Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!sco!johnd
From: johnd@sco.COM (John DuBois)
Newsgroups: sci.electronics
Subject: low-current power supplies
Message-ID: <2588@scorn.sco.COM>
Date: 21 Feb 90 23:44:47 GMT
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Reply-To: johnd@sco.COM (John DuBois)
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Organization: The Santa Cruz Operation, Inc.
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     I often have need of a low voltage (typically 12V or so), low current (up
to 20mA or so) DC power supply for some circuit.  Other equipment in this power
range that I've seen often uses a resistor-diode-zener type voltage drop to 
power it from 120VAC.  Of course that ends up wasting most of the power, which
doesn't really matter in this range except that it neccessitates a large
(several watt) power resistor and somewhat restricts the type of enclosure that
can be used, so the heat can be dissipated.
     It's easy enough to use a capacitive voltage drop, at the expense of
another diode to recharge the capacitor on the other half-cycle and an inrush
limiting resistor.  This is getting to the point where it is almost worth it
to use a small transformer instead.
     My question is, is there a better way of doing this?  Are there any 
power supply chips that run directly off of 120VAC, and if so, what technique
do they use for voltage drop?  Are there any really small power transformers
available (say, the size of a 1/8W audio transformer)?  It seems it would be
difficult to get the neccessary reactance in the primary, unless you used
*really* fine wire.
     I'm not worried about the power supply output being hot; most of the
circuits would need to be hot anyway (AC load controllers).  The main
considerations here are size and cost.
     Thanks for any info!

	   John DuBois
	   johnd@sco.com