RF protection circuit

Discussion in 'The Projects Forum' started by samrat, Sep 24, 2008.

  1. samrat

    Thread Starter Active Member

    Apr 2, 2008
    My group has built a 50 watts power amplifier to boost the 4watts exciter that our school currently broadcasts on(FM89.6). We have built the power amplifier, also making our power supply from the LM338 regulator but we are left with a easily built protection cct to cut supply if the reflective power exceeds the amount allowed. My request is for a simple cct diagram that will be able to be made with readily available components such as a fuse. We have 3 weeks left for our final presentation and we hope that someone will help us with this information.:confused:thanks
  2. DickCappels


    Aug 21, 2008
    Congratulations on building that 50 watt amplifier!

    I don't have a ready-made circuit, but here is a thought:
    Build an SWR bridge with two DC outputs (rather than switching the detector between the two outputs) and use a comparator to determine when one output is X times the other -its ok to use a resistive divider on one output. The output of the comparator would trip a latch, such as a D flip flop when the ratio is exceeded. The output of the latch would drive a transistor that grounds the refernece ( or "ADJ") input on the LM338, thus dropping the output voltage to 1.2 volts.

    A power-on reset circuit (could simply be a resistor or two and a capacitor. for the latch would both assure that the latch was in the proper state when the circuit is first powered on and provide a convenient means of resetting the fault detector.

    There is a nice little circuit based on something like the above in an old Motorola Application Report, AR510, if you can dig up a copy. It doesn't appear to be on the web.

    Or try this link for another circuit:
    Last edited: Sep 26, 2008