[Boatanchors] 12 volt supply question

[Frank Barnes]

I have used a 650 watt computer P.S. for many purposes with no real
problems other than having to:
1) Filter & Shield for the RFI that it generates
2) Put a resistor load on one of the outputs so that the thing would be
stable (as mentioned in an earlier email).

But I think the best solution is the old-fashioned linear supply:  Take a
big, (say 450 watt) and free UPS transformer and apply 120 VAC to its 120v
side.  Rectify the output using a sturdy bridge rectifier and filter it
well. The output will be about 12v x 1.414 = ~16 volts which will feed a
low-dropout regulator chip. Run this ~16v output into a 12 volt regulator
chip (or a variable voltage one) that is placed above ground to deliver 12
- 15 volts (adjustable, using a pot).

Use pass transistors in the negative rather than the positive lead - these
can be bolted right to a heavy chassis used as a heat sink.  Here's a good
reference for doing this:
http://www.qsl.net/wb4kdi/PowerSupply/UpsideDown.html

Derate the 450 (or larger) watt UPS transformer by about 1/3 as they are
not intended for long periods of use.  Mine powers a 100 watt Yaesu without
getting hot.

Add a $5 Chinese digital voltmeter from eBay - makes a very nice output
indicator.  Bend your own aluminum case or put it in a metal UPS case.  Or
a wood case with Masonite panel.  Or breadboard it and keep the cat away
from it.

Others can make a good case for the switching supply but I find them hard
to repair if a unique component fails.  My linear supply is built of junk I
found in recycle yards - total cost is about $15, mostly for a good can of
spray paint and the voltmeter.  On the other hand, computer P.S. can often
be had free, are small and don't need much modification other than changing
a resistor (sometimes a variable one) to raise the voltage.

Lots of articles on the internet about using computer p.s.
Frank
W4NPN

On Wed, Nov 30, 2016 at 11:03 PM, Alex [Temple Boatanchors] <
temple.boatanchors1 at miwww.com> wrote:

> ATX PC power supplies are very inexpensive ($20-$30), highly efficient and
> very lightweight, and basically use the same concept as those modern
> lightweight "wall-wart" wall transformers. But due to their switch-mode
> nature they generate a great deal of RFI in the HF regions, specially the
> lower frequencies bellow 10-15MHz or so, but should be fine for use with
> gear on 6M, VHF, UHF and up. The level of RFI perceived in the receiver
> much depends on how well the radio itself and coax to the antenna is
> shielded and grounded, and the distance between the gear and power supply
> is a factor as well. Also, the output is 12V, not the required/desired
> 13.8V that most mobile DC operation gear needs to attain full output power.
> But with small modifications by changing one or more resistors (varies from
> model to model), in most cases one can "fool" the regulator to raise the
> voltage as needed without tripping the protection circuit. Another issue is
> that the power supply usually requires a minimum load on one or more of its
> outputs for stable operation of the regulator circuit, as one or more of
> these outputs are used as a reference voltage. A resistor or appropriate
> incandescent light bulb might do the trick as a load.
>
> I use a 700W ATX power supply that I modified for 14.4V output to manually
> charge a 150Ah lead-acid backup battery bank. It easily will charge it at
> 20-25Amps and generates minimum heat as long as its internal fan is
> running. Basically these power supplies can be used as very inexpensive
> power sources or charger. But while charging it basically wipes out my
> reception on 80 and 40 meters. Then again the internal power supply of my
> nearby Apple TV does pretty much the same, only at a lesser degree than the
> charger.
>
> Just search for "ATX power supply modification", there is quite a bit of
> info out there.
>
>
>
> At 07:17 PM 11/30/2016, you wrote:
>
>> OK gang ..   Can someone provide a definitive answer to this question?
>>  I see all kinds of conflicting opinions and would like to know what the
>> thinking is here.  Or, if someone here is using them for that purpose.
>>
>>
>> Computer power supplies.   Can they or can they not be used to power ham
>> rigs such as two meter rigs, or even
>>
>> low power HF rigs?
>>
>
>
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>


-- 
Frank Barnes
W4NPN
Chapel Hill, NC
Grid Square FM05
Cell 919.260.7955