measuring inductance with a meat-and-potatoes DMM

[Mark Hammer]

Flipping through the catalog for Active Electronics the other day, I noticed that they carry a DMM made by MTP for $70 that includes inductance measurement in its toolbelt of features.


Are there reasons why I might want to avoid this, or is this a sound purchase until it breaks?

[RedHouse]

Are there reasons why I might want to avoid this, or is this a sound purchase until it breaks?

Hey Mark, try a quick search. There has been a lot of threads here this past year on why a standard DMM won't do in this case.

IIRC it's to do with the freq at which the measurement takes place, and someting about the series resistance.

[belwar]

Inductance (L) is frequency dependant. I.e. it changes based on the signal that is fed to the item you are measuring. When a device that measures inductance is turned on, it will send a signal at a specific frequency (Hertz) through the pickup and then the inductance at that specific frequency will be reported back. Knowing the inductance at one specific frequency doesnt tell you very much.. You can really only use that number to check consistance between several pickups. My fear with an item like this is that you dont know what frequency the L is being measured at. It could be 20hz, or it could be 200khz.

I bought an Extech LCR meter thinking I would learn alot about my pickups. I really havent. It measures at 120hz or 1khz which isnt usefull enough to get any good information.. The human ear can hear frequencies between approx 20hz and 20,000hz (20khz) so this meter covers only a small range of that frequency. Honestly I regret buying the darn thing now.

The best device you could get would be one that would give you a bunch of different Inductance readings between 20 and 20000 hz. Then you could plot them on a graph and get some idea of whats going on. I believe you need an occiloscope for that though - Im not an electronics guy so im not sure.

I recently bought an Audio Oscilator that can be manually sweeped from 20-20000hz which I use for determining resonance peak, and my next task is to see if I can use it to determine L.

b.

[Possum]

...

On the contrary, the Extech should be in every pickup maker's tool box. Its the only thing that spots coil shorts for instance. Even without a higher test frequency its super useful for my work. A frequency sweep is useful but its not so great for giving digital numbers when tiny changes happen in your pickup design. If you are only testing one type of pickup and nothing else, well it might not be so useful, but for pickup makers who make a wide variety of designs as well as proprietary work, I'd be lost without mine, I use it every day. The more you use it the more you'll be using it over time. My first LCR meter was a DMM was a Meterman that had inductance capabilities, but over time it was frustrating to use because it couldn't be set on either test frequency and would switch from one to the other if it was hitting near 4 henries, so alot of information I got wasn't real accurate. The Extech was like opening a magic door for me. I did alot of experiments just to learn about coils, what different types do compared to other types, what different winds do. I did the frequency sweep thing too, but never found them to be all that useful in the long run, plus how you set up your driver coil etc. is real hard to make perfectly the same each time, so you get confliciting results sometimes. I tried Joe's resonant peak setup but found it horribly unreadable most of the time, it just didn't work very good for hand wound coils, because hand winding tends to have real slow peaks, vs. machine wound, the DMM just wouldn't see these slow peaks very well, so it would look like there was a never ending peak.... If anything I would like an LCR meter with a 10khz test signal, at least the ACR would probably be accurate enough for my purposes. They are pricey though but its on my list of tools coming soon.....

[RedHouse]

...an Audio Oscilator that can be manually sweeped from 20-20000hz which I use for determining resonance peak, and my next task is to see if I can use it to determine L.

b.

That's the approach I'm taking also. It's way more usefull to plot a curve than have a basic two number reading. Then you can actually see what changes, when making changes, rather than just know that it changed and verify that with one or two readings.

[-Elepro-]

isn't it most simple with white noise and A-B spectrum analyzer?

[salvarsan]

isn't it most simple with white noise and A-B spectrum analyzer?

Yes. It is expensive, too.

-drh

[salvarsan]

I bought an Extech LCR meter thinking I would learn alot about my pickups. I really havent. It measures at 120hz or 1khz which isnt usefull enough to get any good information.

An LCR meter won't tell you shit about the pickup sound
and anyone here who led to you believe otherwise
should be spanked and send to bed without cookies.

An LCR meter is for quality control on a batch of pickups
made from the same component stock, nothing more.


It will also help you quickly identify defects like
winding cut-throughs and shorts to magnet poles
even when the DC resistance is only 150 ohms low
and still in spec.

-drh

[-Elepro-]

Yes. It is expensive, too.

-drh

you can with a pc.... try this ..... (last version doesn't work well with my pc...)

[David Schwab]

you can with a pc.... try this ..... (last version doesn't work well with my pc...)

Too bad there isn't a Mac version (or even Java). It looks interesting.

[Mike Sulzer]

Inductance (L) is frequency dependant.

Inductance (L), Capacitance (C), and resistance (R) are three concepts that describe the behavior of circuits that can be represented by lumped elements.

L does not vary with frequency. However, any real "component" needs to be described by all three concepts. Some useful real components can be described predominantly by one of the three over some frequency range.

Guitar pickups are complicated little beasts. At low frequencies they can be described very well by an inductance in series with a resistance. A good L meter measures both (the low frequency measurement). At higher frequencies, pickups have a complicated impedance. An L meter gives you some information at the higher frequencies, but not really enough to tell you everything about the components necessary to describe a pickup.

[tedmich]

Too bad there isn't a Mac version (or even Java). It looks interesting.

Mac the Scope may suit you, although its not cheap (officially...)

[voodoochild]

Excuse me for my ignorance but it seems to me that once you understand how everything works together (materials, tension, TPL, etc) you can wind pickups until they sound like you want them to. In regards to testing them why not just read it on a meter for resistance and check the output with a tuning fork. The whole meter with inductance etc, opened a can of worms for me and I ended up ditching it. If you have a short in the coil it won't have enough output or it will read totally off on the resistance. All can be done with a $2 tuning fork and a $20 multimeter IMO. On the other hand I could totally see the use of an inductance meter if you were trying to copy another pickup, active pickups, or rewinding. None of which I do so it pretty much deems itself useless for me.

[David Schwab]

Mac the Scope may suit you, although its not cheap (officially...)

Yeah, I haven't been able to find it, unofficially that is.

[Dmac252]

Certainly off topic here, but I visited your site. I hope not being presumptuous but where do you het your SGD covers made ?