Friday, January 4, 2013

More Destructive Testing

But, . . . hopefully NOT.

With the breadboard FAN Control Circuit designed and working, I decided to do some more Destructive Testing of the PA-47 (see previous post).

The two tests that I had in mind were to; subject the PA to High VSWR, by; first, opening the end of the 18 inch coax, then second, putting a "short" on the end. These are the two extremes.

Initial Test - Normal Operation
  • The PA and exciter were configured for normal operation of 12.6VDC.
  • The Exciter provided 12 mWatts of drive at 10.140MHz to the PA input connector.
  • The PA with Low Pass Filter were connected to a proper 50 Ohm Dummy Load via a 18 inch coax cable.
  • DC input was 12.6 VDC at 2.54 Amps, or  32.1 Watts.
  • Voltage measurements are taken at the output of the Low Pass Filter.
  • The typical Low Pass output voltage is 75 Volts PP, or 14.1 RF Watts output.
  • The FAN Control Circuit turns the FAN slowly.
  • All is normal.

The Open End Coax Test
  • DC input was 12.6 VDC at 1.34 Amps, or  16.8 Watts.
  • The measured Low Pass Filter output voltage was 160 Volts PP.
  • The danger here is the reflected voltage will exceed the voltage rating of the RF Power FET.
  • Results: No apparent harm was done.

The Shorted End Coax Test
  • DC input was 12.6 VDC at 3.11 Amps, or  38.2 Watts.
  • The FET was getting HOT!
  • Immediately the FAN Control Circuit put the FAN into Full High Speed mode.
  • The measured Low Pass Filter output was 35 Volts PP (note: this is about 18 inches from the short).
  • After about 10 seconds, the FET went into a kind of internal thermal shutdown on its own (I did not expect it to do its own shutdown).
  • DC input was now 12.6 VDC at 3.54 Amps, or  43.6 Watts.
  • Measure RF output went down to about 10 Volts PP.
  • The PA was left to cook for about 30 seconds. I am not sure how long this could continue without permanent damage.
  • Results: FAN at High Speed, FET was in Internal Thermal Shutdown, everything was getting HOT!


The FAN Control Circuit saved the day, no Blue Smoke was emitted. I think the FAN Control Circuit is a very good addition to the PA-47 !

After Cool Down, the 50 Load was reconnected and normal operation was resumed.

Although, .  . . the idle current is now 80 mAmps, where as before the tests, idle current was typically only 20 mAmps. Maybe the quality of the FET has diminished?


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