Re: SCR based coil


The transistors used in deflection circuits can switch 15 amps of
current and stand-off a potential difference of 450 volts without
breakdown.  The devices are not designed to dissipate large amounts of
power (neither are SCR's).  

In a transistor switching circuit, when the transistor is "on" there is
a voltage drop of six to seven tenths of a volt across the device
(assuming it is driven hard enough). The device dissipation is in the
area of ten watts when switching 15 amps . . . When the device is off,
there can be up to 450 volts across it, but with no current flow, the
dissipation is zero.

IF the drive circuit were properly designed (and the transistors not
operated in their linear region) it should work . . .  There is
something called the Safe Operating Area that takes into consideration
voltage, current, temperature, and switching speed.  

The real killer is probably frequency.  The TC wants to see 100 to 500
KHZ.  Even using a very short burst of energy driving the coil in a
Class C mode of operation, storage time and switching speed are probably
going to be too much for a transistor designed to switch at 15 KHZ.

An SCR has no linear region, as you state, it is either on or off.  If
the power source is AC or pulsating DC, an SCR would have an advantage. 
Diodes and SCR's are still subject to storage time however, so frequency
would still be a problem. 

The bipolar drive can probably be used for a TC, but at relatively low
power, and probably not with inexpensive transistors.  Or, it may be
just the thing to drive a really large coil at a very low frequency.

bob misiura