--- On Wed, 5/20/09, Charles Lowery <clow...@va.net> wrote:
> From: Charles Lowery <clow...@va.net>
> Subject: [Repeater-Builder] Help with power supply
> To: Repeater-Builder@yahoogroups.com
> Date: Wednesday, May 20, 2009, 1:27 PM
> I purchased a Uniden 28 volt power
> supply today on ebay. It lookes like the Astron LSRM-25A or
> 35A both front and back (I assume Astron made it). With out
> seeing it I assume it works OK (cost $2.00 + shipping). Has
> anyone converted one of these to 12 volts? I have no
> schematics for it and can not find one on the net. If
> converted what would be the available current?
>
> Charles, NM4V
>
>
It is usually easy to make a 28 volt suply put out 12 volts if it has the 723
voltage regulator in it. The problem is unless you can change the transformer
wiring or whole transformer to put out about half the origional voltage you can
only draw about 1/4 the origional current and even at that it will not be very
efficiant. You want to put about 3 to 5 volts more into the pass transistors
than the output voltage. If you just adjust the resistor values the
transistors will have to dissiapate much more heat.
Thank of it as you orgionally will have about 32 to 36 volts going to the
transistors. That is 4 to 8 volts you have to drop. At 10 amps this is 40 to
80 watts. If you do not lower this voltage and go to 12 volts output then you
dissipate about 200 to 240 watts. This will probably be much more than the
transistor/heatsink combination can dissipate.
