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Peak voltage detector schematic

Discussion in 'Electronic Design' started by Ignoramus20878, Nov 29, 2005.

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  1. I would very much like to make some sort of a "peak voltage
    detector". A device that, when plugged into a circuit, after a
    while, builds up an adequate representation of peak voltage. That
    would be again for my IGBT inverter.

    That's for peak voltages under, say, 1 kV.

    I tried making one from just a cap and diode, but multimeters would
    discharge it too quickly and getting reliable readings is difficult.

    After reading the art of electronics, something like this comes to
    mind:

    Make a small capacitor charged by diode (and a series resistor to
    limit current). After a while, voltage would build up there.

    Connect a voltage divider (say, 1 megohm and 1 kOhm in series). To the
    cap. That would be a 1:1001 divider.

    Connect that as input to some sort of FET based amplifier (that takes
    voltage and does not require input current) that produces same
    voltage, but at higher current. That would be the input ot a regular
    multimeter.

    The multimeter reading would need to be multiplied by 1000 to get the
    actual voltage.

    Alternatively to all this, does anyone know if higher end multimeters
    like Fluke 8050A (which I have) can measure voltage without
    discharging the cap so much? I have one lying around somewhere.

    i
     
  2. The Phantom

    The Phantom Guest

    Such a divider wouldn't be much of an advantage since the typical input
    resistance of a modern DVM is about 10 megohms. The DVM would be 10 times
    slower at discharging the cap than your divider.

    You could put 9 resistors of 10 megohms each in series to make a 90
    megohm resistor and then put the meter at the bottom of the string for a
    total of 100 megohms resistance across the cap. Then multiply the reading
    by 10. If you have a couple of DVMs, use a second one on the ohms range to
    measure the input resistance of the other (set to volts range) to make sure
    it's really 10 megohms. If it's not exactly (to 1% or so) 10 megohms, use
    the measured value to figure your divider ratio. You might also measure
    the 10 megohm resistors in the string to see what their total is, and use
    that value in figuring your divider ratio.

    The 100 meghohm load on a 1 uF cap should be a slow enough discharge to
    get a reasonable reading.
     
  3. John - KD5YI

    John - KD5YI Guest


    Maybe I'm wrong, but I thought most multimeters today came with 10 megs
    input impedance. That's 10 times more than you propose.

    You have an oscilloscope, as I recall. Can't you set up the triggering to
    give a trace when the voltage exceeds some value you set with the trigger
    level control?

    Good luck.

    John
     
  4. ehsjr

    ehsjr Guest

    AIRC, Art of Electronics shows a peak detector with op amps.
    Why not use what they show?

    Ed

    <snip>
     
  5. I have two handheld MMs, one is a nice but old Extech and another is a
    $3.99 harbor freight unit.

    Both discharge the caps rather quickly, I can barely get the reading.

    I also have a Fluke 8050A, which seems to have 100K resistance. (I
    downloaded its manual today).
    I suppose I can, yes. I am open to suggestions. Tek 2245A.

    i
     
  6. Sure, I guess I missed it or did not get to it. Do you know what
    chapter or page it was on?

    i
     
  7. The Phantom

    The Phantom Guest

    I explained what to do in another post:

    " Such a divider wouldn't be much of an advantage since the typical input
    resistance of a modern DVM is about 10 megohms. The DVM would be 10 times
    slower at discharging the cap than your divider.

    You could put 9 resistors of 10 megohms each in series to make a 90
    megohm resistor and then put the meter at the bottom of the string for a
    total of 100 megohms resistance across the cap. Then multiply the reading
    by 10. If you have a couple of DVMs, use a second one on the ohms range to
    measure the input resistance of the other (set to volts range) to make sure
    it's really 10 megohms. If it's not exactly (to 1% or so) 10 megohms, use
    the measured value to figure your divider ratio. You might also measure
    the 10 megohm resistors in the string to see what their total is, and use
    that value in figuring your divider ratio.

    The 100 meghohm load on a 1 uF cap should be a slow enough discharge to
    get a reasonable reading."

    If the DVM you would be using to measure the voltage at the bottom of the
    divider isn't 10 megohms, then find out what it is by measuring it with a
    second DVM, and use the actual input resistance of the DVM to figure out
    what your divider ratio is. If the actual input resistance of the DVM is
    Rdvm and if you have used 9 resistors of 10 megohms in series, then you
    would multiply your reading by (Rdvm + 90,000,000)/Rdvm.
     
  8. GregS

    GregS Guest

    Is an oscilloscope out of the question? It would be easier to record peaks.
    Most ohmeters only measure up to 10Meg, but there are some that go to 100 Meg. Perhaps their
    impedance is also higher. In any case, adding series resistance will increase input impedance.

    greg
     
  9. The Phantom

    The Phantom Guest

    It absolutely would. That's how voltmeters work; you're just increasing
    the multiplier resistance (as it's called).

    Think about it. If your DVM has 10 megohm input resistance and you are
    measuring 1 volt, the same current is seen by the meter as if you add 90
    megohms additional and then measure 10 volts. If the meter can do the
    first case, it can do the second case.
     
  10. The Phantom

    The Phantom Guest

    An easier way to calibrate would be this: Take one of your power
    supplies and set it to its highest voltage, maybe 20 or 30 or even 50
    volts. Measure it with a DVM, and write down the voltage. Put the 90
    megohms in series with the DVM and measure the voltage. Divide the reading
    without the 90 megohms by the reading with the 90 megohms. That is your
    multiplier factor and it takes into account the input resistance of the DVM
    and the actual values of the 90 megohm string.
     
  11. Thanks Phantom. I am not sure if a divider with such high resistance
    (10 MOhm) would produce currents that are high enough to allow the DMM
    to reliably measure voltage. I will try your suggestion however.

    i


    --
     
  12. ehsjr

    ehsjr Guest

    Page 217
     
  13. Jim Thompson

    Jim Thompson Guest

  14. thanks, will check tonight.

    i
    --
     
  15. Jim, sorry if I am missing the obvious, but this peak detector on page
    4 would quickly droop (discharge cap) if connected to a multimeter,
    right? I am talking about the part starting with U3.

    i
     
  16. Jim Thompson

    Jim Thompson Guest

    For just peak detector purposes, delete the rectifier sections and
    input your (attenuated) signal into the plus input of U3.

    Increase R4 to suit your required droop rate.

    Use another OpAmp section, connected for unity gain, to buffer
    "PEAK"... then your multimeter will do no loading at all.

    ...Jim Thompson
     
  17. That's exactly what I am wondering about, what opamp would be most
    suitable for this purpose (infinite input impedance, voltage driven or
    whatever it is called). To avoid any droop altogether. Something in
    DIP package or just a piece with contacts would be best.

    i
     
  18. Jim Thompson

    Jim Thompson Guest

    My favorite OpAmp (when I last did discrete designs) was the TL084
    (TI) or MC34084 (Motorola).

    ...Jim Thompson
     
  19. Yes, looks just what I need, thanks a lot!

    i
     
  20. One more question, is the output of his opamp going to be enough to
    reliably read it with a multimeter? It looks like yes, but I want to
    double check. I really want to build a nice PVD.

    i
     
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