Electronics > Projects, Designs, and Technical Stuff
OK to have heatsink "live" in linear power supply?
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grantb5:
I'm building a kit which contains a linear +15/+5/-5/-15 PSU, basic old school 78xx and 79xx TO-220 stuff.

AC comes in, goes to transformer (and fuse), then diode bridge. 7805/7815 parts are side-by-side and have a heatsink that is large but not connected on the PCB to anything other than mechanical stability. No traces, not even ground. The 78xx regulators TO220 tabs attach to that heatsink with a screw each. I have no worries about that.  7915/7905 regulators similarly share a heatsink that is again "floating" other than being attached to the TO220 tabs with screws and again to the PCB with large but floating tabs. This means the heatsink for the 79xx regs will be at the unregulated negative rail.

Should I be worried about this? Is this dangerous?  Or maybe it could be "noisy"? I didn't design it, I'm just building and wondering if it's worth the trouble of isolating the 79xx's from the heatsink. The heatsinks are threaded so I would have to drill them out maybe.

Edit: the entire kit is housed in a metal case when completed (so one wouldn't come in casual contact with the PSU).
Nauris:
Should not be any problem. It is low voltage after all, I remember seeing some device with heatsink at mains potential even.
ArthurDent:
Although there is nothing wrong with having a 'hot' heatsink, I prefer to keep any larger piece of metal at mains ground or common potential and use an insulated mounting kit and heatsink compound for all regulator tabs that aren't at common potential. If you start probing around in the circuit and have forgotten that the heatsink isn't at zero potential, you may have a problem.
Ian.M:
Yeah. However grounding it may not be so simple.  A mica washer or silpad introduces extra thermal resistance, so the negative regulator heatsink may need to be bigger to maintain the same max junction temperature as the positive regulator.  If you have to leave it 'live', a  warning sticker on the heatsink: "CAUTION: NOT GROUND" would be advisable.
Zero999:
Yes, it's no problem. I've also seen a mains powered device with a life heatsink before and considered it to be perfectly safe, because it was inside an insulated enclosure with sufficient separation from anything anyone could touch.
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