Re: [TCML] solid state varaic

[jimlux <jimlux@xxxxxxxxxxxxx>]

Paul Bidmead wrote:
> Scott,
>
> this is how most large ups's are built the difference from them to vfd's 
> is that the output V/F is adjustable
>
> this being simple in theory the practical implication can be quite 
> unstable / expensive in comparison the the cost of a simple variac... 
> not saying it cant be done but for the general hobbyist it can be quite 
> daunting
>
>
> Paul B
> > Greetings all,
> >      So I'm 
> > wondering if it would be possible to make a solid state variac in a 
> > similar manner, the advantage would be light weight and I would assume 
> > it would be much cheaper when dealing with high powered systems, the 
> > disadvantage is I don't think it would be possible to get the 140/280V 
> > output like you can with the real thing. 


Yes.. I've done it.. I used a bridge rectifier in series, with the FET 
as the pass device in the middle driven by a 555 biased by a couple 9V 
batteries.  (hook the FET from the + to - terminals of the bridge, the 
two AC terminals go to line and load)

It works, but...

You need a fair number of components to make it work well and smoothly 
vary.  It generates copious RFI and is quite sensitive to destruction 
from transients.  The variac is a pretty convenient device.. robust, 
cheap, etc.  Where I used to work, we'd buy 5A variacs for about $60 
each in quantity, and we couldn't get the FET equivalent down to that, 
by the time we got the parts, assembled, etc.


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