Message-ID: <13553.200012181440@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 18 Dec 2000 14:44:01 +0000
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Subject: LF: Guard Circuit
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<?color><?param 0100,0100,0100>Dear Andy, LF Group,<br><br>Aha... wondered why you hadn't mentioned it before -  unfortunately, the guard circuit is not shown in the simplified  diagram in the LF handbook.<br><br>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:<br><br>W<?/color>hen the tank circuit current (determined by the load <br>impedance) exceeds a certain ratio with the supply voltage, the <br>voltage across the link winding forward biases the guard circuit <br>diodes, and returns power to the DC supply. This effectively <br>clamps the voltage across the tank circuit, and due to the <br>impeda
nce transforming properties of the LC circuit, appears to the <br>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:<br><br>With 50ohm load, 60V supply -<br>Total PA supply current for 3 PA's = 21.8A = PSU supply current<br>Total guard current = 0.1A<br><br>With short circuit load -<br>Total PA current = 34.9A<br>Total guard current = 28.3A<br>Supply current from PSU = 6.5A<br><br>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!<br><br>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.<br><br>Cheers, Jim Moritz<br>73 de M0BMU<?color><?param 0100,0100,0100><br><br>