A simple way to get an idea of your actual current needs is to figure
your approximate total power consumption. Let's say your amp puts out
1500 watts and that it is 60% efficient. That's 1500/.6=2500 watts input
power. Then let's add filament power, say 50 watts for example. That's
2550 watts. Then you have a few other things in the power supply least
of which is not your bleeder if you have one so let's add another 100
watts or so which brings us to 2650 watts. Now let's assume your power
supply is 90% efficient (it could be more or less, this is just an
example) so we have 2650/.9=2944 watts. On 120 volts the current would
be 2944/120=25.5 amps. We better not do that. Now let's try 240 volts
which would give us 2944/240=12.3 amps. If your line voltage is low and
you have 220 volts the current would be 2944/220=13.4 amps. From this
example it is obvious you can't run a full legal limit amp on even a 20
amp, #12 wire, 120 volt circuit much less any 120/240 volt, 15 amp #14
wire circuit. At 220/240 volts you still want a minimum of #12 wire and
a 20 amp circuit to provide a comfortable operating margin. I've ignored
other current consuming devices like a blower that may take an
additional amp or two so be mindful of all the little things that can
bump the current up. If your wire from the main breaker panel to the
shack outlet is more than 20 feet long, bump the wire size up (you don't
need to increase the breaker size) so you don't experience that I*R
wasted power. If you are considering real QRO you can see how you need
to plan for a little more fire in the wire.
73, Tony W4ZT
Steve Thompson wrote:
>
>
> Alex Eban wrote:
>
>> ...guys, losses are ALWAYS quadrupled!
>> Losses follow the V^2/R law or, more appropriate I^2*R law. And
>> current is
>> average current, not peak current!
>
> Being picky, it's the rms current, not the average. In a cap input PSU,
> the rms current is often 1.5-2x the average you read on a meter, or
> calculate working back from the dc power going into your PA stage. The
> heating effect of that current is going to be 3-4x higher than the
> average value predicts.
>
> Steve
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