Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

High voltage not reaching Detector

  1. Sep 28, 2014 #1
    Hi, I'm working on fixing a preamplifier connected to a semiconductor detector. The high voltage bias supply is working ok, and the preamplifier preamplifies. I've confirmed this with a digital oscilloscope and a square wave generator under many different regimes. The preamplifier components are are all in place, and working, as shown here
    http://muralev.narod.ru/archiv/docs/Dosimetry/PUG_schematics.JPG
    X2 is the High voltage bias supply and X1 is the exit to the detector.

    The proper functioning of the detector requires that " an applied voltage of typically hundreds or thousands of volts must be imposed across the active volume" (Knoll, Radiation detection and measurement).
    All I can see is a flat line with very little noise, and the "behaviour" of the electronics, is... calm. I even attached the crocodile clips from the square wave generator to the bare wire at the input end of the preamplifier (after the input condensator of course), to see that it amplified in actual operating conditions, and it worked!

    All I know is that the high voltage doesn't actually reach the detector, because I've measured it at several points. What I could not measure is the resistance of the resistors, since they are just too high for my multimeter, but, from what I can infer, there are three resistors between the bias supply and the detector : 100M 220M and 2x1.0G+-20%
    Now, the voltages I measured went like this:
    -1004V
    100M
    -143V
    220M
    -35.5V
    2.0G
    -5.204V

    In short: the voltage actually reaching the GeLi detector is -5.2 Volts, and NOT -1000 Volts.
    I am puzzled: what should I do?

    I am afraid to damage the electronics, and there MUST be a reason why these very large resistors are there. My understanding is, they're there to limit the current, and they also function as filters, together with the condensators, but still: why is the voltage getting to the detector is soo low? What am I not getting here?

    Additional links

    http://muralev.narod.ru/archiv/docs/Dosimetry/PUG_raspolojenie.JPG
    http://muralev.narod.ru/archiv/docs/Dosimetry/PUG.pdf (russian)
     
  2. jcsd
  3. Sep 28, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    This is inconsistent - the voltage drop at the smaller resistor should be smaller. One of the resistors or filter capacitors (C3, C") could be broken.

    I don't see the GOhm resistors in your sketch, but the same issue appears there.
     
  4. Sep 28, 2014 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    with what are you measuring that voltage ?
    Remember Heisenberg's Principle of Indeterminacy, anything you measure you perturb.

    Most DMM's have input resistance of 10 megohms
    so the act of connecting one downstream of a high megohm resistor makes a voltage divider
    with the meter as the bottom resistor

    A 10 meg meter with one gig upstream will report low by about 100X.
    Attenuation of a voltage divider = Rbottom/(Rbottom+Rtop)
    10meg/1010meg = .0099

    You should be able from your readings and Ohm's Law to figure out what is the input resistance of your meter.
    Looks to me like it's around 15 12 megohms.
     
    Last edited: Sep 28, 2014
  5. Sep 28, 2014 #4
    I thought You were supposed to measure the voltage between a given point in the circuit and the ground, which is what I did. But, alas, it turns out measuring high voltages in circuits with gigantic ohmages is trickier than I thought. Well, what do you know.
    I think I'll try to measure voltage drops as well, that is: using the meter parallel to the resistors, maybe this way I can get something useful out of it.
     
  6. Sep 28, 2014 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    In the days before digital meters it became second nature. So i have a fifty+ year head start on you there.

    Analog voltmeter is a current measuring device with series resistance to give full deflection at full scale voltage. In vacuum tube days a good voltmeter drew fifty microamps full scale so we had to be always aware of 'thevenin equivalent' of the circuit we were studying. It was just something we accustomed ourselves to keep not too far below the surface in our thinking. We'd sometimes check a high ohm resistor by reading the microamp scale and calculating expected current through it.
    Tube circuits use generally higher voltages and resistances than transistors, So as tubes slipped into the past, so did that habit.

    So - think of your DMM as a 10 meg resistor and and if the voltage you measure is about right for the calculated voltage division ratio you probably have a satisfactory component.
    When you get real brave you can select microamps and do the same. A kilovolt will drive a microamp through a gig-ohm. Resistors usually fail open, but first verify by voltmeter it's not shorted.

    We had a lot of trouble on one particular model of circuit board. A high voltage test point was located adjacent a metal can transistor with collector tied internally to the can. More than once a small slip of the probe discharged the HV filter capacitor through that unfortunate transistor....

    Ttrobleshooting is fun, isn't it ?

    thanks for the reply.

    old jim
     
    Last edited: Sep 28, 2014
  7. Sep 28, 2014 #6
    I had the (mis)fortune of not once, but many times discharging my high voltage bias supply myself, while I was trying to measure it, and it seems like I'm not there yet.
    I think I made a huge mistake. The bias supply, which is attached to the motherboard has a 36M resistor. My voltmeter was showing me that I was getting too low of a voltage at the exit, so I replaced it with a 5.1M. Now I saw exactly what the propriatery software was telling me the voltage should be, so I carried on with my investigation. How stupid of me.
    Also, I vaguely recall being thought in the school at physics lab the something like this: Your voltmeter will sometimes lie to you. If you want to know the voltage, measure the current and then calculate the voltages, given the size of the resistors.
     
    Last edited: Sep 28, 2014
  8. Sep 28, 2014 #7
     
  9. Sep 28, 2014 #8
    What are you using to measure the high voltage? You need a probe/meter with extremely high impedance, or you'll load down the high voltage to a very low value. Are you using a genuine high voltage probe?

    Eric
     
  10. Sep 28, 2014 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    [/PLAIN] [Broken][/PLAIN] [Broken]

    i guess we can't ascribe intent to a machine...
    but
    It'll sure fool you.
    One needs to be somewhat aware of his test equipment's inner workings so he knows when he's approaching its limits.
    I once made an awful mistake by not recognizing an autoranging DMM had switched scales from millivolts to volts .
    Too much automation . I did my best work with a Simpson 260 and analog 'scope.

    But i'm a dinosaur.

    old jim
    [/PLAIN] [Broken][/PLAIN] [Broken]
     
    Last edited by a moderator: May 7, 2017
  11. Sep 28, 2014 #10
    Nothing wrong with dinosaur technology at all, Mr. Flintstone! But I you need a VTVM, not a Simpson!
     
  12. Sep 28, 2014 #11
    The general point is this: If you want to measure voltage, then the resistance of the voltmeter should be at least an order of magnitude larger than the resistance of the circuit. I happen to try to test a circuit of Gigaohm resistors armed with nothing more than a general purpose multimeter.
     
  13. Sep 28, 2014 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Oh well, if jety89 does not have the proper equipment to measure voltages in the circuit, then there are still backup plans. Apply a known voltage to a resistor, measure current to see if the resistor has its nominal resistance. If all resistances are correct, measure current in your final circuit, calculate the voltages.
     
  14. Sep 28, 2014 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    lol i know firsthand, that hurts !

    You've got it now. And when that order of magnitude just isn't there, you know how to work around it.
    We learn by doing.

    If you have two DMM's, use one set for Ohms and measure the resistance looking into the other set for DC Volts.
    Then set one for milliamps and see how much current the other drives through an unknown when it measures ohms. Try various ohm scales.
    And find what voltage it applies to an open circuit(well, to the substantial input resistance of the other DMM)
    Good little details about your equipment to know.
     
  15. Sep 28, 2014 #14
    I think it is settled then. First thing in the morning. It is now 00:13 where I live. Thanks a lot!
     
  16. Oct 1, 2014 #15
    It took me two month of painstakingly troubleshooting every detail, but, Today it finally worked. My detector complex is up and running, and, from what I saw, it is even better than previously, yippee!
     
  17. Oct 1, 2014 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Congratulations ! Glad to hear of your success !
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: High voltage not reaching Detector
Loading...