Elevating Signal/Power ground from Chassis Ground

[Deke609]

Thanks PB. Hmm. I was hoping the fact that the chassis wall would be exposed to external (cooler) air would offer some advantage. And I thought for sure that adding a copper bar would offer superior performance. I'll need to read up on heatsinks and heat dissipation formulas.

cheers, Derek

[Paul Birkeland]

My general rule of thumb is that a chassis plate is about as good as a 40C/W heatsink.  If I have a component that is barely too hot running in free air, then tabbing it down to the chassis can resolve that kind of issue.  If I have a component that is overheating on a decent heatsink, then connecting it to the chassis will make things much worse. 

PJ provided me some information about this some time ago, though I don't have it handy.  If you have a multimeter with a thermocouple and a DC bench supply, you can get TO-220 resistors and do the measurements yourself of the case temperature in free air vs. the case temperature mounted on an 1/8" piece of AL.

[Deke609]

The heatsink is 4.5C/W.  You are dropping about 3.7V across the 1085 at 1.3A, which is 4.8W, we'll call it 5.  This gives 22.5C of temp rise on the heatink and 78C or so working backwards to the junction of the regulator.  The junction shouldn't run any hotter than 125C, and we shoot for less than 100C.

How about the Ohmite  FA-T220-64E @ 3C/W for a TO-220 package? Datasheet attached.  That makes 15C rise instead of 22.5.  I don't know how you calculated backwards to the junction, but assuming a fixed ratio of approx 3.5X, that makes 52.5C rise instead of 78.

Haven't measured whether it will fit the existing board - but I could McGyver my own out of protoboard or try to learn Eagle PCB layout if necessary.

[Paul Birkeland]

You're going to have to buy it and see if it will even fit on the board.  Forced air cooling makes a lot more sense IMO, though fans may make their own issues.

[Deke609]

Yeah, that looks to be both the more effective and simpler solution. So I've ordered two 140mm 12V fans that the manufacturer states produces less than 16 dB of noise at full power - which seems pretty quiet.  They can also be powered @ 7V or 5V, presumably for even quieter operation. I've decided to power them via a separate DC power adapter. They're 25mm thick. But with 5.25" high chassis and some 1.5" feet, I should be good.

cheers and thanks, Derek