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From: karn@envy.bellcore.com (Phil Karn)
Newsgroups: sci.engr,sci.electronics
Subject: Re: use of inverters with modified sine wave output with computers, etc
Keywords: inverter, modified sine wave
Message-ID: <1990Nov6.175144@envy.bellcore.com>
Date: 6 Nov 90 22:51:44 GMT
References: <29237@boulder.Colorado.EDU>
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Reply-To: karn@thumper.bellcore.com
Organization: Packet Communications Research Group (Bellcore)
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In article <29237@boulder.Colorado.EDU>, gordon@boulder.Colorado.EDU
(GORDON ALLEN R) writes:
|> Does anyone know of any long term effects of using such an inverter,
rather
|> one which produces a real sine wave...

"Modified sine wave" inverters produce a 50% duty cycle square wave
with the same peak and RMS voltage as a sine wave. The output voltage
follows the sequence 0, +170V, 0, -170V, ... (170 is sqrt(2)*120, the
peak voltage of a 120 V RMS sine wave).

Computers and other devices with switching power supplies don't care
at all. They directly rectify and filter the AC power line, so as long
as the peak waveform voltage is the same (and it is) they'll work
fine.

Most consumer electronic devices with power transformers also feed
rectifier/capacitor filters, so the modified sine waveform will have
little effect on them too. There is one exception: line-driven clocks
(such as the one in my VCR) will often run at the wrong speed (usually
twice as fast). My guess is that they're looking for zero crossings in
the AC waveform, and the 4 edges per cycle of the "modified sine wave"
fool the zero crossing detectors.

Another unit I had problems with is my old Pioneer SX-780 receiver. It
has a speaker protection relay with a circuit that, among other
things, watches for AC power failure by sampling the power transformer
output before the rectifier. This circuit cuts off the speakers before
the DC from the power supply begins to collapse, avoiding the turn-off
thump that would otherwise occur. This circuit is also fooled by the
"modified sine waveform", causing the relay to click on and off
continuously (no permanent damage to the unit, though).

I've heard claims that conventional core-coil flourescent ballasts
will not last as long on modified sine waveform inverters, but I have
no direct experience with that. In any event, electronic ballasts are
lighter, more efficient and not that more expensive so I buy them in
all
my compact flourescents.

Phil