RLC meters

It's a lot harder than it looks. Better to buy one on eBay. Otherwise try Elektor or "Silicon Chip" magazine (Australia).

did you look at the AD7745 ?

Does "harder than it looks" imply that you know how to do it but would rather not tell?

Opinions about what I "should" do are much less interesting than helpful suggestions about how others have done what I intend to do. Absent that, I'll do it anyway.

All the kit versions I have seen have very limited ranges and the like. Silicon Chip has a magazine format book with designs for a cap meter and for an inductance meter. The cap meter measures in three ranges up to 2 uF. The inductance meter has two ranges covering 10 uH to 20 mH. I suspect that the cost of both kits will exceed the eBay price for a very comprehensive RLC meter.

I'm an old guy - started with tube radios. Been there, done that, learned what's worth doing. I no longer fix popup toasters and pocket transistor radios for money and I know what is economically reasonable and what is not. But go ahead, good luck to you.

# .001 mHy (1 nHy) to 100 mHy (most units measure to 150 mHy) # .010 pF to 1 mFd (most units measure to 1.5 uFd)

PIC based, might be a good idea to take a look at it to get ideas.

Ed

Thanks! I also started with tube radios and I no longer do much of anything for money. I guess I wouldn't fix a toaster or a transistor radio either. However, I'll spend half the winter making something I could probably buy, if I think I can make better than I can buy and might enjoy doing it -- and probably learn a few things along the way. I kinda know what I'm doing some days, having been a practicing engineer for 40 years.

Recent checks on EBay have turned up either meters I wouldn't be satisfied with, or instruments way more than I want to afford as in

graphic user interface yada yada.

Very interesting. Looks like a kit. It doesn't have all the features I want, but if the kit is anything like reasonably priced I'd get one anyway. I'll check this out. Thanks!

Perhaps think about using parallel known C's and looking for resonance with the scope. Use two constant current sources OR'd together. Switch \\ +----[CC.1]--->--+--Scope |___ ___| | | | | )|| | C=== )||DUT | | )|| | | | 0v-+----------+-----------+------0v

[CC.1] is a low power 0-100KHz bipolar CC-source, say 100mArms or so capability. [CC.2] is a high power DC CC-source, adjustable up to say 5 to 10A. Switched in/out, as required.

With no DUT: Scope measurement of the voltage at various frequencies gets the Cstray of the test equipment.

With a DUT: Resonance with various C's and frequencies gets the Inductance and the Cstray of the DUT. A guess of the Q at resonance gives a rough idea of the losses.

Looks a little clunky and old-fashioned, but cored inductors are better tested with some *welly* going through them.

--
Tony Williams.

I have an old Heathkit LC meter.

It uses an internal signal generator to set a frequency appropriate to the L range to be measured.

The unknown L is connected in series with a variable capacitor and the C is tuned for a voltage peak at their junction. A fairly high impedance detector is used to very lightly load the junction.

The C is calibrated, and the L can be readily calculated.

In other words, it inserts an unknown with a known, and determines the resonant frequency...

In the past, for SMALL inductance values, I have used it in a parallel tank using an MC1648 VCO, and counted the frequency of oscilation. That works really well with

50 nh and up...... Select a frequency range to shoot for where the expect inductive, or capacitive, reactance is in the ball park of 100 ohms or so. That seems to work well for practical values of L and C......

It ain't rocket surgery, and will give you ample opportunity for experimentation... Have fun.

Andy W4OAH

My ARRL handbook has several circuits for measuring R, L and C (2004 edition, section 26.25). Typically they convert the unknown value to a voltage that can be measured.

Very small R, L and C tend to be difficult, but away from that area, 10% accuracy can be achieved. If you want better, then bridges are the way to go - but as others will point out, you can buy better for less than your component cost ..

Dave

Something like this:

or these:

These are low cost units. Really high performance units go for $10K+.

Tells you something about the difficulty of building one.

Ordered one today. Thanks for the tip!

I've ordered a simple low-cost meter for now. Most of the high-buck jobs have a lot of features I don't care about -- but I can see that building a precise instrument is not a trivial undertaking. Still might be fun to try. I'll have the simple one to use while I'm screwing around discovering why a better one is so expensive -- or perhaps how and why it needn't be.

Thanks for the leads, Phantom!

I think you'll be well pleased. I've read a number of reviews on it - all were positive, and I love mine. Ed

I see that you're considering a vintage GenRad.

Have a look at item no. 190028769647 on ebay. I notice that the seller says at the bottom of the description that he also has a unit that will make 10 KHz measurements. Also look at the seller's other items.

You could try finding some downloadable service manuals for some of the better RLC meters, and see how they did it.

Maybe you could find some at

.

If you're not sure what models to download manuals for, you could check the archived old catalogs, in the reference library section at

(after free registration, which has proven to be harmless).

Good luck.

- Tom Gootee

"He who lives in a glass house should not invite he who is without sin."

Ed, I concur...

I bought Neil Heckt's LC meter kit about 1 year ago and have been mighty pleased with it.

Ross Herbert

That does look interesting -- and I could have my daughter in the UK grab one for me. But I've already ordered the kit that ehsjr mentioned and recommends, so I'll see how that works out. Could have used it today.

That site doesn't seem to be working. I also tried it earlier looking for a manual or schem on a Tektronix 2235 scope that won't display both channels in either alt or chopped though it can add them.