Measuring 25kV

Discussion in 'The Projects Forum' started by PulseLED, Feb 5, 2014.

  1. PulseLED

    Thread Starter New Member

    Jan 24, 2014
    cant post in general this is a test

    EDIT: sorry I thought I'd be able to delete, only off topic is working. its "500 internal server error" I cannot search the forum either for same reason.
    EDIT 2: I can't even make another post here now so this is the only one I have to work with!

    I'm trying to construct a 1/1000 voltage divider from a 25kv capacitor to a 0-100v panel meter. The below image shows what I have and the proposed dividers

    What I don't understand is how the resistance of the panel meter is significant? I see people constructing the divider on the left and testing it by measuring across the second resistor, them I see circuits where the meter resistance is used in place of the second resistor.

    Which one is correct? the divider on the left is easy for me to do with what I have on hand using 13.5W of resistors, for the one on the right I will have to buy.
    Last edited: Feb 5, 2014
  2. Georacer


    Nov 25, 2009
    Hello PulseLED,

    First, please read this thread which might explain your trouble posting.

    Secondly, 25kV is not a voltage level to be trifled with. Depending on how this thread evolves, it is very likely that it will be closed for safety reasons.
    inwo likes this.
  3. PulseLED

    Thread Starter New Member

    Jan 24, 2014
    Thankyou for moving the post.

    After looking into the divider problem, the error of just using 375MΩ in series with the meter should be insignificant when accounting for the inherent error of the resistors used anyway, and only metering 2 sig figures.

    I just ordered 3x 100MΩ and 3x 25MΩ 5% error resistors 60W overkill in total and will go with them.
  4. crutschow


    Mar 14, 2008
    What is the voltage rating on the resistors?
  5. PulseLED

    Thread Starter New Member

    Jan 24, 2014
  6. inwo

    Well-Known Member

    Nov 7, 2013
    The NST is safeley limited to a few mA but, once rectified and capacitor is added, 25KV is lethal.
  7. Georacer


    Nov 25, 2009


    Jan 20, 2014
    For each resistor in series, the voltage rating i derated.

    If one build divider with let's say, 100 10M resistors, each resistor only need to cope with 250V.
    Almost any resistor can handle that.

    The drawing You have attached tells me that there is only 10mA total current available.

    First: In no way lethal.
    Second: VERY difficult to meassure with any accuracy with any analogue panelmeter directly.

    You need to have an electronic meter with very high impedance to make as little load to the capasitor cirquit as possible.
    Maximum current You should load it with would be in the region of 100uA
    100V at 100uA gives an impedance of at least 1MOhm.
    25KV would then require 250MOhm total series resistance.
    Total wattage is only 2,5W

    Even better is to have 2,5GOhm total resistance and a normal DVM should be good to meassure the voltage in speak.
    But then the meters impedanse have to be in series with the 2,5GOhm resistance up to the 25KV.

    I would go for 20 pcs 10MOHm, + 10 pcs 4M7 and then 3 pcs 1MOhm to ground.
    Put a digital panelmeter in paralell to one of the 1M resistors and You have å good meassurement of that 25KV.

    Larger resistance values often are too unstable, unless You are willing to pay the price for good instrumentation resistors.
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
    You will have a very, very low impedence voltage source for your meter to measure. Your meter is like a resistor in parallel with your circuit. You will have to account for that in any measurements you make.
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
    check out the hvm-5 high voltage probe with meter fron EICO, there should be a schematic on the web somewhere, with a parts list. basicly a voltage devider with a low current meter acorss the bottom resistor. heathkit and others used to make these meters for measuring the high voltage on tv picture tubes.
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Agree with last post.
    In CRT days I used a HV probe to measure the anode voltage which varies with 10KV to 30KV
  12. MaxHeadRoom


    Jul 18, 2013
    Since the demise of CRT TV's you may find a tech willing to get rid of his.
    Looks like ebay 221311861970.
  13. bertus


    Apr 5, 2008
  14. loosewire

    AAC Fanatic!

    Apr 25, 2008
    I have two high voltage probes ,a 25 kv and a 30 kv Not a lot use for them ,give me

    a test point on something new.What are you trying to measure.
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    I used a x1000 Tektronix's probe to measure the B+ ... could be anywhere from 15.5kV to 21kV, depending on the equipment model.

  16. tindel

    Well-Known Member

    Sep 16, 2012

    Did those things even work? They claim a bandwidth of 100MHz, but their impedance drops 20dB per decade over 1kHz (Sheet 1-26 of the following pdf). As I see it - the usable bandwidth is 1kHz if you don't need to consider the phase shift with another signal. I guess maybe you could design an amplifier to produce the correct gain to offset the effects of the probe to really get 100Mhz linear bandwidth.

    I have an oscope (Tek 564) that is having problems on it's high voltage circuitry that I'd love to see what's going on... but alas - I don't have a appropriate probe. I too have thought about building one, but I'd only make mine to withstand about 10kV. Of course, bandwidth would be questionable with a homemade probe too. I've also thought that a source of error would be the PWB that the resistors are mounted on... as it would be high impedance too... but I'm not sure how high.

    It seems that to really build one of these things right you'd need to build a some test circuitry to verify your bandwidth and attenuation at a minimum.

    Finally when you do hook up a homemade HV probe... how are you going to be 100% sure that the plastic you've encased your probe in will not leak at high voltage and still kill you? I think you'd have to megger it - more equipment to build or buy.

    Here's a 100M resistor which will give ~40dB - DC attenuation directly into a 1Meg input of a scope... if you dare.

    Might as well just buy one of those off of ebay for $300 - $500... out of the range for most home hobbyist for a seldom used toy.

    Sorry about my ramblings.
    Last edited: Feb 8, 2014
  17. RichardO

    Well-Known Member

    May 4, 2013
    I think you missed a tricky part of the specification:
    "Bandwidth (--3 dB) Test conditions: Test oscilloscope bandwidth must be
    ≥100 MHz, Zsource = 25 Ω"

    Note the specified source impedance is 25 ohms! Yeah, right -- I can't even imagine any 40 KV signals likely to have anywhere near that kind drive at 100 MHZ. OK, maybe the output stage of a commercial FM radio station...
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    They worked, but I always connected that probe to the end of a long broom stick handle, and still wore gloves. The ground wire (that four - six inch visible wire) was extended with a longer wire.

    When working with higher voltages, my only instructions to people are ... the ass you save maybe your own. So, your knowledge needs to be more than casual.

    Why not use a voltage divider and a normal x10 probe to do your measurements?
  19. tindel

    Well-Known Member

    Sep 16, 2012
    Good catch on the spec Richard O. I missed that. But what does it mean?!?! This isn't an active probe - is it? It doesn't seem like it is from the description. It doesn't say it is either.

    Joe that's a pretty good idea rather than go straight to a scope.

    Another thing I've thought about: 10kV/100Mohm = 1mA... at those kind of voltages, won't that load the supply of a CRT pretty good changing what I see on the scope, possibly? What is the cathode current into a typical CRT? I just don't know.

    I hope I'm not hijacking this thread too much since it's mostly on topic.
  20. strantor

    AAC Fanatic!

    Oct 3, 2010