Re: [TCML] IGBT variac dimmer?

[David Petzer <davepetzer@xxxxxxxxx>]

Ed is correct, for a reasonable amount of power you would not be able to operate the transistors(or IGBTs or MOSFETS) out of saturation.

For a high power system you would need to produce your sinusoidal waveform via PWM control and passive filtering. 


For single phase this could be a two opamp triangle wave generator and a mains frequency, amplitude adjustable sine wave produced via a transformer and pot. These are fed into a comparator, some digital logic and a high and low side gate driver. Oh and the main switching devices of course.


A good book on the subject is "Pulse width modulation for power converters:Principals and Practice by Yazar: D. Grahame Holmes,Thomas A. Lipo,T. A. Lipo". I believe it is available free on Google books."

David Petzer




________________________________
From: Ed Phillips <evp@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Saturday, 17 September 2011 10:01 AM
Subject: Re: [TCML] IGBT variac dimmer?

Problem with the idea is the power dissipation in the transistors when 
they're not saturated.  At less than maximum power instant smoke!  I've 
used the series full wave rectifier and shunt transistor for low power 
voltage regulator functions such as feeding variable voltage into a 
small filament transformer and it works OK.

Ed

David Petzer wrote:

>Hi Scott,
>
>IGBTs (or fets or transistors) could definitely do the job of a variac, that is to produce a sinusoidal voltage at mains frequency  of variable magnitude. One circuit topology would be to rectify the mains AC then use an IGBT half bridge to produce the desired sinusoid.
>
>You could do more with the IGBTs as well because you would be able to vary the magnitude and frequency of your sinusoidal out put, or even produce non sinusoidal functions like ramp or square waves these could produce some interesting results for Tesla coil outputs.
>
>As for protecting from over load there are a host of solid state methods for protecting the switching devices from this.
>
>As an aside I believe that the humble dimmer switch contains a thyristor (SCR) so the wave form which it produces is a chopped version of the mains AC, not a sinusoid of lower magnitude.
>
>David Petzer
>
>
>________________________________
>From: Scott Bogard <sdbogard@xxxxxxxxx>
>To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>Sent: Friday, 16 September 2011 4:35 AM
>Subject: [TCML] IGBT variac dimmer?
>
>Greetings all,
>     Just wondering if anybody has played with the idea of using IGBTs to
>build a sort of solid state variac that would function like a dimmer
>switch.  I can't imagine it would be that difficult, and IGBTs have proven
>their worth in TCs, so it would just be a matter of using external
>ballasting to keep the current levels acceptable for the IGBTs even at full
>power.  This might actually be cheaper than trying to source super big
>variacs for pole type coils, if one can find cheap used IGBTs as is
>seemingly the case on eBay.  Any thoughts?
>
>Scott Bogard.
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>
>  
>
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