Gradual drop in voltage upon powering off power supply

Discussion in 'General Technical Questions and Answers' started by KC1MUU, Jun 28, 2020.

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  1. KC1MUU

    KC1MUU Ham Member QRZ Page

    I have a Kenwood TM-V71 indoors with a power supply that when switched off drops the voltage gradually when powered off. It probably takes about a second to drop from 13.8v to 0v. The radio shuts off around 9v. Curious if anyone thinks this is a problem as opposed to a straight switched on/off. Is this going to damage the radio at all? When turned on, it powers up to 13.8v immediately. It is convenient to switch off the whole surge protector that is why I am asking.

    Hope everyone is having fun at field day!

  2. WR2E

    WR2E Ham Member QRZ Page

    That is pretty typical of a DC supply. What you are seeing is the stored energy in the output filter capacitors being discharged by the bleeder resistors.
    K8BZ likes this.
  3. N3PM

    N3PM Ham Member QRZ Page

    I usually shut the radios off first, then the PSU. Power up sequence is PSU then the radios.
    I just have something about surges and spikes.
    Mike N3PM
    KA0HCP, KU4X and WR2E like this.
  4. N4MU

    N4MU Ham Member QRZ Page

    That is normal. The power supply drops in voltage due to "bleeder resistors" in the circuit specifically for that purpose. The quick on effect is also normal. You should however turn the radio OFF FIRST, then the power supply off. Turn the power supply ON FIRST, then the radio. This helps to protect the radio from transients while being powered up and is gentler on the radio.
    I see you're a pilot...maybe...from your avatar. What do you do with the avionics in the aircraft? Turn them on before starting? NO! Same applies to your little radio...just a lot less $$$ to fix if it gets damaged compared to the aircraft equipment. 73, Bill, CFII
    K8BZ, KA0HCP and KU4X like this.
  5. KC1MUU

    KC1MUU Ham Member QRZ Page

    Thanks everyone. I totally understand how this normal, just was inquiring about whether it would harm the radio at all. I know the safe way is to shut off the radio first, then kill the PS.

    N4MU, no I'm not a pilot unfortunately. Maybe someday. The avatar was from spending a day with a medical transport helicopter service...probably the coolest thing I've ever done.
  6. N4MU

    N4MU Ham Member QRZ Page

    Glad you had fun! My best days were going flying and then collecting money from my "passenger". Talk about a win-win! Don't do that anymore...boohoo... eyes aren't what they used to be. Nothing else is either!. You know, helicopters can't really fly don't you? LOL. 73. Enjoy!
  7. KC1MUU

    KC1MUU Ham Member QRZ Page

    That sounds like a dream! thanks again, 73

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