johnd@sco.COM (John DuBois) (02/22/90)
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.comgrege@gold.GVG.TEK.COM (Gregory Ebert) (02/24/90)
I once made a low-power supply by making a transformer which was placed in SERIES with an AC load which was active while I needed the power. You need to make about 20-30 turns of appropriately gauged wire (to handle the AC loads current) on a metal object (a 1/2"x4" bolt is good. Then wind a few hundred turns of #30 (or smaller) wire on the core. Hook-up your load, with the 'x-former' in series, and use a meter to measure the output voltage. Hopefully, you will have 'extra'. If so, start unwinding until you are about 10% over. Your low-power load will 'bog-down' some of the remainder. You may have to iterate a bit. Hope this helps..... Oh, by the way, this won't zap, sizzle, or fry !!!
markz@ssc.UUCP (Mark Zenier) (02/24/90)
In article <2588@scorn.sco.COM>, johnd@sco.COM (John DuBois) writes: > > 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. ... > 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? ... > 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. Maxim makes some 8 pin minidip 120 vac to 5 Vdc power supply chips. Max610, 611, 612. Active has them in the catalog for $3.70 - $3.85. I think they use a capacitor as the dropping element. markz@ssc.uucp
armoror@ucscb.UCSC.EDU (Space Cadet) (03/03/90)
Thanks! This is exactly the information I was looking for. Does anyone have an address for Active, or any other company that carries them? I don't find it in my catalogs. John In article <505@ssc.UUCP> markz@ssc.UUCP (Mark Zenier) writes: >Maxim makes some 8 pin minidip 120 vac to 5 Vdc power supply chips. > >Max610, 611, 612. Active has them in the catalog for $3.70 - $3.85. > >I think they use a capacitor as the dropping element. > >markz@ssc.uucp
ankleand@mit-caf.MIT.EDU (Andrew Karanicolas) (03/03/90)
In article <1597@darkstar.ucsc.edu> armoror@ucscb.UCSC.EDU (Space Cadet) writes: > > Thanks! This is exactly the information I was looking for. >Does anyone have an address for Active, or any other company that >carries them? I don't find it in my catalogs. > > John > Active Electronics 133 Flanders Rd. Westborough, MA 01581 617-366-8899 fax 617-366-1195 Active Electronics 2010 Duane Ave. Santa Clara, CA 95054 408-727-4550 fax 408-727-1114 Active Electronics 13107 Northrup Way Bellevue, WA 98005 206-881-8191 fax 206-883-6820