inductance meter and frequency of measurement

[dai h.]

I was thinking of buying one (since I don't have anything that can measure L) and looking at some of them, I noticed there is a difference in the frequency used to measure the L. It seems the cheaper ones use a lower frequency such as say, 250Hz. Is being able to (or using) a higher frequency a sign of higher quality (of the meter)?

[Enzo]

I'd say the results are what matters. If one meter can measure at 1% and another at 5%, what difference does it make how they did it? And one meter might have a slightly better accuracy, but the other a much better range for your needs. For example, a lot of meters measure capacitance these days, but I recall looking at some that only went up to like 2uf. That might be great for working in RF, but lousy for power supply and audio. SO even if it were somehow "better," it would be less useful. Crossover coils and other audio chokes, plus power supply chokes come in a wide range of values.

Are you looking at an inductance meter? Or at a multimeter with an inductance range? If the latter, then what is the overall quality of the unit? If a dedicated I-meter, then how large an inductance can it measure.

There are inductance meters, and there are also speaker impedance meters.

One feature that some I-meters have is a ringer. This sends a pulse through a coil and watches to see how many cycles of ring result. This is used to determine if turns are shorted. A good coil will ring quite a few cycles, while even one shorted turn will damp out such oscillation quickly. It is sort of a higher tech version of the tester on RG's site. Useful, but not a deal breaker for me.

I have had an I-meter in the shop before - one of those fancy Sencore jobs. The ONLY thing I EVER used inductance measurement for was trimming the value of generic replacement coils in crossovers. They came from the suppliers as 3mh, 4mh, 5mh. If I needed 3.6, I unwound a 4mh part way. Other than that I never used it. That was a different shop, I do not have one now.

The Sencore also did caps. It measured them, checked their leakage at voltage, and even measures ESR. But tell me, when was the last time you really needed to know ESR? By the time I took a cap out of a circuit and set this thing up for an ESR reading, I could have just replaced the cap and moved on, saving me time and the customer money. I think ESR is one of those things you absolutely HAVE to know, once you have a way to measure it. The 20 years you did without will just be forgotten.

I do have an ancient Eico LCR bridge here that could be pressed into service, but it requires a known good sample to balance against for inductance.

www.fairradio.com often has surplus test gear like this, and some new. Under PRODUCTS go to Bridges. There are some new handhelds, and down the page is an LRC bridge "used repairable." That means not working, but complete and fixable. They'd likely send specs if you asked. The surplus market is always interesting for test gear. Nice military stuff cheap.

[dai h.]

Are you looking at an inductance meter? Or at a multimeter with an inductance range? If the latter, then what is the overall quality of the unit? If a dedicated I-meter, then how large an inductance can it measure.

thanks Enzo. I was looking at a couple of things. A kit with limited range (100mH max.) and a DVM with an L reading function (about USD$36, 600mH max.), both inexpensive but limited range so not good enough. Another DVM including an L function (about $101, 40H max., "pulse freq. (square wave?) for inductance measurement: Typical 550Hz"). Lots of functions incl. freq. temp., hfe measurement and a low price which is nice but since price is low and considering country of manf.(S.Korea?--not that I think Koreans, Chinese, etc. are incapable of making quality stuff mind you) I wonder about the reliability in the long term stuffing all that into one meter and doing it for a low price. Then a couple of LCR meters up to about $270 with the most expensive one able to use 10kHz to measure inductance (what got me wondering what sort of connection between the measurement freq. and quality).

Couple of situations I've encountered inductors (speaker inductors for exp'ing with reactive dummy load, wah inductors, now pickup simulation using small transistor radio Xfrmers as inductors where L is not stated) where I didn't have the facility to measure, then there are of course pickups in gtrs. and basses and it was my understanding that L tells more than DCR.

[dai h.]

did a bit of googling and apparently it's possible to make a (Wheatstone) bridge enabling the measurement of inductance. Found a design for one here:

http://www.geocities.jp/hy210304/bridge.html

and apparently someone built it and tried it successfully taking some pickup inductance measurements showing here:

http://www.geocities.jp/dgb_studio/pu_etc.htm#LBR

not sure if I could pull it off but doesn't look particularly difficult. He left out the 100uH and 1mH range which I guess makes sense since it seems like such small inductance measurements wouldn't typically be needed plus very high precision (0.1% and below) 1 and 10 ohm Rs (needed to obtain good accuracy at those settings-- 100ohm to 1Meg Rs with 0.1% tolerance are readily avail. at about US$1 ea.) don't seem to be very easily obtained. So I guess that is another option.

Also stumbled on an article which says something about software advances making inductance measurements more readily avail. which maybe explains the L function showing up on cheap meters(?).

http://machinedesign.com/ContentItem...onabudget.aspx

I guess I should do some more research before rushing to buy something.

[Joe Bee]

Without reading through the thread in detail, i'd say "the inductance you need to measure is what matters". For instance I have a very nice PIC microchip controlled DIY inductance meter that runs at 1MHz or so, and it's biggest range is 1uH or so. So if you want to measure iron rather than HF-chockes, 50Hz is all you need, right?

I reality inductance changes with frequency, so if you need accurate measurements, you should probe at a frequency that is close to what you intend to run at. But since you probably just need a ballpark value, you might even be able to get away without a dedicated meter by e.g. looking at the LR time constant with an oscilloscope on single shot triggering.

[Joe Gwinn]

I was thinking of buying one (since I don't have anything that can measure L) and looking at some of them, I noticed there is a difference in the frequency used to measure the L. It seems the cheaper ones use a lower frequency such as say, 250Hz. Is being able to (or using) a higher frequency a sign of higher quality (of the meter)?

A handheld LCR meter may be a better approach. One thing to be careful of is that cheap LCR meters require fairly "pure" components (ones with low parasitics) to read accurately. This can be a big problem with some inductive components with values in the henrys.

The best LCR meter I have found is the Extech 380193: http://www.extech.com/instrument/pro...99/380193.html. They cost about $180.

As for bridges, see the item about the Maxwell Bridge at http://home.comcast.net/~joegwinn/.

[dai h.]

thanks Joe (and Joe)!

So if you want to measure iron rather than HF-chockes, 50Hz is all you need, right?

I reality inductance changes with frequency, so if you need accurate measurements, you should probe at a frequency that is close to what you intend to run at. But since you probably just need a ballpark value, you might even be able to get away without a dedicated meter by e.g. looking at the LR time constant with an oscilloscope on single shot triggering.

I wasn't really sure about this and guessed I would just basically need an audio frequency for the measurement since I'm still pretty much a novice and don't deal with anything complicated like radio freqs (I read about some laboratory grade type bridge and LCR meters that can go up to MHz or something waaay up there and these cost thousands). I also don't have a scope (only a couple of DVMs and a cap. meter) so that wouldn't be something I could do.

The best LCR meter I have found is the Extech 380193: http://www.extech.com/instrument/pro...99/380193.html. They cost about $180.

thanks this is cool, it gives me some sort of reference on quality and an idea what people are using. It looks quite similar in performance to one of the more expensive ones I was checking out:

http://www.toeimusen.co.jp/~dp/lcr/ad-5826-27.html

The AD5826 is about US$225 (and up depending on vendor), so I gather that's the sort of money you have to pay for that level of functionality and accuracy. The 5827 is about $45 more and has phase and some additional measuring frequencies.

Also, FWIW I was searching for precision Rs (0.1% tol. or better since that is what is recommended in the link for the bridge project) and HOLY COW(!) the very low value (1 and 10 ohm) precision Rs are really expensive! Mouser has some Vishay bulk metal foil Rs and they can be in the tens of dollars--like 60 bucks or something like that for some of them. I guess the more precise you want to be the more expensive it gets...yow.

[Enzo]

Is everything else in your system 0.1%? MAking resistors that accurate without making everything else that accurate - including the measuring devices - isn't doing much for you. Looking at lab quality components for hobby level projects is an easy way to inflate the budget, as you found.

Does not the bridge project require examples to match to? I have no bandwidth at the moment to download anyting.

[dai h.]

that's true the article (all in Japanese) :

http://www.geocities.jp/hy210304/bridge.html

mentions that (besides the Rs) the accuracy also depends on the DVM accuracy and a cap (tolerance of the one used) as well.

in the other link where someone built the bridge project:

http://www.geocities.jp/dgb_studio/pu_etc.htm#LBR

he left out the 10 and 1 ohm Rs (the lowest two ranges--1mH and 100uH), so just leaving them out is an option as well as using 1% for those Rs instead of 0.1% or better. Would be a much cheaper option unless I can luck out with surplus or something.

[Enzo]

And consider the accuracy you actually need for the projects. Certainly the B+ choke on a typical Fender doesn't have to be within a percent any more than the filter caps do. Crossovers again are not all that critical. Inductors for somethig like tone stacks or active EQs do set the freq, but how close does that have to be.

I might compare it to volt meters. My DMM and bench meters are pretty accurate - at least last time they were calibrated - but there are plenty of times when a cheap meter works for my needs.

[Liam]

Might be worth looking for a secondhand LCR bridge. That gets you inductance, capacitance and resistance all on one meter. I generally borrow one from work, but I'll bet they can be found on ebay.

Liam

[dai h.]

thanks Liam. Reminds me someone on a Japanese bbs put up some links for a couple places that sell used test equipment so I should check that out as well.

[dai h.]

found some more info here (which might be of interest for anyone else looking for info on LCR meters) :

http://www.google.com/search?q=lcr+m...nics-forum.com

also, this is the experiment I was trying (a situation where it would be useful to have a meter) :

http://www.diystompboxes.com/smfforu...?topic=60396.0

[atulbhagwat]

Here is LCR with 200-250USD range and with 100Hz to 25 kHz user settable with twin test voltage and all secondary parameters
http://sites.google.com/site/knohowlcrmeter/

Might be worth looking for a secondhand LCR bridge. That gets you inductance, capacitance and resistance all on one meter. I generally borrow one from work, but I'll bet they can be found on ebay.

Liam

[ScottA]

Has anyone tried this one (Tenma 72-8155)?

http://www.mcmelectronics.com/produc...-8155-/72-8155

It seems reasonably accurate and relatively inexpensive @ $52.95

It uses a 1kHz test frequency in the 2mH - 2 H ranges and 100 Hz for the 20 H range.

Here's a link to the manual:

http://www.mcmelectronics.com/conten...ls/72-8155.pdf