"Paul Keinanen" wrote in message
...
On Sun, 12 Sep 2004 16:54:30 -0500, "Craig"
wrote:

I am wanting to build a 24 volt power supply for a project I am working
on.
I was wanting to know if anyone would have a schematic for such a power
supply. The power supply needs to be atleast 25 amp.

From other postings it appears that you need a power supply for an ATV
transmitter (repeater?), one should note that ATV transmitters are
usually operated at nearly constant duty (and FM ATV transmitters
additionally at constant load), while typically amateurs transmitters
are operated at (much) less than 50 % duty cycle. I the ATV power
supply, the temperature balance will be reached during each operation
cycle and in the interest of reliable operation, this temperature
should be as low as possible.

Thus, when looking for various designs in the amateur literature,
select one with rated capacity well above the transmitter requirement.
Use larger heat sinks than specified or use a fan. Some
overtemperature protection is also needed, especially if a fan is used
and it fails.

The 24 V 10 A design in the ARRL Handbook is a good starting point. It
uses four 2N3055 series pass transistors, thus 2.5 A/transistor. Even
at full load, the average voltage drop across the transistor could be
8 V, so each individual series pass transistor (and heatsink) is
required to dissipate 20 W. These are quite reasonable figures for the
2N3055.

Upgrading this to 25A should require a larger transformer, capacitor
and rectifier. Ten series pass transistors with larger heatsinks
should keep the individual transistor loading at the same level.The
short circuit measurement resistor value should be reduced.

The worst case current gain for the 2N3055 is specified as 20 @ 4A, so
the total base current for the ten 2N3055 would be well below 1.25 A,
which the single 2N3055 driver could well handle and still assuming
hfe=20, the uA723 would have to supply 63 mA, well within
specifications, however, the power dissipation might be a problem.

Those are worst case specifications for these transistors at larger
currents, but looking at the Motorola 2N3055 transistor, the typical
hfe=50 at 2.5 A at room temperatures or above, thus the total base
current for the ten 2N3055 would be 500 mA and at that current the
single driver 2N3055 will have a hfe 100 at or above room
temperatures, thus the uA723 would have to supply 5 mA and there would
not be any power dissipation issues.

Of cause, when using typical values (instead of worst case values) to
build a one-off project, one should verify by measurement that the
typical values apply. This is specially important when "no-name"
components are used.

Using larger storage capacitors than specified will reduce the voltage
drop during each cycle, thus a slightly lower secondary voltage can be
used, which drops the total power dissipation.

Paul OH3LWR


Craig -

Paul and Eric and provided a good background on how to adopt the ARRL
design.
You can substitute the 2N3771 transistor for the common 2N3055 pass
transistor (this is what Astron has done on many of their latest models).

Greg
w9gb