[time-nuts] +12 Volts 1A (plus a bit) supply?

Bob Stewart bob at evoria.net
Thu Jun 27 23:40:39 EDT 2013

Oops, I sent this as a reply to Richard, but I had meant it for all.  Also, in the interim, Bob Camp had advised me that my OCXO load current was most likely only 750ma instead of 1000ma.  I checked it and it was about 690.  Moral of the story: check it, don't just rely on the datasheets.  =(  I've corrected the numbers before posting this.

Hi Richard and everyone,

Just a reminder for you and me of the topology.  I have a +20V supply 
readily available.  It needs to power a Trimble 34310-T, VE2ZAZ's 
disciplinizer board, and a Motorola Oncore UT+ GPS timing board.

I've finally looked at the various datasheets, and the loads are a bit 
different than I had guestimated.  The OCXO takes about 750ma for 
warmup and then about 250ma steady state after that.  The 5V load should be around 360ma, and will be derived from the 12V supply through a 
7805.  So, (12-5)=7 times .36 = 2.5 Watts to dissipate on the 5V 
circuit.  That's maybe a bit high for one 7805, so guess I'd better put a separate 7805 in for the UT+?  The disciplinizer board has a 7805 on 
it, already.

So, the 12V current load is initially (750+360) =~1110ma, going down to (250+360) = ~610ma afterward.  That means I need to dissipate (20-12)=8 times .6 equals 4.8 watts on long term average on the 12V 
line.  OK, that's better than I thought, so a TO-3 regulator on 
the HP heatsink should be more than OK.  I hope my math has been right, 
above.  Using a TO-3 gets rid of the worries about the switcher.  And, 
I'm just about ready to order my last (hopefully) order of parts for 

Thanks for everyone's valuable input on this.  Now my brain hurts.


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