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undervolt-resetting of a Yeasu VX5r from weak power supply

Discussion in 'Electronic Basics' started by tom, Mar 11, 2005.

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  1. tom

    tom Guest

    I have an HT that draws 1.9 amps when transmitting, and the voltage can't
    sag more than 2v or the HT does an undervoltage reset. I just bought a
    samlex 12v, 4amp power supply to run my HT but there's a problem. The
    samlex 12v output voltage begins to sag as soon as the mic is keyed and the
    amount of sag increases steadily until 3-4 seconds into the transmission the
    HT does an undervolt-caused reset because it's input voltage has sagged 2v.
    I might not be able to return the samlex to the shop and replace it, and I'm
    wondering if there's something that can be done to enable me to use it some
    way, somehow to power my HT. The HT will work fine on a reduced
    transmission power setting but then I can't reach repeaters. I have another
    wall -wart plug in adaptor that puts out 1700mA at 12v.
    -Is there some way I can combine the two power supplies together?
    -In parallel maybe? Parallel would probably work if they were batteries
    because I know in series the voltage would exceed the max of 16v.
    -How about some kind of arrangement with a transformer?
    -If I get a lead-acid, 12v, 10amp/hour battery, how complicated acircuit
    would I need to charge the battery from the samlex? is this a possibility?
     
  2. mike

    mike Guest

    If you're a ham, you need to learn how to figure this stuff out.

    I can't tell from your post whether you measured stuff, or just guessed.
    If you can't measure volts and amps, your first priority is to buy or
    borrow a meter that will.
    So don't get offended If I go too slow. Divide and conquer is the strategy.

    First, check for operator error. Is there a current limit control
    somewhere in/on the power supply that's mis-adjusted? It's possible to
    have wire that's too small and a couple of inline fuses contributing to
    the voltage drop. Measure the voltage under load at the power supply
    terminals and as close to the handheld terminals as you an get.
    Also consider the possibility that if your handheld battery is dead, the
    charging current plus the transmit current may exceed 4A. I'd say, not
    likely, but worth a think.

    Next thing I'd do is check the power supply. Use a big resistor or a
    car headlight or a couple of tail-lights to put a 4A load on it.
    Measure the current and voltage. If you get rated current at rated
    voltage, it's probably good. If not, fix it. If you just bought it,
    take it back.

    Does the VX5 have normal battery life when run on batteries? That shows
    that the final is probably not blown drawing excessive current.

    But it is possible for the final transistor breakdown voltages to have
    been reduced by overheating and draw too much current on 12V.
    Verify the external power supply load by running it off a car battery
    and verify with your ammeter that it doesn't draw more than 1.9A. If
    it's drawing more than 4A, it will drag down your power supply and
    probably go up in smoke. Maybe a 2A fuse during the test to reduce the
    smoke.

    Ok, so they're both good...must be interaction.
    Put the 4A lights on the power supply. Run the HT on internal batteries
    and waive the rubber ducky around the leads to the lamps while
    transmitting. If the lamps dim, you know you have RF getting into the
    power supply. Try output bypass caps and ferrite cores on the wires.
    If that don't fix it, you'll have to get out the radio amateur handbook
    and learn about keeping RF out of your power supply. This should not be
    a problem with any modern commercial power supply.

    It's almost never a good idea to parallel power supplies. It's always
    a BAD idea when one of them is broken. If your 4A supply ain't broke,
    you don't need to parallel. If it is broke, you need to fix it.

    If your parallel lashup gets more broke and puts out 20V, you'll have a
    handheld pile of burnt junk.

    mike



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  3. tom

    tom Guest

    I just got back from the shop --- I up-gunned the power supply to a 7-10
    (amp) model and it works fine now, at all power settings and antenna
    variations, without a single glitch. This model of yaesu has a transient
    current spike, according to the shop's guru, that is similar to what happens
    when you start an electric motor. At the first instant, the current drawn
    is much higher than the rated draw, but just briefly. Some power supplies
    can cope, others can't. What's interesting is that I tried a new, hi-tech,
    completely solid-state, 8-10amp rated unit (that was 1/4 the size of the
    samlex and twice the cost), and it reset every time! But the cheaper, lower
    tech, lower rated, older fashioned 7-10amp samlex works fine --- because the
    solid state unit can't handle the 'amp-transient' or whatever you call it
    (whatever the proper term is, I'm pretty sure it isn't "amp-transient"!).
    At least, all of this is according to that shops electronics guru. However,
    til I hear another explanation that's a better fit for the facts, I'm going
    with the "amp-transient" theory. Anyone got a better one? Mike?
     
  4. mike

    mike Guest

    Sounds like you have the explanation. Probably could have worked around
    it with a big electrolytic on the output of the power supply.
    I sure don't like the idea that the handheld can fold up a 10-amp power
    supply. That's gotta be a reliability problem for the handheld.
    Think I'll cross the VX5 off my wish list.
    mike

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  5. cl

    cl Guest

    I'm wondering if perhaps it was RF that got into the power supply and caused
    it to shut down or go flaky enough to cause the drift in voltage. I find it
    hard to believe that a Handheld screwed up a power supply like that "just"
    on current draw. RF getting into a power supply can do some weird things.

    cl
     
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