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Measuring the high voltage plate Voltage

Discussion in 'Ham Radio Discussions' started by KA5ROW, Dec 9, 2010.

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  1. KA5ROW

    KA5ROW Premium Subscriber QRZ Page

    Measuring the high voltage plate on RF amplifiers.

    How can you measure the plate voltage on a amplifier?

    I looked at some HV probes that can be used with multimeters, but there seems to be a problem with them handling the current. So what to do to accomplish this.

    This is what I have looked at.

  2. AI3V

    AI3V Ham Member QRZ Page

    That probe will work.

    Keep in mind it will only be accurate with a meter that has a 10MΩ input impedance.

  3. K1ZJH

    K1ZJH Ham Member QRZ Page

    Probably work with the correct meter...

    But note that is safety rated for use on devices with HV
    supplies that are designed to deliver very low currents.

  4. W1VT

    W1VT Ham Member QRZ Page

    You might consider using a series network of resistors to scale down the voltage. The really cheap way is to use the existing bleeder resistors. Perhaps the most accurate way is to buy a lot of precision resistors and connect identical resistors in series--and then calibrate them with a lower voltage. You should measure the voltage across the resistor closest to ground. They should all heat up identically.

  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you're measuring voltage, that has nothing to do with current.

    A HV meter of any kind won't draw any current to speak of.

    Measuring "plate current" or other stuff from a HV supply is not related to measuring the voltage, at all.

    As W1VT pointed out, any DC voltmeter can become a high voltage meter with enough resistance in series with it. Just be careful -- a lot. It's best to connect the test instrument(s) to a HV source while that is still shut OFF, then turn on the source while you, your fingers and all body parts are nowhere near the high voltage.
  6. K8ERV

    K8ERV Ham Member QRZ Page

    Also be aware that some resistors-mostly carbon I think- change their value with high voltages. Don't know why.

    TOM K8ERV Montrose Colo
  7. AG3Y

    AG3Y Guest

    Use a high voltage probe that is designed for your meter. Nothing else. I cannot believe that you guys are advocating a string of resistors. What if the guy touched a bare wire lead on one of the resistors closer to the plate cap?

    Once you are dead, there is no turning back !
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    A HV probe usually is just a string of resistors (or possibly one very long HV resistor) with a plastic cover over it.:) I've built HV probes before. Mount all the resistors on a PCB then coat that with a conformal coating, then a layer of epoxy, and stick it in a PVC tube with end caps. It works, although simply buying one is probably cheaper.:p
  9. AI3V

    AI3V Ham Member QRZ Page

    There are similar divider probes from many manufacturers.

    Typically they have a long glass/metal film precision HV resistor inside.

    A resistor, as in one :).

    Each manufacturer picks a resistance value so that one of the existing meter scales are accurate.

    I have a Triplett probe here (paid a whopping $3), I built a simple meter that consists of a microammeter and 3 switchable range resistors, gives me a choice of 15, 30, and 60KV fullscale readings. I built in when I was repairing CRT TV's and needed to be able to measure the 2nd anode voltage.

    A minutes work with Mr Ohm's law and you can work up a calibration chart to use just about any probe with any meter.

    Nice thing about the commercial units is the fancy insulated handle with leakage rings- reminds you to be careful.

    Electrostatic air cleaners like the ones installed in many furnace/air conditioning units have similar resistors that can be salvaged and made into HV probes.

  10. K8ERV

    K8ERV Ham Member QRZ Page

    He would become a fast learner?

    TOM K8ERV Montrose Colo
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