Measuring capacitance using multimeter

[Rulycat]

I was talking to my physics teacher about how to measure capacitance today, and he suggested i charge up a capacitor with a 10 volt battery (assuming 10V or more capacitor) and discharge it through a 10kohm resistor (think it was 10kohm). Time how long it takes to reach 3.7V and use this data to calculate the capacitance.

Is this right?

[PeteS in CA]

Re: Measuring capacitance using multimeter

It can be done that general way. It takes 1 time constant to discharge a capacitor to ~37% of its initial voltage. The formula for the time constant is T = (R)(C). So you would know the "R" term, and you would be measuring the "T". From that you can calculate "C". Do not forget, however, that your meter (or oscilloscope) is part of the circuit. You have to select "R" to be low enough for the "meter's" resistance to be irrelevant, but high enough to get an observable time. Using an oscilloscope will improve your observation (and a digital meter's refresh rate will mess up your time observation), and a 10X probe is commonly 10M-Ohms (a 1X probe is just 1M-Ohms, IIRC).

[Rulycat]

Re: Measuring capacitance using multimeter

Thanks for that PeteS. I have a £15 multimeter and not the cash for an oscilloscope. I have some cheap caps (470uf, 1000uf, 4700uf) that I'd like to test to see if they're near what they say they are as they are a fly-by night brand.

[PeteS in CA]

Re: Measuring capacitance using multimeter

An analog volt meter might work almost as well as a scope, depending on how good of a measurement you want. Also, do you have access to equipment at your school?

[Rulycat]

Re: Measuring capacitance using multimeter

I only have a digital one, there is an oscilloscope at my school I could perhaps use in the Physics department, but if I can get a reading within 10% of the value I'm happy.

[Topcat]

Re: Measuring capacitance using multimeter

I check mine with a Sencore LC102. To get an accurate capacitance reading, it MUST be checked out of circuit.....anymore, with my stash of caps, if I remove it I just replace it..... I will get curious every now and then on some I pull that APPEAR ok (mainly teapo and OST), just to see how far out of tolerance they are. Most the time, they're way out of spec.

[Rulycat]

Re: Measuring capacitance using multimeter

I check mine with a Sencore LC102. To get an accurate capacitance reading, it MUST be checked out of circuit.....anymore, with my stash of caps, if I remove it I just replace it..... I will get curious every now and then on some I pull that APPEAR ok (mainly teapo and OST), just to see how far out of tolerance they are. Most the time, they're way out of spec.

Cheers for that Topcat, I just need something crude now to give me a rough idea of the capacitance. I have some uses for these off-brand caps but I need to make sure the capacitance is about right.

[Rulycat]

Re: Measuring capacitance using multimeter

Just charged up a 25V 1000uf cap and as it's connected to the multimeter the voltage gradually drops to 0 over 5 minutes or so. Is this normal, and will it affect my capacitance calculation?

Got some resistors today from Maplin as well. 0.25W, assorted sizes from 10R to 1M0.

Should I use a 10K or a 1K? Or neither?

[popeye]

Re: Measuring capacitance using multimeter

I was talking to my physics teacher about how to measure capacitance today, and he suggested i charge up a capacitor with a 10 volt battery (assuming 10V or more capacitor) and discharge it through a 10kohm resistor (think it was 10kohm). Time how long it takes to reach 3.7V and use this data to calculate the capacitance.

Is this right?

your teacher is as suggested one way to get a rough idea as a city and guilds engineer I would sagest as you seem to very keen go online to your local library and type in electronics you will find a lot of books that will take you through the basics one of those basics is about capacitors you can learn the formula's and the correct way to measure capacitance you may even find a circuit for an lcr bridge you can build that is an old way of checking caps inductors and resisters that is very accurate you may even have one in your physics department. You can find a lot of information on the web for free I know what it is like when funds are tight but if you persevere you can end up repairing items for a profit and then your hobby will self fund its self.
I will try to find some of the text books in pdf form and will attempt to upload them to badcaps for you


popeye city and guilds engineer.

[Rulycat]

Re: Measuring capacitance using multimeter

your teacher is as suggested one way to get a rough idea as a city and guilds engineer I would sagest as you seem to very keen go online to your local library and type in electronics you will find a lot of books that will take you through the basics one of those basics is about capacitors you can learn the formula's and the correct way to measure capacitance you may even find a circuit for an lcr bridge you can build that is an old way of checking caps inductors and resisters that is very accurate you may even have one in your physics department. You can find a lot of information on the web for free I know what it is like when funds are tight but if you persevere you can end up repairing items for a profit and then your hobby will self fund its self.
I will try to find some of the text books in pdf form and will attempt to upload them to badcaps for you


popeye city and guilds engineer.

Thanks popeye, my teacher has given me an old 80's book on electronics, as my school no longer offers the course so he let me have it. It has the equations for capacitance and so on and some stuff on other things like transistors and specially computers, such as 4 bit multipliers etc. But for capacitors it is too vague without mention of ESR and so on. Thanks for the offers to upload the PDFs and I will read them when I get back from work. (checkout job - not too fun!)

We've a branch library a few miles away, I can have a gander at what's on the shelves there. My objective at some point in the future is to be able to repair PSUs to a competent level as well as design my own circuits because I find often I have the parts from dead ones but not the knowledge to use them to assemble my own. It should save me money if I can repair things like this, and perhaps make some on the side as an income source when I go to university in a few years.

[PeteS in CA]

Re: Measuring capacitance using multimeter

Just charged up a 25V 1000uf cap and as it's connected to the multimeter the voltage gradually drops to 0 over 5 minutes or so. Is this normal, and will it affect my capacitance calculation?

Approximating a bit, your cap will be effectively discharged in 5 time constants. So the effective time constant you observed is 60 seconds. Using T = (R)(C), if your cap is truly 1000uF, your effective meter resistance is ~60K ohms. That seems really low, less than 3K ohms/volt if the cap was charged to 25V! With a meter resistance of 60K, a 1K resistor should be used to swamp out the meter resistance. The problem with that is that the time constant of a 1000uF cap and a 1K resistor is 1uS, which you won't be able to observe using a meter.

I'm not using my 1970s-vintage steam-powered calculator, so check my very mental Math.

A 10M-ohm 'scope probe would let you use a resistor in the 100K-1M range. And the 'scope would then introduce you to a phenomenon known as "switch bounce".

If things like meter and 'scope probe impedance seem arcane and less than practical, consider a trouble-prone circuit in the systems I work with every day. It's a ~2 minute timer that uses a .47uF cap being charged through a 60M-ohm resistor (I think those are the values). The cap is a (evil) large form-factor SMT monolythic ceramic capacitor, which are susceptible to cracking. Also, any conductive residue around either part will screw up the operation of the circuit. So, if I try to observe the charging of the cap, my 'scope probe forms a 60M-ohm -- 10M-ohm series circuit, with the 10M-ohm being across the cap. Because of my 'scope probe the 2-minute timer cannot work and I cannot tell whether the cap is leaky ... Gotcha!

[Rulycat]

Re: Measuring capacitance using multimeter

Going to Maplin again to get some more parts tomorrow, most notably a proper switch and some electrical wire (currently using wire savaged from an ATX PSU) so does anyone have advice on what other stuff to get to fill out my parts bin?

For the Americans Maplin is like the U.K's version of Radio Shack. (but better)

http://www.maplin.co.uk/

[jamesbo]

Re: Measuring capacitance using multimeter

I've been to Maplin, it's a lot better.

[lipyphil]

Re: Measuring capacitance using multimeter

Omg..I have now understood time constant reading this post. I have struggled with this concept for a while. Is it also right to say that the time constant measures time it takes to charge a capacitor to about 67% of its maximum charge? Folks am new to electronics and trying to understand capacitors,inductors and resistors.