805, rated for a maximum current of 1A. When you draw 400mA and you have an input voltage of say 9 V, then it will dissipate 4V*400mA, which is just 1.6W. This doesn't seem much, but without a heatsink (which you installed now) it will get too hot. Fortunatley htese devices are quite well protected against overheating, current and even against short ciruiting them, so they are nearly impossible to destroy. But they do cut off the power when they get too hot, which is what you saw in your MIDIbox. So, no harm is done. If you actually want to draw more than 1A (by increasing the number of LEDs), you will need to use a stronger power supply.

Best regards
Reiner

Thanks for the background info, I sleep better now. 1.6W doesn't sound much, but it burnt like hell. I'll keep an eye on it when I add more LEDs and a small character LCD.

I put heatsinks on my core module's 7805 regulators even though I had no intentions of driving LED's. I guess I tend to be on the conservative side.

Eric

It wouldn't hurt adding a tiny amount of thermal pasta (not much!) - helps on heat transfer.

Imre,

I think you said it was a MIDIbox kit. If that means that the regulator is inside a box, the heat has to go somewhere. If the heatsink is in an enclosed box it may still get too warm. If it is a plastic box attach a small piece of flat metal to the inside of the box and attach the 7805 tab to that. That should be plenty of sink for 2 Watts or so. Thermal compound doesn't hurt either but a thin layer is prefered.

Did you ever let the smoke out of an IC. They run on smoke and mirrors but when you let the smoke out they quit.

Thomas

The panel is mounted inside the console unenclosed (...).