Re: [TCML] SCR tesla coil

[Carl Noggle <cn@xxxxx>]

SCRs only tuirn off when the forward current drops to zero.  That's why 
you need a reverse diode to conduct the second half of the sine wave.  
The drive needs to be on until all the oscillating is done, or at least 
until you are sure that the next positive half-cycle can't turn it back 
on by overvoltage or dI/dt.

---Carl





> I would have thought topology would be as follows.
>   
> Two SCR's in a half bridge arangement driving a primary coil with a resonant DC blocking capacitor. The upper SCR is fired to conduct current charging the capacitor and the lower SCR is fired after the current zero crossing to conduct current discharging the capacitor. Process is stopped before the SCR current limit is reached.
>   
> Anti parallel diodes could be added to prevent over volting the SCR's. Two or three in series would probably be needed to ensure the SCR is properly turned off before the freewheel diodes turn on.
>   
> I do not know if SCR's turn off quickly enough for Tesla coil frequencies though.
>   
> - David
>   
>   
>
>
> ________________________________
> From: "bturner@xxxxxxxx"<bturner@xxxxxxxx>
> To: Tesla Coil Mailing List<tesla@xxxxxxxxxx>
> Sent: Tuesday, 22 November 2011 1:50 PM
> Subject: Re: [TCML] SCR tesla coil
>
> Has a lot to do with the trigger (gate) capacitance I would figure. Other
> point is that an SCR only conducts in one direction, thus defeating the
> whole idea of a spark gap. Perhaps in some sort of a PWM DC style,
> changing the duty cycle of the current pulse into the primary tank
> circuit...
>
> - b
>
>
>
> > Dear List
> > I understand the rate of change in current with time (dI/dt) in the
> > switching device in a TC is considerable. Is it still the case that only
> > The SCRs capable of handling such severe duty are those with
> > "interdigitated" gate construction e.g GTOs or has modern SCR technology
> > changed this. Would a modern modern hockey-puck SCR such as the one
> > described  here
> > http://www.ebay.co.uk/itm/Capsule-thyristor-N195CH12-390A-ITav-/320794404043?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4ab0d624cbwith
> > ratings of 1200V and 390A be of running at least a small TC would the
> > turn-on still be too slow?
> >
> > Jolyon
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>
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