In short, if the 128D uses 60W, I need a transformer with a VA of
60 or higher, your recommendation around 100?
>>
>>
>> I visited Radio Shack yesterday, and look for a step-up transformer
>> for my European C-128D. They had everything neatly labelled, but
>> how do I find out how many watts it's rated for? It has a measurement
>> called 'V/A Cap' which goes from about 70 to over a thousand. The
>> prices goes up with it.
>
>OK... VA (not V/A...) is Volt-amps. It's called the 'apparent power'
>rating of the load.
>
>Remember that in a DC circuit you calculate the power by multiplying
the
>voltage (in volts) by the current in amps. So the VA unit does have the
>right dimensions to be power.
>
>In an AC circuit, things are not so simple. I'm going to stick to
>sinusoidal waveforms here, BTW and let someone else handle the nasty
>cases. It turns out that if the voltage and current are not in phase
with
>each other (and they won't be unless the load is perfectly resistive),
>then the useful power you can get out of the load - the so-called true
>power - is given by V*I*cos(phi) where phi is the phase angle between
the
>voltage (V) and current (I) waveforms. V and I are the rms values of
the
>voltage and current, as usual.
>
>cos(phi) is called the 'power factor' of the system.
>
>But, the size of the transformer you need is really determined by the
rms
>values of V and I, no matter what the phase shift is. So that, for
>example, if you have a load giving out a true power of 50W, with a
power
>factor of 0.5, then you need to use a 100W transformer to supply it.
>
>What does this mean to you?
>
>I think it's fair to say that 0.5 is an excessively low power factor
for
>any micro. 0.66 would be pretty low as well. So if you (say) see that
>your machine takes 100W, then a 150W or 200W transformer would be
ample.
>A 100W transformer _might_ be OK, but it might be marginal.
Practically,
>I'd probably pick smalllest transformer with a VA rating in excess of
the
>wattage of the computer, and if it doesn't get excessively hot in use,
>it'll be fine.
>
>-tony
>
>
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Received on Tue Sep 22 1998 - 21:19:32 BST