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RE: SCR based coil



Reinhard and All,
RE: transistor specs clarification
The spec for power transistor dissipation relates to the collector to
emitter voltage
 when the transistor is ON (CE saturation via hard drive reduces this value
but slows
 the turn off) NOT the voltage seen by the LOAD ex: 2v * 15A = 30W.  Vce ON
is in tenths
 of a volt for low voltage transistors rising to a volt or two for those
rated for
 higher standoff voltages. This voltage spec applies when the tranisitor is
OFF and
 only leakage current flows Ex: 450V * 1ma = .45W. Transistors (generally)
switch much
 faster than SCR's but drive control is more complicated and critical. The
HV transistor
 dissipation limit is a function of (limited by the) SOA: Safe Operating
Area from the
 specific device data sheet graph. FETs & IGBT's are even faster.
Dale

-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com] Sent: Wednesday, January 27, 1999
9:39 PM
To: tesla-at-pupman-dot-com
Subject: Re: SCR based coil  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