Doh, It's late here, that should have been "...bolting the 7812 to the existing TO3 heatsink.." Robert.
________________________________ From: Robert Atkinson <robert8...@yahoo.co.uk> To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Thursday, 27 June 2013, 22:09 Subject: Re: [time-nuts] +12 Volts 1A (plus a bit) supply? Hi Bob, As adrian has said, 20V off load is not really excessive. Assuming you are using the blue wires and associated bridge rectifier, this was designed to run a pair of +5V and -5V regulators, a 7805 and 7905. This means if anything the voltage is a little low for 12V output. Try bolting the 7805 to the existing TO3 heatsink bracket. The problem is heatsinking not the requlator per se. Personally I'd use an LM317 as it's rated for 1.5A and is available in TO3 (but the TO220 LM317T will fit too). It has lower drop-out and better protection than a 7805 and only needs a couple of resistors to set the voltage. As it's not constant 1A and you are not (hopefully) using the isolated 5V supply winding (red wires) the transformer should be big enough. Avoid switchers if you can, they tend to increase phase noise on the OCXO output. Robert G8RPI. ________________________________ From: Bob Stewart <b...@evoria.net> To: Time Nuts <time-nuts@febo.com> Sent: Thursday, 27 June 2013, 20:49 Subject: Re: [time-nuts] +12 Volts 1A (plus a bit) supply? Hi Hal and other responders, I believe that the OCXO uses an ON-OFF type of regulation. So, no, it doesn't use 1A all the time. Maybe I should just put an ammeter in the line and see what kind of duty cycle it has after it's at temperature. I tried a 7812 regulator with a small dissipator on it. It goes into overheat mode almost immediately and the output voltage starts quickly doing down. The case does have provisions for a single TO-3 transistor conducting heat to the back of the case with no radiating fins. I had briefly considered using a zener and a 2N3055. I think that instead of worrying this to death, I'm just going to use a brick to get it together, and then if something comes up later that I like, I'll put that in. I did like the looks of that PT5102 but it's at end of life, and 1A is right at it's limit. I've given some consideration to seeing whether I can pull a few turns off the transformer secondary, as well. Bob - AE6RV ----- Original Message ----- > From: Hal Murray <hmur...@megapathdsl.net> > To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency > measurement <time-nuts@febo.com> > Cc: hmur...@megapathdsl.net > Sent: Thursday, June 27, 2013 2:24 PM > Subject: Re: [time-nuts] +12 Volts 1A (plus a bit) supply? > > > b...@evoria.net said: >> The transformer gives me about 20 volts DC out. Dropping 8 volts at 1 amp >> is just a lot of power to void with a resistor. I'd like to avoid > having >> that much waste heat in the unit. > > Does it really take 1A after it's warmed up? _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
