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
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.
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
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.
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