Keying an old tube amplifier: soft start or simple solid state relay?

Discussion in 'Amateur Radio Amplifiers' started by CT2FZI, Nov 10, 2019.

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  1. CT2FZI

    CT2FZI Ham Member QRZ Page

    Hello and greetings QRZ,

    I would like to ask you why not simply using a DC-DC solid state relay outside a (for example) TL-922A and use a more expensive "soft key" kit to reduce the voltage across the radio key relay?

    One reason might be the need to power the relay (extra PSU if you radio has already an internal PSU).

    But is the SS relay introduced delay so bad, even for full QSK?

    Will SSB be more forgiving?

    Thank you in advance for your reading and help.

  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    Such a solid-state relay will work, but it is not intended to operate with so small currents that are around in the TL-922 keying circuit.

    A more suited type would be the Clare PLA140 MOSFET relay that easily handles 400V and 200 mA which gives a quite good margin.

    One thing that is not provided in the TL-922 circuit is a transient protector that clips the turn-off inductive transient caused by breaking the current in the relay coils.

    A suitable suppressor is a VDR resistor with about 180 V clipping voltage over the MOSFET relay.

    CT2FZI likes this.
  3. CT2FZI

    CT2FZI Ham Member QRZ Page

    Hello Karl-Arne, really good tip about the VDR.

    I searched for the Clare PLA140 MOSFET and saw its Reverse Input Voltage is max 5v. Then I could not connect it to +12V on the radio PSU (since the ground will be common to both PSU and TRX, the TRX keying will be easy with 12V and the above SSR).
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    The reverse voltage rating of the LED is of no concern.
    as it would never become reverse biased.

    Just connect a current limiting resistor of 1000 ohms in series with the diode between the anode and +12 V.

    CT2FZI likes this.

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