"I believe the original problem was that the raw unregulated voltage may be marginally too high for a conventional three-terminal to take safely"
Hi Ed, Not really. The voltage is in line with the product specs for a 7812 (35V max), as is the current I had projected (a bit over 1A). However, the voltage difference (20-12) times the current (assumed to be 1A) is not within the dissipation ability of a small free-air heat sink. When I hooked things up for a test, the 7812 immediately went into overheat and started reducing the output voltage to compensate. It would be OK (according to the datasheet) if I bolted it to the chassis, but at my original post, I had not worked out the actual current load through the device. And it turned out that the datasheet I was using for the OCXO had overstated the current draw by some 30%. After working out what my actual needs were, I compared that to what HP was heat-wasting in the 37203A PSU, and realized that my needs were smaller. The collective quickly convinced me that a switching/bucking device would be too noisy, so I've decided to use a 78S12CT, which is a TO-3 cased 12V regulator, to pull down the 20V I have available to me. So, essentially, I didn't know what I was doing, as I have never done this before. After understanding how to do this, I decided to get a TO-3 cased device, which will fit the heat sink available to me in the 37203A case I'm putting it in. For me, this has all been a good discussion. The one thing that is missing is how to quantify the heat sink needs for a linear regulator. Any thoughts? IOW, is there some way to project how many square inches of heat sink needed for X watts to dissipate? Bob - AE6RV ----- Original Message ----- > From: ed breya <[email protected]> > To: [email protected] > Cc: > Sent: Sunday, June 30, 2013 1:47 PM > Subject: Re: [time-nuts] Regulator choices > > I believe the original problem was that the raw unregulated voltage may be > marginally too high for a conventional three-terminal to take safely. I have > often encountered this problem, which is due to the wide input range possible > considering the worst-case line voltage tolerance, transformer regulation, > transformer selection limits, and possible surge voltages. If you drop the > voltage with extra stages or series devices, you may run out of headroom, but > if > you don't, then it may run dangerously close to the maximum input rating of > the regulator. If adding to, or reusing existing power circuits, there's > often already some degree of protection from MOVs or gas-tubes, but these are > very coarse, so are unlikely to be effective. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
