[CQ-Contest] Power Inverters

[Tom Rauch]

 > I am told that to run a computer properly, you need a true sine wave
 > output inverter.  Don't know if that's just urban legend, but it's often
 > repeated on the RV newsgroups.

It would depend on the supply Bill.

I know very little about computers (except they waste my time), but
I work daily with circuitry used in power supplies.

Most AC line operated switching supplies rectify the CREST of the
sine wave, which is 1.414 times line voltage. If the supply is
designed to operate at 100 voltas AC minium, it actually
REQUIRES a peak voltage of 1.414 times 100 or 141 volts from the
mains.

If you have a square wave supply, the peak is the same as the
average. With a 120 VAC **square wave** supply the computer
would see a shortfall of 20 volts or more on the minimum voltage it
needed.

There would be a way to fix this by stepping up the voltage, but
you'd have to be sure the supply only looked at peaks! Anything
connected to the inverter requiring an RMS voltage of 120 volts
would burn out.

There is no easy way to filter the output of a switching, except
perhaps you might try a large "Sola-transformer". That transformer
will saturate on peaks and "clip" the input waveform into a square
wave. The secondary then steps up the voltage and rounds it off
with a large capacitor that filters harmonics.

The problem is a Sola-transformer is very inefficient and runs very
hot, and the inverter might not like the peak currents drawn by the
transformer as it saturates.

I've thought about this, and I don't see a good solution other than
using a sine-wave inverter or a computer supply that will accept a
square wave and making sure the square wave voltage is correct.
I'm positive MANY computer supplies will run just fine on square
waves, but the voltage would need to be stepped up so square
wave peaks fall in the range the supply normally requires from sine
waves.


73, Tom W8JI
w8ji at contesting.com


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