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From: ron@brl-sem.UUCP
Newsgroups: sci.electronics,rec.video,rec.audio,rec.ham-radio,rec.autos.tech
Subject: Re: Mobile 120VAC sources
Message-ID: <654@brl-sem.ARPA>
Date: Thu, 26-Feb-87 12:43:48 EST
Article-I.D.: brl-sem.654
Posted: Thu Feb 26 12:43:48 1987
Date-Received: Sat, 28-Feb-87 06:44:12 EST
References: <278@dopey.AMD.COM> <672@twitch.UUCP> <3235@elroy.UUCP>
Organization: Electronic Brain Research Lab
Lines: 41
Xref: utgpu sci.electronics:324 rec.video:580 rec.audio:737 rec.ham-radio:588 rec.autos.tech:439

In article <3235@elroy.UUCP>, bob@elroy.UUCP (Bob Yen) writes:
> an inverter); I need some input on a good unit.  The ones by Tripp-Lite
> put out square waves, whose RF radiation screws up TV reception.

> I was wondering how the mobile amateur radio people handle 120vac
> input requirements (or is everything 12v).  I would think a home
> stereo + inverter would give better fidelity $ for $, at the
> expense of some inconvenience.
> 
Well these days everything is transistorized so even if you have
a fixed transceiver chances are that that wall plug is connected
to a power supply that puts out low voltage DC.

Back in the (good?) old days, when tubes were popular there were
several methods of getting the high voltage necessary.  The cheapest
probably was the vibrator.  Popular in old dash board car radios as
well.  The vibrator turned the DC into a rather crude (yes square
wave) AC, which could then be run through a transformer.  Mobile
transmitting equipment was sometimes equipped with a "dynamotor,"
essentially a 12V DC motor with an AC generator on it.  This of
course made sine waves but was big and clunky and since it had
moving parts was more prone to failure.

Enter the marvel transistor:  They hadn't quite managed to make transistor
power amplifiers yet, but they were able to make transistor multivibrators.
This (as you've descrived) is the square wave generator that is the heart
of the inverter.   The way to avoid the emmissions, lots of filtering.
Even traditional SCR dimmer circuits put out a lot of noise that would
be intollerable without a filter on the output.  However, artifacts of
the T-power were still present, in that you could here in the signal the
frequency of the vibrator which used to change pitch with fluctuations in
the voltage of the car electrical system.  (Of course, even DC rigs often
pick up whine from the cars alternator).

I suspect if you really wanted to you could modify a cheap intverter to
be much less annoying with a few RF chokes.  By the way, the output filters
on even the best Theatrical Dimmers (where you need to keen the buzz out of
the sound system) are pretty simple.  Just a few turns of the output wire
around a toroid core on it's way to the connector.

-Ron