[NLRS] NEED 24-28 Volt Power Supply

[John P. Toscano]

christenson at charter.net wrote:
> 
> I have picked up a Military T.E. Systems 225-400 MHz amp that has a power req. of 24-28V DC. 10W in 100W out. I am going to use it on 222. 
> 
> I need to know what would be an adequate amperage for a power supply and some information on building a pass band filter.
> 
> Anyone with a 24-28V power supply for sale or where I could find one CHEEP or any ideas on how to build one please let me know. Also the infor on low loss pass band filters
> 
> Thank You,
> 
> Kris KC0REO

Kris:

I don't recall that anyone replied to this query, and I noticed it as I 
was going through my saved messages to "clean house".

I was recently in the market for a 24-28v power supply, to run one of 
those surplus 902 MHz telephone amplifiers.  I'm told that the high 
power one requires more than 20 amps, so I was looking for something 
with a LOT of power output.  (I now also have a 2304 MHz PA that wants a 
supply in that voltage range, so the PS can do double duty.)  The best I 
could come up with in brand-new equipment at a reasonable price was a 
HY3020E, a variable 0-30v at 0-20a supply with digital readouts of both 
voltage and current. This is a switching power supply, so it is of 
reasonable size and weight. Only after I bought it, did I discover that 
there is an HY3030E model, which is basically the same but 30 amps out. 
  They appear to be very rare, however.  On eBay, I eventually found a 
fair number of used beasts of various sizes, conditions, and power 
outputs.  Most were boat anchors to say the least, and the one I 
eventually bought cost nearly as much to ship as to buy, but in this 
case, it wasn't an eBay shipping scam, the beast was over 100 lbs and 
had to be shipped in a wooden box.  There were many of a more manageable 
size with lower power output than that one.

Now, as to how much current you need, that's one of those "good 
questions" to which there is not a simple, obvious answer.  For 
starters, if the PA was 100% efficient (which it definitely WON'T be), 
you'd need a minimum of 100 watts at 25 volts or 4 amps (for nice round 
figures).  At 50% efficiency, 8 amps.  At 25% efficiency, 16 amps.  At 
10% efficiency, 40 amps.  You get the idea.  So what is a typical 
efficiency for a linear (SSB-capable) PA?  I don't know offhand, but 
others on the list can probably hazard a better guess.  What I can say 
is that my RF Concepts 110 watt PA wants at least 20 amps at 13.8 volts 
per the front panel meters on my RS-70M power supply, and it has always 
worked OK with a 30-amp (at 13.8 volt) power supply.  So, I'm guessing 
that since your supply voltage is almost twice as high, you will need 
about half as much current, so a 15 to 20-amp supply would probably work 
OK, implying 20-25% efficiency of the PA.  Probably the best thing to do 
is hook it up to a really beefy power supply and measure the current, 
then no guessing would be needed.  Of course, while doing the test, you 
want to transmit in FM or continuously-keyed CW, not just look at SSB 
peak current consumption.

I hope that helps some.

73 de W0JT