From: neal@cmptrc.lonestar.org (Neal Howard)
Newsgroups: sci.electronics
Subject: Re: A much better power inverter design
Date: 11 Sep 92 21:20:26 GMT
Distribution: usa
Organization: CompuTrac Inc., Richardson TX

In article <BuE4p8.5Bq@acsu.buffalo.edu> v064mb9k@ubvmsb.cc.buffalo.edu (NEIL GANDLER) writes:
>
>   I am a sophomore engineering student and have been researching and
>trying to develop a new type of car amplifier that runs cooler and
>the taim was at the power supply. 12v simply is not enough voltage
>to power speakers to large amounts of power unless multiple amps
>and speakers are used. So most manufactuers use mosfet power supplies
>that step up the 12vdc to higher levels by first providing a 60hz
>oscillating signal and using power mosefets to switch the 12v power
>to a step up power transformer which results in a larger square wave ac
>voltage output. This is rectified and the result is a dual polarity
>high voltage dc power supply for power amplifiers. But the mosfets although
>amazing devices have internal resistances that cause them to heat up a lot
>under heavy current loads. So I tried a mechanical method using 2 high
>speed quality relays. The design amazingly worked with little problems,
>like noise from the 60-hz modulated relays, a small amount of magnetic
>intereference which can be solved and slightly warm relays. The only
>major problem I can see is reliabilty. How many cycles can a relay take
>before they burn out, thats the only question. I must do more work and
>improve on this. Another small problem is irregularity, relays are not
>perfect bounceless switches and spikes and irregularities on the scope
>show up frequently on the square wave ac output. I have yet to try
>this with a power amplifier. Overall the major advantage is cost including
>no expensive mosfets, no large metal heatsinks and little waste and its
>small size makes it perfect for a car. The only real source of heat is in
>the step up transformer. I am not an enginneer yet and I doubt that I
>found something new. I am open to any questions, opinions and comments
>and would like to know if I have something here. Thankyou
>
>Neil Gandler

Neil,
 What you have described here is very similar to the power supplies used
 in old 1960's era tube type CB radios for automobile use. Some of these
 included the old E.F. Johnson models I (whiteface) and II (blackface) as
 well as many Heathkit, Lafeyette (sp?), Regency Range Gain I and others.
 They used a device which was called a "vibrator" (no joke) which was basically
 a type of relay with a normally closed set of contacts wired in series with
 the relay coil. When the coil was energized, it would pull the relay
 armature down, break the current, armature would then pop back up, re-energize
 the coil, etc. etc. etc, making a second set of contacts attached to the
 armature open and close repeatedly to provide a pulsed switch in line with
 the 12V primary of the power transformer. These power transformers usually
 also had a 120V primary so the same radio could be used either off of 120V AC
 at home, or 12V dc in a car or truck just by using a different power cable.
 They usually had a big round multi-pin power cable jack on the back of the
 chassis. These vibrators were metal cylenders about 4 inches tall and 1.5
 inches diameter and had a 4 pin base like an ancient radio vacuum tube.
 Eventually, "solid state" vibrators were available as plug in replacements,
 containing a pair of power transistors and a small transformer and a few
 resistors, capacitors, etc inside the same size can. These worked much quieter
 and lasted longer (the points in the mechanical ones burned out every few
 months) but were prone to overheating. Before all solid-state CB's were
 very abundant, manufacturers began incorporating their own solid state
 oscillators on board the CB using larger power transistors and big finned
 heat sinks. I believe some units even came close to having a good sine-wave
 output of the oscillators ( the Regency Range Gain II and it's SSB cousin)
 had pretty reliable 12V power supply circiuts. Needless to say these things
 would drain a car's battery in a hurry (something like 15 to 20 amps) if the
 engine wasn't running. The idea of using a electro-mechanical has been around
 for a pretty long time and does work pretty reliably for such a simple design.
 To make the contacts last longer, parallel a capacitor across the arcing 
 contacts (probably any where from .1 to 5 mfd, go as large as you can to keep
 the unit self oscillating while yet minimizing sparking).

 My 2 cents worth of power supply trivia.
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= Neal Howard                         | '91 Harley Sportster XLH-1200 =
= email: neal@cmptrc.lonestar.org     | DoD # 686                     =
= CompuTrac, Inc. (Richardson, TX)    | '69 Cougar XR7-GT390          =
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