[ARC5] 28 volt transformer question ?

[millerke6f at aol.com]

Hi
A great source for low voltage magnetics are the transformers removed from computer UPS units.  Most of them have secondary voltages in the 15-18 vac range.  Amperages range from 6 amps to 50 amps depending on the UPS capacity.    Most have tapped primaries so you can push the secondary voltage a couple volts by using the 105 vac primary.  I've even had some success removing secondary turns to bring the secondary voltage down so a Voltage Doubler scheme can be used as a brute force 28 VDC scheme.  with BIG computer grade caps and cheap 50 amp diodes you can get RMS X 2.8 minus 1.2 vdc for two diode drops  to get a no load volage of around 28 VDC and with big caps the loaded voltage wont drop that much.
Bob


-----Original Message-----
From: Robert Eleazer <releazer at earthlink.net>
To: arc5 at mailman.qth.net
Sent: Mon, Sep 7, 2020 10:06 am
Subject: Re: [ARC5] 28 volt transformer question ?

 I have a ARC-116 220-400 MHZ AM transceiver I bought onebay years ago and a ARC-114 FM 30-75 MHZ transceiver I got for free.  Ijust knew I could build a 28 VDC power supply for them with stuff I justhad laying around. My first attempt used a 26CT V transformer I bought fromFair Radio in 1974.  Turned out that it was more like 31 VCT with 122VACinput.  I ran though several three terminal regulators before figuringout that it was never going to work; the input voltage to the regulator was toohigh.  I finally built a regulator circuit featuring a passtransistor and that worked if I used a Variac to drop the inputvoltage.  Then I removed the transformer and installed an XL connectorand hooked in a much bigger electric wheelchair charger a neightborgave me.  It worked then.  I added a mike premap using one of the kitsthat Fair Radio sells.  It was a rather awkward package,though. Then I found a discarded stereo amp and built anotherpower supply on a Lear aircraft radio power supply chassis.  After usingsome of the transformer extra secondaries as reverse buckers, putting in a regulator IC, adding a really big pass transistor and heatsink salvagedfrom the stereo, adding a fan, and then built a copy of the mikepreamp circuit; it worked fine, too.  Handles 3 amps 28 VDC outputwithout breathing hard. But it ain't easy to turn commonlyavailable parts into 28 VDC power supplies, even given cheap IC regulatorsfor $1.00 and really high output pass transistors and transformers available intrashpiles by the road.  I do not wonder at all why the guys in the 40'sthrough the 80's did not try to run the gear on 28 VDC; they did not even havethe junk we can get now.
WayneWB5WSV              

|  | Virus-free. www.avg.com  |

 ______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: https://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20200908/4675bc98/attachment.html>