Measuring AC Impedance with a LCR meter

[RedHouse]

The Extech is about $200.

Very few pickup makers are going to build their own LCR meter. As a practical matter, if one does not already have a solid electronics background, this project is out of reach.

The cheap solution is already published, the Maxwell-Wein Impedance Bridge, but the schematics of the Extech are also available.

Joe Gwinn's website homepage

Can't tell if that's just lazy or flippant attitude, lets see, twenty six bought the 1st Elepro gauss meter kit, about a dozen the second, maybe half dozen bought-in to the CNC winder, dismissal of the subject is silly, you're not actually paying attention. More than "very few" appear interested in DIY gear around here but there we go again.

[salvarsan]

Can't tell if that's just lazy or flippant attitude, lets see, twenty six bought the 1st Elepro gauss meter kit, about a dozen the second, maybe half dozen bought-in to the CNC winder, dismissal of the subject is silly, you're not actually paying attention. More than "very few" appear interested in DIY gear around here but there we go again.

Brad,
You may be taking his response the wrong way.

Joe is saying that measuring inductance is a large chore no matter how you do it.

The Maxwell-Wien bridge is antique technology and well-understood by engineering types but it's likely beyond the skill or will of most other people.

Replicating the Extech inductance meter is more difficult and may not even be desirable since some of its measurements seem suspect. Given that the heart of it is a 16-bit CPU and that the source code was not furnished, building one is a day job, not a hobbyist endeavor.

It can be done, I'm sure, with a small amount of analog circuitry external to a PC sound card, and some programming to do an end run around the vexingly bloated GUI to actually get to the sound card.

[FunkyKikuchiyo]

Can't tell if that's just lazy or flippant attitude, lets see, twenty six bought the 1st Elepro gauss meter kit, about a dozen the second, maybe half dozen bought-in to the CNC winder, dismissal of the subject is silly, you're not actually paying attention. More than "very few" appear interested in DIY gear around here but there we go again.

I have to agree with Joe. Most people who either have the knowhow to make an LCR meter or are ambitious enough to try it probably already own one. Maybe there are people who could build them in their sleep who have never owned one, but that doesn't seem likely to me. Yes, there are people on this board who could do it... but how many of them would be first time LCR meter buyers?

There is also the consideration of your own time and labor costs. Sure, you could make your own light bulbs for free, but if it takes you three hours to make a light bulb and one at the hardware store costs $2, you're in a way paying yourself $.67/hour to make light bulbs. A $199 Extech starts to seem a bit more reasonable at that point.

[Mike Sulzer]

Here is a bit more on why a frequency variable inductor is the incorrect model of a pickup.

The physical model suggests that an R and L in parallel. Could this be equivalent to a frequency variable L?

At first thought, this might make sense. Consider the L and R in series that go in parallel with the main coil. At low frequencies the impedance of the L is low and the R dominates. The L has little effect. At high frequencies the impedance of the L is large and it dominates, appearing in parallel with the main coil. Parallel inductors are equivalent to a smaller value inductor. So you might think of this as an inductance that that decreases with frequency.

However, this falls apart if you look at the effects on the frequency response of the circuit. The R and L in series decrease the output from the pickup in a frequency selective manner: they act as a shunt impedance loading the series impedance of the main coil inductance and resistance. If you try to include the series R and L in the main coil as a frequency variable inductance, you lose this effect: you would predict the incorrect frequency response. You could get it back by making the voltage source in the model frequency dependent in just the right way. But in order to figure out how to do this, you would need to make the better model first. Why not stop with the one that works?

My previous post (quoted above) is in error. I got the model mixed up. It is only the series resistance of the main coil that forms the series leg of the voltage divider. This means that the R and L added in parallel with the coil to account for eddy currents will act as an L in parallel with the main coil at a high enough frequency so that the inductive reactance is much larger than the series resistor. At lower frequencies the resistance will dominate. If this resistance is quite small (and it probaly is not small enough) we would just have an L in parallel with the main coil at all relevant frequencies.

[Joe Gwinn]

Can't tell if that's just lazy or flippant attitude, lets see, twenty six bought the 1st Elepro gauss meter kit, about a dozen the second, maybe half dozen bought-in to the CNC winder, dismissal of the subject is silly, you're not actually paying attention. More than "very few" appear interested in DIY gear around here but there we go again.

OK. Time to get down to brass tacks.

There are a number of people in the group who could design and build a LCR meter from small parts, but those people know from experience just how much trouble it will really be, especially if one is hoping to achieve better than 1% accuracy, the same as the Extech.

The people who bought kits from Elpro did not design and debug the circuits, Elpro did. I would venture that most people buying the kits from Elpro could not have designed those kits.

But in answer to your request, I published two solutions, The Maxwell-Wein Impedance Bridge, and the actual schematics for an Extech.

The Maxwell-Wein Bridge is now obsolete, but was for a century the national-standards-lab accurate way to measure the inductance of low-Q inductors, and it works very well for this. The problem is that it is very slow and fiddly to make measurements using such a bridge. When the Extech came along, I used the bridge to prove that the Extech got the right answer when measuring pickups, switched to the Extech, and never looked back. (The background being that most handheld LCR meters got wildly wrong answers, by orders of magnitude, when used on pickups.) But this is the low-tech reliable solution to the problem for people who have more time than money.

Now, anyone with the solid electronics background could build an instrument from those schematics, but as Salversan mentioned, there remains a simple matter of programming. And there is a lot in that circuit that isn't obvious, and appears to be intended to compensate for this or that imperfection. All of this would have to be understood in great detail, if one will achieve errors below 1%.

Now one alternative I did work on was a V-I circuit, which allows one to measure complex current through and voltage across a component under test. I was not achieving anything like 1% accuracy, but the V-I circuit approach is how high-end LCR meters work, and there are many patents from Agilent and the like on how to do this. The fact that there are so many patents implies that it isn't quite as simple as the textbook claimed.

One could marry a V-I front end with a soundcard to measure the complex impedance of a component (such as a pickup) as a function of frequency. Mike Sulzer had been doing something like that, but focused more on getting the broad view of impedance versus frequency than measuring with 1% accuracy.

[Mike Sulzer]

Mike Sulzer had been doing something like that, but focused more on getting the broad view of impedance versus frequency than measuring with 1% accuracy.

That is exactly right. For one thing, the absolute accuracy depends on knowing the value of one resistor in the circuit to significantly better than 1%, to be safe. I have not measured the value of this resistor with several high quality meters, just my normal digital multimeter. You cannot trust it until you do it right. If one did know the value of that resistor, then it is likely that the measurements over some ranges of the parameters would be good to 1%, but that is not proven, and it is almost certain that the measurements are not that accurate over the full desired ranges of the parameters.

For what I am doing !% accuracy is not the issue. Reasonably good relative response over frequency is.

[Harrysong]

Thanks everyone for your help. I should be buying my first Extech LCR meter soon. Here it cost U$S 347.-

[salvarsan]

Thanks everyone for your help. I should be buying my first Extech LCR meter soon. Here it cost U$S 347.-

The Extech LCR meter is a rebranded Tecpel LCR-612 from Taiwan.
20000 Counts Hand Held LCR Meter Dual Display Auto-Ranging(LCR-612 ) - Tecpel Co., Ltd.

Maybe they are closer to you and cost less.

[Harrysong]

The Extech LCR meter is a rebranded Tecpel LCR-612 from Taiwan.
20000 Counts Hand Held LCR Meter Dual Display Auto-Ranging(LCR-612 ) - Tecpel Co., Ltd.

Maybe they are closer to you and cost less.

Good to Know

Thanks Salvarsan!

[Joe Gwinn]

Thanks everyone for your help. I should be buying my first Extech LCR meter soon. Here it cost U$S 347.-

Good lord. Where are you located?

More generally, it's helpful if you update your user profile to say where you are (city and country), because for many questions the answer depends on location.

[copperheadroads]

Good luck finding a used Extech ,there's only 1 listed on eBay , new or used

[FunkyKikuchiyo]

Good lord. Where are you located?

More generally, it's helpful if you update your user profile to say where you are (city and country), because for many questions the answer depends on location.

So many online/mail order places sell these things, it doesn't seem like it should cost too much, even if there are weird customs/duties. I've found that some companies that don't carry ANY LCR meters except one typically will carry the Extech. It seems weird that you couldn't shop around a little and get below that figure.

[Harrysong]

Good lord. Where are you located?

More generally, it's helpful if you update your user profile to say where you are (city and country), because for many questions the answer depends on location.

I´m in Uruguay. South America.

[RedHouse]

...The people who bought kits from Elpro did not design and debug the circuits, Elpro did. ...

Must have missed my point, go figure. I was making a point (sarchasm) about all the fuss and churn about silly crap that goes on here and yet we can't... as a group... make a simple kit to address one of the most fundamental parts of this hobby/interest/business. The Eleopro reference was just to highlight how many are actually willing, and able, to build a kit because you had said "few people...", again debugging isn't the point but never mind, I see we're back to the 1% accuracy notion again.

... I would venture that most people buying the kits from Elpro could not have designed those kits....

Yes I know Joe, that's the point of a kit isn't it?

[Steve Conner]

There are several LCR meter kits available on EBay already, so why bother designing one.

Also as was pointed out, the real parameters of a pickup are too complicated for the internal assumptions made by these meters. A humbucker has two self-inductances, one mutual, and at least three capacitances. To give a reading, the meter's firmware has to stuff ten pounds of crap into an eight-pound bag.

If there was a niche for a kit, I'd say a LR meter that worked at 100Hz where the capacitances are negligible.