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Re: 2 simple questions - SCR devices - continued



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Some thoughts on the comments below:
1. SCRs are not a resistive device - because they are a semiconductor, they do 
have a pretty constant voltage drop across them (as I recall, 2 diode drops 
when conducting, e.g. 1.4 volts).  Large SCRs look like hockey pucks (and are 
called hockey pucks) and are used in arrays to switch DC to AC so that HVDC 
power lines can be converted back to AC for distribution.  they can handle a 
thousand amps or more easily. If we figure a peak current of 1000 amps, then 
the dissipation across the SCR is 1.4 KW at the instant of the peak.  If we 
are driving the tank circuit with 100V from a bank of capacitors, then the 
loss of 1.4 V is no big deal (97.6 v instead of 100v - there's that magic 
megawatt again) 

Another comment someone made was that driving a tank circuit through an SCR 
would back bias the electrolytics used for current source - the SCR will, of 
course, turn off as soon as the current goes through 0, so this will not 
happen.  

I don't have any SCR reference material handy, but I seem to recall that even 
farily small SCRs can handle several thousand amps (peak), and that the large 
ones peak over 10,000 amps - If my memory is correct, they should be ideal to 
simulate a spark gap.

However, at low voltages, you're really limiting the peak amperage.  I don't
know what the on-resistance of a SCR is, but let's assume 10 ohms.  At 100v,
that's only a maximum of 10A it can conduct.  In addition, at the high
frequencies that a TC operates at, all conductors only conduct on the
outermost few mils of the material (the skin effect), which increases the
resistance of all conductors several orders of magnitude over the normal DC
resistance.

Hope this helps,
Chris C.


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