[Prev][Next][Index][Thread]
Re: SCR based coil
Nick wrote,
>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
Reinhard replied.
>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.
It's not quite as simple as that, a transistor may have 450 volts across it
when
it is switched off, no current so no power. It then switches and the
voltage across
it drops to under a volt as the current goes up to maximum, again very
little
power is dissipated. Most power is lost while the transistor is switching
when
you have both high voltage and high current, it is therefore important to
use
transistors with a very high switching speed.
At the moment I'm using a pair of irf 740 mosfets (120w devices) to handle
about
400 watts and they are barely warm. Because switching speed is important I
reckon
that these devices offer more bang per buck than bipolars.
Have fun,
Alan Sharp (UK)