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High-accuracy Digital LC Meter

Here's a handy piece of test gear you can build for yourself - a Digital LC Meter for measuring inductance and capacitance over a wide range. It's based on an ingenious measurement technique, delivers surprising accuracy and is easy to build.

By Jim Rowe

ANY MODERN DMM’s (digital multimeters) have capacitance measuring ranges, especially the up-market models. So it’s not hard to measure the value of capacitors, as long as their value is more than about 50pF or so.

Below that level, DMMs are not very useful for capacitance measurements. Dedicated digital capacitance meters are available, of course, and they generally measure down to a few pF or so. But if you want to measure things like stray capacitance, they too are of limited use.

Click for larger image

It’s even worse when it comes to measuring inductors. Very few DMMs have the ability to measure inductance, so in many cases you have to use either an old-type inductance bridge or a ‘Q’ meter. Both of these are basically analog instruments and don’t offer either high resolution or particularly high accuracy.

It’s different for professionals who for the last 20 years or so have been able to use digital LCR meters. These allow you to measure almost any passive component quickly and automatically, often measuring not just their primary parameter (like inductance or capacitance) but one or more secondary parameters as well. However, many of these you-beaut instruments also carried a hefty price tag, keeping them well out of reach for many of us.

Fortunately, thanks to microcontroller technology, that situation has changed somewhat in the last few years with much more affordable digital instruments now becoming available. These include both commercial and DIY instruments, along with the unit described here.

Main features

As shown in the photos, our new Digital LC Meter is very compact. It’s easy to build, has an LCD readout and fits snugly inside a UB3 utility box. It won’t break the bank either – we estimate that you should be able to build it for less than $75.

Despite its modest cost, it offers automatic direct digital measurement over a wide range for both capacitance (C) and inductance (L) with 4-digit resolution. In fact, it measures capacitance from just 0.1pF up to 800nF and inductance from 10nH to 70mH. Measurement accuracy is also surprisingly good, at better than ±1% of reading.

It also operates from 9-12V DC, drawing an average current of less than 20mA. This means that it can be powered from either a 9V alkaline battery inside the case or from an external plugpack supply.

How it works

The meter’s impressive performance depends on an ingenious measurement technique which was developed about 10 years ago by Neil Hecht, of Washington state in the USA. It uses a wide-range test oscillator whose frequency is varied by connecting the unknown inductor or capacitor you’re measuring. The resulting change in frequency is measured by a microcontroller which then calculates the component’s value and displays it directly on an LCD readout.

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