[time-nuts] Regulator choices

Bob Stewart bob at evoria.net
Sun Jun 30 19:35:07 UTC 2013


"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 <eb at telight.com>
> To: time-nuts at febo.com
> 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.


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