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RE: SCR's
Subject:
RE: SCR's
Date:
Sat, 15 Mar 97 11:29:02 UT
From:
"William Noble" <William_B_Noble-at-msn-dot-com>
To:
"Tesla List" <tesla-at-pupman-dot-com>
I used to build electronic ignitions for cars - I would charge a 1 to 2
microfarad capacitor to about 450 V, The capacitor, an SCR,and the coil
primary made up the HV circuit. When the breaker points opened, I would
trigger the SCR, dumping the capacitor into the primary of the ignition
coil.
The coil would ring, shutting the SCR off, and the capacitor would start
charging again. The Delta Mark 10 ignition uses a similar circuit.
This is a
lot like a tesla coil (and the spark from the ignition coil would jump
about 4
inches), which is why I started thinking about SCRs - clearly in the
above
circuit there is a pretty fast current rise - of course it depends on
the
reactance of the automotive ignition coil (I used a 6V coil).
(snip)
>
>I'm not sure that SCR's can switch fast enough. I tried using MOSFET's to
>switch large currents into a primary (no capacitor - instead I drove the
>switch at the resonance frequency. I got a 2" spark and I killed 4 or 5
>FET's - the problem was that as the voltage rose in the secondary -
>it was fed back to the primary. Result overvoltage killing the FET's.
>
>FET's have very low on resistances, 0.1 ohm, switch very fast
>ie 50nS and handle high peak currents (40A), and they can be switched
>off. Unfourtunately you would need a lot of them to handle 1000A at
>10kV - back to the spark gap lads.
>
Have fun,
>
>Alan Sharp (UK)