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Building a homemade LCR meter

Bishnu Bhatta

May 28, 2016
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I'd be very happy if people in here helps in explaining me about the principles underlying in measuring the Inductance, capacitance and resistances of the respective LCR electrical elements as well as guide me through the process to build them :)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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You could start off reading this.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Welcome to Electronics Point!

Before we get into a long and involved discussion, please tell us about your electronics background and experience. Why do you want to build instead of buy?

If you want to be an electronics hobbyist, as most of us who respond to questions here in the Forum are, you need certain tools to "play" with. Among the most important of these is a decent digital multi-meter with at least three and a half digits resolution. Many of these are available with a capacitance measuring function, and all of them measure resistance. Finding an accurate inductance-measuring instrument is fairly easy and many of these use the same circuitry to also measure capacitance.

I was recently gifted with a very nice combination inductance and capacitance measuring instrument that is commonly available for purchase on the Internet as a kit or fully assembled. Try this Google results page for links to sources. And here is a review of a typical DIY kit. When shopping around for LCR meters, know what range of inductance, capacitance, and resistance you want to measure and compare that against the specifications of whatever meter or DIY circuit you are considering buying or building.

The measuring procedure for resistance is quite simple: a constant-current source is applied across the unknown resistor and the resulting voltage across the resistor is linearly proportional to its resistance. Resistances from nearly zero to several meg-ohms can be measured with virtually any digital multi-meter.

The measuring procedure for reactive L and C generally uses an alternating current to measure the reactance of the component at a specific frequency. The reactance measurement is then converted (by a microprocessor) into inductance or capacitance. In multi-meters that provide a capacitance measuring function, a constant current is often applied to the unknown capacitor (as in resistance measurements) and the time required to charge the capacitor between two voltages is measured. This time is proportional to the capacitance.

The instrument that Steve linked to in post #2 is the most basic LCR measurement instrument. It uses a bridge circuit to compare an unknown LCR component value against a known precision LCR standard. Nice to have one of these around if you can afford it, especially if you are interested in metrology.
 

Bishnu Bhatta

May 28, 2016
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I'm the undergrad student currently studying Electrical Engineering degree. I've been given or assigned this project to complete. I couldn't find the solution anywhere so thought to post it here. Thank you HEVANS for ur kind reply. After searching ur given links and exploring more on internet I found this circuit. Could you please help me out through this, how the circuit works and how circuit measures LC. That would be so kind of you.
 

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Colin Mitchell

Aug 31, 2014
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That's an old design. You can get a much-simpler design.
And you can buy the whole thing on eBay for $12.00 incl a transistor tester with a screen to show you the results. And it tests resisatators, SCRs and your love potent too.
 

hevans1944

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Jun 21, 2012
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The explanation of "how this circuit works" can be found here. It is basically an LC oscillator whose frequency is measured with a microprocessor and converted to an inductance or capacitance value that is then displayed on the LCD. When measuring capacitance, a fixed inductor is switched in to form a resonant LC circuit. When measuring inductance, a fixed capacitor is switched in to form a resonant LC circuit. The LC circuit determines the operating frequency of the oscillator which the microprocessor measures and converts to a capacitance or inductance value. Some programming of the microprocessor is required, but you can purchase a "kit" with the microprocessor already programmed. Won't learn much 'lectrical inga-neering that way, but at least you would have a working LC meter.
 
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