Determining a short with an LCR meter

[kevinT]

Besides identifying unusally low DCR in a coil, what other way(s) can you tell that you have a short in a coil using an LCR meter?

Does it have something to do with the AC?

[cbarrow7625]

It has everyting to do with the AC! That is what the 'L' (inductance) part of the meter is using to measure the coil. It usually will measure the inductance at 1kHz.

If you have a shorted turn, the inductance will be unusually low. Of course, like DCR, this is only valuable if you know what it is supposed to measure in the first place.

Why do you think you have a short? Do you have any other coils like it that you can measure?

There are calculations for both inductance & dcr that you can do to determine what the resistance & inductance is supposed to be for any given configuration. Do you know how many feet of wire were wound onto the coil? Knowing that and the ohms / ft of the wire gauge will tell you what the DCR should be.

The inductance calculations are way more involved. You can Google "calculate coil inductance" (or something similar) and find the info you need to calculate it out.

Chris

[Spence]

If you have a short the DC reading will wander. A sure sign.

[Joe Gwinn]

Besides identifying unusally low DCR in a coil, what other way(s) can you tell that you have a short in a coil using an LCR meter?

Does it have something to do with the AC?

The most sensitive approach using the Extech LCR meter is to measure inductance and AC resistance at 1 KHz. A short will cause the AC resistance to be much higher than usual. This works even if the DC resistance change is too small to be detected.

[kevinT]

The most sensitive approach using the Extech LCR meter is to measure inductance and AC resistance at 1 KHz. A short will cause the AC resistance to be much higher than usual. This works even if the DC resistance change is too small to be detected.

Joe,

Could you give an example. Sort of walk through a scenerio so that we can get an idea of what to look for....I'm by no means an electrical engineer here

Let's say i have a bobbin that reads 4k Ohm DCR. What would the inductance and ACR readings normally be (ball park of course) and what would they read if there was a slight short?

Or if you have a better example, please provide.

[Joe Gwinn]

Let's say i have a bobbin that reads 4k Ohm DCR. What would the inductance and ACR readings normally be (ball park of course) and what would they read if there was a slight short?

First of all, the DC resistance tells you nothing about the inductance. All we know is that with pickups, which are usually would with #42 wire or thereabouts, more resistance means more turns means more inductance. But if one wound the same pickup with #44 wire and double build insulation, the same number of turns would more-or-less fill the same space, so while the inductance would be the same, the DC resistance would be higher.

What I call "excess" AC resistance (being the difference between AC and DC resistances, the AC resistance always being the larger of the two) is proportional to the eddy current loading on the coil being tested

In an air coil well away from any metal, AC and DC resistances are equal or almost so. If you bring a piece of metal near, the AC resistance will rise while the DC resistance stays the same. The excess (ACr-DCr) represents the energy lost to eddy-current heating of the metal. With a sufficiently powerful AC signal, the metal will become hot to the touch. This is used industrially, and is called "induction heating" (http://en.wikipedia.org/wiki/Induction_heating and http://www.ameritherm.com/aboutinduction.php).

Back to pickups. It's pretty much impossible to compute what the AC resistance of a variable-reluctance sensor like a guitar pickup ought to be from first principles, unless one uses one of the very large and expensive software tools designed for the purpose (http://www.ansoft.com/device/sensor/...duct=simplorer), so in the pickup world one does it by comparison. Measure the inductance, Rdc and Rac of every pickup you make. Be very suspicious of any unit (by serial number) whose excess resistance is out of line with other units of the same model.

[David Schwab]

But if one wound the same pickup with #44 wire and double build insulation, the same number of turns would more-or-less fill the same space, so while the inductance would be the same, the DC resistance would be higher.

I just got a roll of 45 AWG heavy build. I'm curious to see what I get with it.

That was a great post Joe. Now I have to get one of those Extech LCR meters.

[Possum]

Extech

They've been discontinued but you might still find some...
45 heavy, I hear you can get 90K on a strat coil, that ought to sound like, uh, 90K

[kevinT]

First of all, the DC resistance tells you nothing about the inductance. All we know is that with pickups, which are usually would with #42 wire or thereabouts, more resistance means more turns means more inductance. But if one wound the same pickup with #44 wire and double build insulation, the same number of turns would more-or-less fill the same space, so while the inductance would be the same, the DC resistance would be higher.

What I call "excess" AC resistance (being the difference between AC and DC resistances, the AC resistance always being the larger of the two) is proportional to the eddy current loading on the coil being tested

In an air coil well away from any metal, AC and DC resistances are equal or almost so. If you bring a piece of metal near, the AC resistance will rise while the DC resistance stays the same. The excess (ACr-DCr) represents the energy lost to eddy-current heating of the metal. With a sufficiently powerful AC signal, the metal will become hot to the touch. This is used industrially, and is called "induction heating" (http://en.wikipedia.org/wiki/Induction_heating and http://www.ameritherm.com/aboutinduction.php).

Back to pickups. It's pretty much impossible to compute what the AC resistance of a variable-reluctance sensor like a guitar pickup ought to be from first principles, unless one uses one of the very large and expensive software tools designed for the purpose (http://www.ansoft.com/device/sensor/...duct=simplorer), so in the pickup world one does it by comparison. Measure the inductance, Rdc and Rac of every pickup you make. Be very suspicious of any unit (by serial number) whose excess resistance is out of line with other units of the same model.

Thanks Joe. This really helps.

[David Schwab]

They've been discontinued but you might still find some...

Yeah, I saw someone mention that here. Are there any other brands that work with pickups?

45 heavy, I hear you can get 90K on a strat coil, that ought to sound like, uh, 90K

Ouch! Well I'm sure not planning on winding something that hot! I was really looking for 43 or 44 single, and found this on eBay. It's a 2 pound roll. I got it for $47. Figured it could be interesting to work with.

I've been winding high inductance/low resistance pickups, so I'm curious how this would figure into the equation, since it will be higher resistance, and lower inductance. I think a little would go a long way....

I'm just getting around to trying out different windings and stuff. I wound two identical pickups, one with 3K turns per coil, and one with 4K turns, 42 on each. Besides the obvious output difference, which I expected, the hotter pickup actually sounds brighter in the bridge position! This is on a bass. I think it has a lower resonant peak which is accentuating the highs. The other pickup sounds much flatter and more middly. I'm trying to get the lower one brighter. (the customer wanted a bassy neck and bright bridge pickup for his bass... which is the opposite way that I usually make them.)

Boy, you could spend all day doing this stuff... But you know that already.

[Possum]

skinny wire...

Yeah I just snagged 13 lbs of single build formvar 44 gauge myself, I was running out of the poly stuff and got sort of a deal on it, will probably last forever. Let us know how that wire turns out. Haven't wound much with the formvar to know if its much different yet....

[Scott S.]

A good alternative to the Extech type meter is an old impedance bridge.

Much larger physically, but no no worries about whether it will measure pickups correctly, which is an issue with the digital ones (discussed at length here)

They go way cheap on ebay, good ones are ESI 250 DA (what I use) and ESI 250 DE and the General Radio 1650s, and there are other good ones too.

Here is a refurb for $100 BIN that has been checked out by a tech

http://cgi.ebay.com/Crescent-ZM-30-U...QQcmdZViewItem

[David Schwab]

Wow... that thing would sure make you look like a mad scientist!

[David Schwab]

Re: Skinny Wire

Yeah I just snagged 13 lbs of single build formvar 44 gauge myself, I was running out of the poly stuff and got sort of a deal on it, will probably last forever. Let us know how that wire turns out. Haven't wound much with the formvar to know if its much different yet....

Well..... wound two coils tonight with the 45 AWG... actually four if you count having to rewind them!

Whatever feel I learned working with 43 is useless! I broke a lot of wire starting out. Then it got easier. It's delicate stuff.

3000 turns gave me 5.33K... 4000 turns gave me a 7K coil!

I wont get to hear it until tomorrow... Oh, the suspense.

[kevinT]

cool. can't wait to hear the results david.