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Measuring the high voltage plate Voltage

Discussion in 'Discussions, Opinions & Editorials' started by KA5ROW, Dec 9, 2010.

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  1. KA5ROW

    KA5ROW Ham Member 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 Ham Member QRZ Page

    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
  11. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    Words to live by.
  12. K5FH

    K5FH Ham Member QRZ Page

    I use an 80K-6 with both analog and digital multimeters; works great.

    I thought about making my own but just didn't have the time or inclination to scrounge up all the parts :mad: The Fluke is a decent quality instrument and should be more than adequate for your needs. Just remember that the scale factor is 100:1.
  13. WA7KKP

    WA7KKP Ham Member QRZ Page

    use a Simpson 260 series 5 or older

    Back in the olden days, Simpson 260's had the 5kv AC and DC multipliers built in . . . until OSHA determined that anything over 600v was very dangerous to one's health.

    If you can lay your hands on one, set it up to measure plate voltage at the bottom (cold) end of the rf choke -- that's that big coil thingie sticking up between the tubes. Or anywhere between it and the DC output of the power supply. Make sure all plugs in the meter are secure, and keep it well away from the chassis and yourself.

    Read the meter, and you'll have your plate voltage.

    Gary WA7KKP
  14. K2WH

    K2WH Premium Subscriber QRZ Page

    Had to measure 50kvdc once. Bought (50) 1 megohm resistors (I think) put them in series and then put the resitors and board in transformer oil. Worked like a charm. I forget the meter movement value.

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