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  1. #21
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    Re: Bridge Rectifier won't Rectify

    Quote Originally Posted by FvM View Post
    It really depends on the intended probe frequency range, which wasn't specified along this thread. The shown resistor divider primarily suffers from HV resistor parallel capacitance, which is causing large error already in the kHz frequency range. A first order compensation improves the probe function a lot, it's probably sufficient for the relative low frequency signal this thread is dealing with.

    The setup in this thread looks pretty much like "my first DIY HV probe". Don't expect same performance as with calibrated professional products or other expert designs.
    "my first DIY HV probe" - Actually it's my second, the first one was with a 10x10M resistors that didn't work quite well. Maybe I should run a fundme.com campaign and get me some prof equipment, or maybe I could pass the hat around on edaboard

    Anyway, I gave the dividers a once-over again this morning, and I targeted 33KHz so with a 100pF across the 100K and the 100M/100K divider became 1000:1, and max frequency around the 1MHZ mark.



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  2. #22
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    Re: Bridge Rectifier won't Rectify

    you need a small HV cap across each high value resistor, and a compensating C across the 100k, same RC time constant for each I think ...



  3. #23
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    Re: Bridge Rectifier won't Rectify

    Quote Originally Posted by Easy peasy View Post
    you need a small HV cap across each high value resistor, and a compensating C across the 100k, same RC time constant for each I think ...
    Well, I tried adding capacitance (ranging from 33p to 2.2n) across the 100M, and what I saw on the scope was that the full 6Vp-p from the signal generator appeared across the lower 100K resistor, nice clean traces but I thought that defeated the purpose of a voltage divider, so I tried the lower leg instead and the result can be seen in the traces shown. I guess that the 100M has quite high capacitance itself.



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  4. #24
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    Re: Bridge Rectifier won't Rectify

    across EACH HV resistor - not across the outside of all of them ...

    - - - Updated - - -

    essentially you need to build a capacitive divider that matches the resistive divider, so smaller caps across the large value resistors and larger caps across the low value bottom rung resistor - as I said above get the same RC time constant in each case - beware inductive resistors if wanting to see high frequencies



  5. #25
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    Re: Bridge Rectifier won't Rectify

    Quote Originally Posted by Easy peasy View Post
    across EACH HV resistor - not across the outside of all of them ...

    - - - Updated - - -

    essentially you need to build a capacitive divider that matches the resistive divider, so smaller caps across the large value resistors and larger caps across the low value bottom rung resistor - as I said above get the same RC time constant in each case - beware inductive resistors if wanting to see high frequencies
    So I made a 100:1 with 100M+100n/1M+1n and it also works. I was actually using 100:1 until this voltage rise thing threw me off in the wrong direction. The top resistor is made of glass and the bottom is metal film, I would think they are ok as they are not wire wound.



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  6. #26
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    Re: Bridge Rectifier won't Rectify

    To get the same RC time constant you put the smaller cap across the larger resistor ...



  7. #27
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    Re: Bridge Rectifier won't Rectify

    Quote Originally Posted by Easy peasy View Post
    To get the same RC time constant you put the smaller cap across the larger resistor ...
    Yes that's what I did, but I wrote the opposite., sorry. And now I changed caps because the 1n blew up it was only 3KV. So I found some old USSR caps of 22p 12KV put those across the 100M, and a 2.2nF across the 1M.



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