Measuring capacitance with multimeters

Discussion in 'Supporting Physics' started by NorwichJames, Nov 18, 2016.

  1. For an A-level physics practical we wanted students to be able to measure the capacitance of capacitors using a multimeter. The cheaper multimeters don't have a capacitance setting, but we do have some, of two different makes, that do. However, when we went to trial the practical we noticed that the capacitance setting has two thin 'letter-box' type slots near the settings. There are some similar but smaller slots used for temperature, and the multimeters came with a probe for that, but nothing for the capacitance slots. I've had a look through some catalogues but can't see any capacitance probes sold separately either.

    Does anyone else use the capacitance setting on multimeters, and if so did an attachable probe come with them?

    Thanks,
    James
     
  2. I have been looking into this also, though I dont have any 'capacitance meters' to start with and didnt know such a thing existed and nor did any of the teachers. From the research i have been doing into capacitance meters I have learnt of the setting on multi meters. I think that the two slots you mentioned are there to poke the two ends of the capacitor into.
     
    NorwichJames likes this.
  3. Thanks for that, and poking the capacitor ends into the slots does seem to work, so that's that mystery of the non-existent leads solved. Unfortunately the practical we wanted the meters for was the type where students make a model capacitor from foil plates that are clamped and moved closer rather than electrolytic capacitors, but that gives me something to work with

     
  4. Thanks for the link. The meters we've got differ a bit from the one in that article, notably with these thin slots for the capacitance range. I did try the normal probes when I first looked at them and couldn't get any readings, but will check again though.

     
  5. I made our own connectors from bits of copper sheet, but the multimeters have a very low range of values, more useful for electronics type capacitors than the ones usually used in physics. I had to find some tiny capacitors.
     
    NorwichJames likes this.
  6. I might end up doing something with similar with copper. Yes I'd noticed that the upper limit was rather low, so I suspect they will only end up being used for this one lesson, which is a shame.

     
  7. Out of interest does anyone use or have experience with using something that is just a 'capacitance meter'? and if so what did/have you used?
    I might need to source some at some point but have no experience with them. i have checked the multi meters I have and they don't have a capacitance setting.
     
  8. We have a coulomb meter, based on the easyread meter case. You can still buy them.

    We used to need to measure capacitance in the old A level but not in the new one. When I was at school we used a capacitance bridge.
     
  9. Former_user

    Former_user Guest

    We luckliy have Capacitance substitution blocks and multimeters with capacitance reading

    However, you can measure (roughly) capacitance using a multimeter (without capacitance capabilities), using below method, but it is long winded and takes a little more time:

    (got this from an old electronics book which I have a pdf version of)

    • connect a resistor with known value( in Mega ohm), a digital multimeter and the capacitor which is going to be measured in parallel.
    • Before you connect the capacitor with the resistor and the multimeter in parallel, charge the capacitor with a known constant voltage source(battery or a good power pack). After the capacitor is fully charged, prepare a stopwatch and let the stopwatch start counting as soon as you connected the capacitor in parallel.
    • Set a voltage reference in such a way that you stop the stopwatch counting after you observed the value of the voltage reference displayed by the multimeter , ie 100mV. Record the time taken. Record the data as follows:

    Initial Voltage, Vo = (voltage of power source, in Volt)
    Resistance, R = (Resistance of the resistor used, in ohm)
    Final voltage, V = (voltage reference, in Volt)
    Time taken, t = (The time when the stopwatch stop counting, in seconds)

    Then use the formula V = Vo x e^(-t/CR)
    ln(V) = ln(Vo)-t/CR
    ln(V/Vo) = -t/CR
    ln(Vo/V) = t/CR

    Finally,
    C = t/[ln(Vo/V)R], in Farad

    Since the capacitor and multimeter have internal resistance, the measured value will be slightly differ from the actual precise value, but good enough as an estimate
     
    NorwichJames likes this.
  10. Hi...you can measure larger value caps with a 12.6V or 6.3V filament transformer, an AC voltmeter, and a 50k potentiometer. Put the R and C in series with the secondary and apply mains voltage to the transformer primary. Adjust the pot until the voltage drops across the R and C are equal. Disconnect and measure the resistance of the pot. Then C = 0.159/f*R, where C is the capacitance, f is the line frequency, and R is the measured resistance of the pot. A capacitance meter is much easier, especially for smaller value caps. Every multimeter will measure resistance.
    Tolerance isn't really measured because it is a range. It is the amount that the component will vary from the specified value, ie a 5% tolerance 100 ohm resistor should measure between 95 ohms and 105 ohms.
     
  11. what is a filament transformer?
     
  12. The youth of today! These were used to power the heaters of thermionic valves.
     
  13. I'm 62

    We had transistors when I was at school.

    We were never allowed to have mains transformers in component form to play with, and we did all our electrics experiments with glass cased accumulators
     
    Andrew Goloskof likes this.
  14. Back in the 1960s at school I was making crystal sets and selling them to make pocket money. The "de luxe" versions had simple amplifiers using Germanium transistors (OC71 & OC72).
    I remember being asked to wire a 12 way rotary switch in order to rapidly switch 12 mains incadescent light bulbs to simulate lightning for a school play... Stage light dimming was lowering zinc plates into an aquarioum containing brine, all at mains potential, not very linear but with practice it worked! Somehow we survied, now at 67 still loving being a physics technician. I'll never forget being told off at a religious studies lesson for connecting a fellow pupil to a home made induction spark coil. Now working with low safe voltages and adhering to CLEAPSS rules and regulations.

    Cheapish multimeters which measure capacitance to low levels are quite good, we have several, also four dedicated capacitance meters from Amazon, not at all expensive.
     
  15. We have a few larger multimeters that do very low capacitance measurements, most have lost the special adaptors so I made more. We have a few more autoranging ones that also do smaller ranges but a bit more useful, they just use normal leads. Then we have a few more that will do quite useful ranges, but are pretty useless for anything else. Now I have enough I keep a dedicated set of capacitance multi (and not so multi) meters away from fiddlers and just for 6th form.
     
    Andrew Goloskof likes this.
  16. About the same here Nick. I have a couple of expensive ones, a few dedicated capacitance only meters from Amazon and the other meters I inherited. We have overcome the problem now of measuring down to a few pico farads and up to a lot of µF. Strictly 6th form only!