Kenwood TM-V71A Switched Power

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KF7EFI, Sep 1, 2019.

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  1. KF7EFI

    KF7EFI Ham Member QRZ Page

    I'm looking to use a Kenwood TM-V71A with a Mobilinkd TNC. The TNC and transmitter will be remote mounted in the body of the vehicle with limited access. The Mobilinkd can be configured to turn on and off when power supply is connected or disconnected. Is there a good place on either one of the ports of the radio or the board inside to get switched power for the TNC? I'd like it wired so when the radio is on the TNC powers up and when the radio is turned off the TNC powers down. The radio has constant power run to it so it can be used even when the vehicle is powered off. For example it looks like the pin 2 of the microphone is 8V power that turns on and off with the radio. However it appears that pin is limited to 100mA which is likely a little low to power both the mic and TNC. The TNC is powered by USB so 5V 500mA or if there is something in the radio that turns on and off 12V and can convert it down as needed.
  2. WB2LBV

    WB2LBV Ham Member QRZ Page

    You could use a 5V micro-relay, powered from the 8V pin through an appropriate dropping resistor, to switch the power to the TNC. It should be possible to find a relay with low enough coil current to work with this setup.
  3. KF7EFI

    KF7EFI Ham Member QRZ Page

    Hmm that might be a good way to do it and be able to get high amperage power. From the spec it looks like the Mobilinkd only draws ~160mA max when the battery is not used. Looking at the radio schematic (MechE here so not the best with EE schematics) it looks like pulling 12V from the SB leg gives 12V when the power is on. That is the power going to the voltage regulators that power the radio circuits (8V, 5V). I might see if tapping in there and adding a 150mA voltage regulator to power the TNC from there does the trick.

  4. WA9SVD

    WA9SVD Ham Member QRZ Page

    As far as I know, the stock Kenwood mics (MC-43) don't use any current from the 8 Volts available. Illuminated mics may use some current, however. Your user manuals should verify that.

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