Wah inductor "mystery" on my hands.........

[plexi12000]

winding up my first inductor and i'm....perplexed!

prroper resistance....30 ohms.  assemble it and put my L meter across it.  270 mh??  how is that possible?  hahaha

new meter w/ new battery.  wtf?

[R.G.]

Resistance is not equal to inductance, or even particularly indicative.

[PRR]

What core material?

Also what frequency are you measuring at?

As R.G. says, L/=R, but there are some practical limits for practical materials. 0.27H on 30 ohms sounds like a mostly-iron core. That's good for low frequencies, will be lossy in much of the audio band.

[plexi12000]

core material is n48.  according to my instructions...inductance should be pretty high...well over 500.  and usually have to scrape off some material from a core to drop it down to 500, or whatever you are shooting for.

so how i'm reading 270...i dunno.  coil winding is nice and tight.  and as neat as i could possibly get it.   

i stopped winding the core at just under 31 ohms.  checked with ohmmeter, of course.  put inductor together and got 270 mH with my L meter on the leads.  bizarro.

first i did this...so i musta screwed up something.  but what? how? pretty 'simple'?!  -lol

[J0K3RX]

Are you not able to roll anymore wire onto the core? If not, then why not just keep winding and stop every so many turns, take a reading... Keep going until your L meter reads 500 or whatever you're wanting? I'm not too sure what you are hoping to gain by measuring the resistance? I would just go by the L meter..

[amptramp]

The inductance goes up with the square of the number of turns.  Add 0.414 times your current number and the inductance doubles.

[R.G.]

core material is n48.  according to my instructions...inductance should be pretty high...well over 500.  and usually have to scrape off some material from a core to drop it down to 500, or whatever you are shooting for.

OK, so you're winding up someone's wah inductor kit. They have simplified the instructions for you in the same way that pickup winding instructions simplify it, in that the wire has a consistent resistance in terms of milliohms per foot, and so there is a rough correlation between resistance and number of turns.

The N48 material is what is used in many aftermarket wahs, but the inductance per turn on a core, even of the right material, is hugely dependent on the air gap being as intended. I suspect that if you measure the inductance with no cores in the coil, one core, and both cores, the inductance increases substantially each time.

If it's a typical potcore, an N48 with no gap will give about half a henry with something like 200-400 turns, depending on the core size and whether it has an air gap ground into the center leg.

Or if there is anything that holds the precision ground halves of the core apart from fitting together perfectly.

So, there is only a rough correspondence between wire resistance and number of turns, and almost no correspondence to inductance, as the amount that the core "multiplies" the air-core inductance of the coil depends on the core material and any air gaps, even how tightly the halves are clamped together.

Were the mating surfaces clean and shiny? Does a straightedge across the diameter of each core half show that the center leg is co-planar with the edges?

[tubegeek]

OK, but, to be completely fair, R.G., you did NOT correctly guess which card he pulled from the deck.

[guitarkill]

Why don't you quit guessing and get something that measures inductance? It makes life a lot easier...

[PRR]

> get something that measures inductance?

He said he did:

...put my L meter across it.

I might disagree that a inductance meter makes life easier. On iron-cores (including ferrites), it usually confuses. L depends on frequency (eddy currents push the flux out of the core). L of an "un-gapped" core depends a LOT on the actual gap you got (R.G. is hinting about grit or poor pressure).

[plexi12000]

instructions recommned 550 turns....more than you need.....so you UNWIND a few turns until the RESISTANCE of the coil...is around 30 ohms.

THEN you shlep it all together...and take the INDUCTANCE reading.  so i ended up with about 340 turns to get 30 ohms!!  put it together and only got 270 mH.

Thats all I'm saying.....i did it according to the guidelines. I'm a pretty particular person. yes, the cores had no gaps....and lined up very straight.

i have no idea.   someone recommended more turns.....but then i'll increase resistance, right?   I have no idea. 

[Jdansti]

You might want to contact the manufacturer of the kit and ask them.

[plexi12000]

I know now what I'm doing wrong.  'guidelines' could be a little more clear...but hey, whatever.