Bill Schmidt, K9HZ has designed a fool-proof control circuit for the uBITx for power control. This circuit prevents bad things from happening by shutting down the radio before any damage is done.
It faults on:
- reverse voltage
- over power
- High SWR
- High PA Current
- High voltage.
It provides a visual indication of WHAT fault occurred, and the individual fault LEDs begin to flicker BEFORE the trip so you can fix the problem before you hit a hard trip.
The fault conditions listed above can be expanded to any number by adding more SCR Trip components (they are set to trip at 1.8V whatever the fault is).
When initially turned on, the transistorised RS Flipflop circuit comes up in the “Operate” mode. If a trip occurs, it flips into “FAULT” and shuts down the PA.
The circuit is reset with the “RESET” button, but ONLY if the fault has been resolved. Turning the power off and on resets the circuit too.
Bill bread-boarded the circuit last week and has been using it on his radio for a while and found that it works flawlessly (yes transmitting and yanking the coax off the back of the radio shuts down the PA nicely!). The circuit and a build list can be found in the BITX20 list’s files section.
Parts List for the uBITx Power Control Circuit
|D6||12V 0.5W zener 1N759, or 1N5242, or 1N6002|
|R14||1K 10-turn POT|
|R15||10K 10-turn POT|
|R16||88K||0.125||Can just use a 100K POT set appropriately|
|R17||12K||0.125||Can just use a 100K POT set appropriately|
|R30||1K 10-turn POT||0.125|
|U1||LM339||(Make sure to connect Vdd and ground!!!!).|
|F1||1A Poly Fuse||50V|
|F2||4A Poly Fuse||50V|