Amp keying?

Discussion in 'General Technical Questions and Answers' started by KW4TI, Oct 20, 2019.

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  1. KW4TI

    KW4TI Ham Member QRZ Page

    This is a question about how relays are actuated by transceivers to insert an external amplifier into the signal path.

    The output voltage to drive amplifier relays from transceivers often outputs 12 volts, which is usually sufficient to drive a few tens of mA to close the relay.

    However, when the transceiver does not use the amplifier, is the output of most transceivers to the relay zero volts, or an open circuit?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Most transceivers do not output 12V to key an external amplifier.

    The "12V" comes from the amplifier, and the transceiver provides a "ground on transmit" to key the amplifier's relay. Even if the transceiver can provide 12Vdc on transmit, that would very rarely be used to key an amplifier, since nearly all commercial amplifiers provide the relay voltage from inside the amp, so they only need a "pull to ground" to activate.

    Modern transceivers can all key 12V at 100mA or more. Many are rated more than that, for both voltage and current. Somewhat older transceivers often used isolated (ungrounded) relays and could key a few hundred volts, sometimes up to an ampere. The old Kenwood hybrids were like that: Pretty big relay with no common ground connection anywhere; they could even key "AC" relays if desired.
  3. KW4TI

    KW4TI Ham Member QRZ Page

    So below is an excerpt from the Icom 7300 manual. If I understand what this means then, the transceiver grounds the inside conductor of the send jack to enable the amplifier?

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, it does. Just like almost every other transceiver.

    The "keying voltage" doesn't come from the transceiver, it comes from inside the amplifier.

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