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Yes please Jim, I am considering an 800W amp so this guard
circuit could be very useful. 73s Laurie
Dear Andy, LF Group,
Aha... wondered why
you hadn't mentioned it before - unfortunately, the guard circuit is not shown
in the simplified diagram in the LF handbook.
What it is - the tank
coil has a link winding on it. This feeds the 'AC' side of a bridge rectifier
made up of 4 fast diodes, via a series leakage inductance tuning capacitor.
the 'DC' side is connected to some big plastic film reservoir capacitors, and
then across the PA supply rail. In fig 3.20 in the handbook, it is assembled
on the 3 heatsinks largely concealed by the tank capacitor boards. I think it
works like this:
When the tank circuit current (determined by
the load
impedance) exceeds a certain ratio with the supply voltage, the
voltage across the link winding forward biases the guard circuit
diodes, and returns power to the DC supply. This effectively
clamps
the voltage across the tank circuit, and due to the
impedance transforming
properties of the LC circuit, appears to the
PA as a non-linear resistance
in series with the load which increases rapidly when the output current
exceeds a certain point. If you monitor the 'guard', PA input, and supply
currents as the load is decreased, the guard and PA currents increase, but the
difference between them (ie. the supply current) decreases. With a dead short
on the output, the PA current is increased by about 50%, eg, after the unit
was re-tuned:
With 50ohm load, 60V supply -
Total PA supply current
for 3 PA's = 21.8A = PSU supply current
Total guard current =
0.1A
With short circuit load -
Total PA current = 34.9A
Total
guard current = 28.3A
Supply current from PSU = 6.5A
With the full
supply voltage and a short circuit load, the PA modules run quite a bit warmer
than normal, but not dangerously so, so this is an impressively rugged
design!
I can send you a photocopy of the full diagrams if you like -
also the details of how I tuned it up for a different frequency and load
impedance, if anyone's interested.
Cheers, Jim Moritz
73 de M0BMU
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