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?