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  • October 26, 2018 at 8:32 pm #6259
    R_A_Ghosh
    Participant

    I am using anode signal model without the compensation capacitor, so after 3mR /he I am getting noise in GM reading and false high counts, why is it happening with lnd7807 tube? .as a solution I may need compensation capacitor…Is this how shall I connect compensation capacitor to avoid noise (find attached)?
    I am using 3.3M R1 as anode resistor, 330k as divider R3
    I am not using cathode signal model as your kit….

    Attachments:

    1. Screenshot_20181027-015813.jpg

    October 27, 2018 at 9:26 am #6262
    R_A_Ghosh
    Participant

    Though my HVDC power supply is clean and well filtered, I did not observe any false count on my monitor. I have to impose the system on high radiation and see if it still works or gets GM tube noise again.
    My problem was that I had simple wiring earlier and everything was fine until 3mR/hr or 450 counts/s, over that it worked well for few minutes and then suddenly a high fixed false pulse count of 4000/s started to come. It fell back to normal when radiation was reduced. I have proper data upto 3mR/hr. I want to measure upto 10mR/hr but the GM tube noise is causing issues. The noise is not from HVDC power supply for sure.

    October 27, 2018 at 10:12 am #6264
    R_A_Ghosh
    Participant

    The power supply I am using has capacity of 500V, 1mA. I am using 100X probe to measure HVDC. As per LND7807 datasheet and some emails sent to me by LND support for 10mR/hr it consumes no more than 10uA current…attaching graph here sent to me by LND support.
    I have a switching frequency of 9KHz of the DC to HVDC supply. The link to make one is here HVDC Power supply

    Attachments:
    1. 7807_currentCurve.pdf
    October 27, 2018 at 10:20 am #6266
    R_A_Ghosh
    Participant

    One more thing is that I have the both biasing resistors on the PCB board and the gm tube is then connected by a foot long wire and kept far. If as per suggested I can take the anode resistor to GM tube assembly close to the GM tube anode and from return wire of cathode i take cathode wiring connection as recommended by you and hence using a compensating capacitor on board, then can the problem be mitigated ?
    I have R anode=3.3M
    R drop at cathode =330K
    Capacitance of tube = 8pF
    then formula 3.3Mx8pF=Ccx330K
    Cc =80pF
    So if i do such that I change the wiring to cathode signal type and use 80pF compensation capacitor.

    December 3, 2018 at 6:51 pm #6302
    R_A_Ghosh
    Participant

    So I finally did that. Used cathode wiring and put 82pF compensation capacitor across R2 and it worked like magic. Got good calibration and now basic instrument working. Rest will be done in due time.

    December 4, 2018 at 10:12 pm #6303
    Radu
    Keymaster

    @r_a_ghosh, would you post a few pics too?

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