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Best way to measure capacitance/inductance?

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cduma

Wed Oct 14 2009, 02:37PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

What is the best way to measure inductance/capacitance without buying an expensive meter? My thought was to create an RC oscilator where R is known and measure the frequency with an 0-scope to measure capacitance. To measure inductance my idea was to create an LC oscilator where C is known and measure the frequency with an O-scope. Any better ideas?

Meatball

Wed Oct 14 2009, 03:50PM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
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O-scopes are going to cost just as much, if not more than a fancy meter.

For someone without an O-scope, a fancy meter might look like a more practical solution.

Other than than, if you already have a scope, your idea should work pretty well in my opinion.

Proud Mary

Wed Oct 14 2009, 04:14PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

cduma wrote ...

What is the best way to measure inductance/capacitance without buying an expensive meter? My thought was to create an RC oscilator where R is known and measure the frequency with an 0-scope to measure capacitance. To measure inductance my idea was to create an LC oscilator where C is known and measure the frequency with an O-scope. Any better ideas?

The Wheatstone Bridge is the classic LCR measurement device, and will be as accurate as the small number of components needed to make it. Where C and L are to be measured, headphones are used to listen for a null in the driving tone, which indicates that the bridge is in balance. Elementary maths must then be used to calculate Rx/Cx/Lx Everyone ought to know how to make and use this simple but versatile circuit, so you can Google the rest for yourself now!

Mads Barnkob

Wed Oct 14 2009, 04:27PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I wont call this expensive, I got one of these and its just as precise as my Fluke/Elfa meter.

Proud Mary

Wed Oct 14 2009, 05:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Posts: 4992

Mads Barnkob wrote ...

I wont call this expensive, I got one of these and its just as precise as my Fluke/Elfa meter.

You wouldn't be able to make much of a Wheatstone Bridge - not a good one with 0.05% resistors and a centre reading galavanometer - for the price of the one Mads has suggested. Just the mahogany baseboard of a righteous bridge would cost more!

uzzors2k

Wed Oct 14 2009, 09:18PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

If you're into microcontrollers making an automatic L/C meter is fun. I just finished mine a few weeks ago.

klugesmith

Thu Oct 15 2009, 01:46AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Mads Barnkob wrote ...

I wont call this expensive, I got one of these and its just as precise as my Fluke/Elfa meter.

Apparently its most sensitive L range is 19.99 mH, a resolution of 10 uH.

For smaller L, an oscillator with known C (like in Uzzors project or "classic PIC16F84 L/C Meter"

works fine. The circuit core with LM311, one C, unknown L, and a few resistors is easy and sufficient, if you have a way to measure frequency. Headphones & musical aptitude, or PC sound card utilities, will do in the audio frequency range.

If you have a 'scope, then square wave generator and (one resistor, or known source impedance, or known scope termination impedance) are also sufficient to determine unknown L or C value to (say) 5% accuracy. You just need to measure the half-time of an exponential decay waveform.

cduma: The key questions are
1) what range of L and C values do you want to measure;
2) what accuracy do you need, and
3) do you want immediate digital readout of the measured value? Or is it OK to get a frequency or decay time, then work the math or consult a conversion chart? [edit] A spreadsheet calculator program is handy if you have a computer or smart phone at hand.

mccoywm72

Thu Oct 15 2009, 02:29AM

Registered Member #2042 Joined: Sat Mar 21 2009, 03:44PM
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Posts: 9

This is currently the assignment in one of my labs. We have to design, then build an inductor for a boost converter. I haven't done so yet, but I plan to make an RL circuit and using a function generator and o-scope watch the input voltage vs voltage across the resistor. These values can then be compared to a numeric simulation to determine the inductance. I realize this requires two devices (a function generator and an o-scope) that you might not have, but this is just another method you could try.

Sulaiman

Thu Oct 15 2009, 06:41PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

mccoywm72, due to core and winding losses being dependant on many variables and non-linear, the best test equipment for an inductor is in it's intended circuit.
Design from manufacturers data and often calculators, make it, test it.
If it runs too hot it's no good, if it runs cool it's too expensive.

Dr. Slack

Thu Oct 15 2009, 06:59PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

the best test equipment for an inductor is in its intended circuit.

I'll second that. Losses increase strongly with frequency, so if loss is important, test at the intended frequency. Inductance varies with current, so test at the intended current. If you can't use the final circuit to test, then a very good alternative is making a resonant circuit with a suitable capacitor. Pick the L and C so they resonate at the right frequency. In a power application, it's usually possible to use the Q of the circuit to get the circulating power levels up while feeding in very little power from a generator. Usually, capacitors are so good compared to cored inductors, you can safely ascribe all of the observed losses to the inductor.

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