Discussion in 'Electronic Equipment' started by Dave, Jun 30, 2005.

1. ### DaveGuest

I am apparently more out of touch than I realized, and need someone in the
know to please answer a dumb question for me. If an LCR meter with the
lowest range of L is 20mH has a 3-1/2 digit display, does this mean it will
display down to 1 microhenry? I'm thinking that it would display .001 in
the range for mH's, which should be 1 microhenry. Does this sound right to
anyone out there who uses such a device? Your help is appreciated.

Dave

2. ### Robert BaerGuest

3-1/2 digit max on a 20mH scale would be 19.99mH, so if one looks
*only* at digits, then the least significant digit is 10uH, *BUT* the
best accuracy one can expect is +/- one digit, making a reading of
0.01mH virtually meaningless.

3. ### DaveGuest

Gotcha. Many thanks.

Dave

4. ### John MilesGuest

I wouldn't count on it without checking with the manufacturer. The
'1/2' digit is for the 0 or 1 needed in the most-significant digit, but
you'll need one more digit to the left of the decimal point for the
0?.00 or 1?.00 place. That leaves you with only two full digits to the
right of the decimal point.

For measurement in that range, I'm aware of only one affordable LCR
meter (really, LC meter) that's useful -- the L/C Meter IIB sold by

-- jm

5. ### DaveGuest

Wow. Too cool. I need one of those...

One NANOhenry. Is this really meaningful? how does it get around stray
inductance etc?

Thanks for the ref. I'm really going to check this out.

Much thanks,

Dave

6. ### John MilesGuest

There's a zero button; to measure L, you short the test leads, zero it,
then try to connect the unknown inductor without moving the leads around
too much. It actually works pretty well in practice, although the 1-nH
LSD isn't exactly repeatable with normal test leads.

Same for C, only you zero the meter with the leads open.

Some people have built their own test jigs to eliminate the instability
caused by the leads, but I haven't had the need to do that, myself. The
meter itself is stable enough to take advantage of its own resolution,
which is really saying something.

Obviously the accuracy is much less than the resolution, but for sorting
or tweaking parts, that's fine.
Note that it doesn't work on caps bigger than 1 uF, or inductors that
get extremely lossy at frequencies in the hundreds of kHz. It's more of
an RF-grade complement to the high-range LCR functions on DMMs than a
replacement.

-- jm