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Re: SCR based coil

Reinhard,

I'm no expert on transistor power dissapation, but I am quite sure the
transistor will only dissapate approximately the collector-emitter voltage
DROP times the current.  Thus, if one uses a Class D circuit (essentially
either completely on or completely off), then the dissapation will only be
a couple of volts times the current during the "on" state.  More power will
be dissapated during the times the transistor is transitioning from one
state to another, but this time could be kept as small as possible (fast
rise/decay times on the rectangular pulse drive).

--Steve

----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: SCR based coil
> Date: Wednesday, January 27, 1999 10:38 PM
> 
> Original Poster: RWB355-at-aol-dot-com 
> 
> Hello Nick, 
>  
> You wrote:
> "I wondered if anyone has thought about using big bipolars for the
>  drive. The transistors used for scan coil drive circuits in TV are rated
a
> 15A
>  at 450V and cost £0.89 each.  They are therefore perfect for use in big
banks
>  to drive a tesla."
> SNIP
> 
> The transistor ratings you mention above are OR values not AND values. In
> other words, the transistors are able to handle either a MAXIMUM of 15A
or a
> MAXIMUM of 450V, but not both at the same time. This depends on the power
> disapation rating of your transistor. For example if it has a 150W
rating,
> then you could switch 15A (max), but only at 10V, etc. 
> 
> For a transistor that could switch 450V AND 15A at the same time, it
would
> have to be able to handle 6750 watts. That would be a VERY BIG
transistor.
> 
> Plus transistors are slow devices if you compare them to a SCR, etc. A
> transistor acts like a variable resistor, whereas a SCT acts more like a
> switch. Using a big trigger pulse on an SCR will turn on the complete die
> (semiconductor surface). This fact plus the fact that an SCR acts more
like a
> switch lets these devices handle very high ratings, eventhough the SCRs,
etc
> are not very large (size-wise).
> 
> coiler greets from germany,
> Reinhard
> 
>