From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: Computer power supplies and radios.
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Destructive Testing Systems
Date: Thu, 28 Jan 1993 05:31:20 GMT

In article <26JAN93.23285391@nauvax.ucc.nau.edu> cvm@nauvax.ucc.nau.edu writes:
>I have seen several posts saying that it is easy to use a computer power supply
>to power a radio.  I remember an old post talking about using the computer
>supply to charge a deep cycle battery and power the radio, but I can't find it. 
>Right now I am running my radio off a deep cycle battery and charging it with
>an automotive charger as needed.  I would like to use the power supply to
>charge the battery/power the radio.  How can I do this?
>
>I have a 150 watt power supply, a group 27 deep cycle battery and a mobile two
>meter radio (Alinco DR-110T).  I know the radio does not draw more that 15 amps
>(the fuses are 15 amps) on high power, but don't know exactly how much it does
>draw.  The power supply does not have any amperage ratings on it, but I am
>afraid it would not supply enough current for high power transmitting.  The
>power supply provides 5, 7 and 12 volts (approx) on the various connectors.  12
>volts would probably be enough to run the radio but not charge the battery. 
>Do I have to modify the power supply to get a high enough voltage to charge the
>battery?  Also I need some overcharging protection.  Any help would be greatly
>appreciated.

Well first the bulk of that 150 watts in the PC supply is at 5 volts.
The 12 volt supply capability is probably no more than 2 amps, or 24 
watts. That won't power your Alinco. As to how much current the Alinco
draws, *measure it*. That's why meters were invented. It's also printed
in the owner's manual. The 12 volt output of the PC supply is 12.0 volts.
A fully charged deep cycle lead-acid "12 volt" battery actually has a
terminal voltage of 13.8 volts, so your PC supply will never fully
recharge the battery and no overcurrent limiting is required. In fact,
to properly charge a "12 volt" battery, a charging voltage of 14.5 volts
is required. The Alinco is designed with those voltages in mind and
will begin to get squirrelly as voltage drops to near 10 volts as
the battery discharges.

There is a further fly in the ointment. The PC supply is a switcher
and won't work properly unless it is loaded to a minimum current
draw. Basically this means you need to put some big power resistors
across the 5 volt line, and perhaps a smaller power rating resistor 
across the 12 volt line as well, to insure proper operation of the 
switch mode regulators. The switcher is a RL circuit with the load 
supplying the R. Unless a certain amount of R is present, voltage 
transients can reach very high levels and do serious damage to solid 
state components. All switchers are finicky about their load, and
typical clone PC supplies are particularly cheaply designed with
little or no protection. It's a bad idea.

Gary
-- 
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