How to determine low pF capacitor polarity?

Discussion in 'Test & Measurement Forum' started by Jm_electrons, Nov 25, 2018.

  1. Jm_electrons

    Thread Starter New Member

    Nov 25, 2018
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    I have an 8pF capacitor charged to 8V. How can i measure which end is positively charged assuming i have a small amount of cash and a local electronics store.
    This seems tricky since it has only a tiny amount of charge.


    Could i for example:
    - see a brief flash from a connected Led?
    - see a momentary needle movement on an analogue ammeter when shorted?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You should see it on a analogue meter, you don't have a electronic version?
    Max.
     
  3. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Unlikely to both questions. There's not much stored energy.

    My RCA analog meter has a resistance of 20k ohms/volt. That would give an RC time constant of about 1us. I'd be surprised if the needle moved at all.

    It's unlikely that you could measure it with a DVM. With an input impedance of 1G ohm, the time constant would be 8ms. The meter wouldn't have a chance to settle before the cap was completely discharged.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    A small, high brightness LED may give a brief pulse visible in a dark room, but I wouldn't bet on it.
    Otherwise, I can't think of anyway to detect that except using an oscilloscope with a 10MΩ, low-capacitance probe, which should give a couple volt pulse with an 80μs time-constant.
     
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  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    For measuring on a very small capacitor, you will need a meter with a VERY high input impedance.
    I have attached a circuit copied from an old magazine about an 100000 megohm meter.

    Bertus
     
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  6. Jm_electrons

    Thread Starter New Member

    Nov 25, 2018
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    Cheers, yeah not enouh charge or energy. Need a big impedance. Looks like an op amp system is the go. Since i need a portable system i'm ganna go for a battery run instrumentation amplifier.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The non-inverting (+) input of a FET op amp may have a high enough impedance to detect the voltage.
    But the input stray capacitances could be a problem.
    You would likely need to mount the op amp on a probe with a short wire from the op amp input to the UUT.
     
  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    One of the side issues here is you have to state how you go from not charged to charged to measurement.
    You must at some point start with no charge, then charge it, then make the measurement. Between the charge and measurement you have to state what happens because that could very well have a large influence on the measurement which in some cases wont be possible because the cap self discharge will eat up all the energy before you even get to make the measurement. It would have to be low leakage or else you would have to make the measurement very very quickly right after the charge.
    Of course a question also is how is it charged. You may be able ot tell what you want to know from more info on that process.

    Since the value is so small we should hope the charge voltage is very large. That gives us more hope in finding an answer.
    However, if the charge is small, then you have to find a more sensitive measuring method.

    You may be able to use an op amp but another interesting idea is to use a live fish. A fish has the ability to detect the presence of an electric field and for example a shark can detect something very low like 6nv/cm.
    You'd have to find a fish that can detect that wont bite you :) and also do some experiments to see how they react to small fields. Would be interesting.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Why do you want to do this?
    Do you have any idea how small is 8pF?
     
  10. nsaspook

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    Aug 27, 2009
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  11. wayneh

    Expert

    Sep 9, 2010
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    Would a charged capacitor potentially respond to an applied magnetic field, and move like a compass needle?
     
  12. Jm_electrons

    Thread Starter New Member

    Nov 25, 2018
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    Thanks guys, looks like i'll be trying a premium high impedance op amp circuit, although i do like the idea of utilising a fish. Yes i know the capacitance / charge is extremely low, it's part of a science experiment that i described in the general topics forum section. Basically i'm making a custom ultra low leakage plate capacitor and charging it with the earths electric field.
     
  13. Jm_electrons

    Thread Starter New Member

    Nov 25, 2018
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    I'm learning a lot of interesting stuff! Nearly ready to go and order some parts (i don't own any electronics).
    Here's a diagram of my measurement, the experiment is slightly changed by still measuring an 8pF metal-ground plate capacitor with a high impedence amplifier.

    Does anyone see any obvious stupid mistakes?
     
  14. danadak

    Distinguished Member

    Mar 10, 2018
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  15. Jm_electrons

    Thread Starter New Member

    Nov 25, 2018
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    Wow! You just gave me an easy to understand explaination of measuring fA currents, that's 1000x more sensitive than I need. I had some similar teflon ideas, this saves me some trouble of generating accidental static charge. The cosmic ray limit for an air gap capacitor was interesting. Electronics just became more fun. Thanks Dana
     
  16. MrAl

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    Jun 17, 2014
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