Probably stupid idea for a guitar pickup inductance meter - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: November 15, 2021, 06:28:03 pm
content: The title says it all. This is not how most inductor meters work as far as I know, so I'm guessing there are good reasons for that.But anyway. I was thinking of maybe using an RCL filter to measure guitar pickup inductance. Since the resistance is known, the idea is, if I can set a fixed, known capacitance for the filter, then I can in theory calculate the series inductance by measuring the resonant frequency. I'm guessing the challenge is to keep stray capacitance from skewing my results, since it looks like that has to be a rather small C value.Is it even worth testing if this may work?

author: CaptDon
author_rank: Super Contributor
timestamp: November 15, 2021, 07:45:51 pm
content: You chose 2.9 Henries for your inductance. I doubt guitar pickups are anywhere close to that much? A much smaller value of inductance means it will resonate with a larger capacitor and stray capacitance will have a far lesser effect. Are you measuring 'in guitar' through the 1/4" jack or direct to the pickup? Also, is it a multi-pickup guitar? How will you handle that? Not sure about your final goal, since your capacitor filter scheme would only have predictable results at one setting of the 5-Way coil combiner switch and a certain series/parallel combination of the guitar's very own volume and tone controls. I play a Peavey T-60, the most under appreciated guitar out there. My Auxiliary Axe is a Casio MG-510 Candy Apple Midi Guitar who's body was made in the Ibanez factory. It plays like a Strat but with the added features of MIDI accompaniment. The Peavey still has the original 'toaster' pickups!! Abalone inlays done by Paul Reed Smith!!! It's my 'Peavey Reed Smith'.

author: dazz
author_rank: Frequent Contributor
timestamp: November 15, 2021, 07:56:01 pm
content: Quote from: CaptDon on November 15, 2021, 07:45:51 pmI've never tried a T-60, but it's a thing of beauty. I would be testing them out of circuit to eliminate as many disturbing factors as possible (cable stray capacitance, etc)Apparently pickups with an inductance in the order of 2 - 4H are common. See these Texas Special for instance:https://www.strat-talk.com/threads/fender-texas-specials-analysis-and-review.403909/

author: Benta
author_rank: Super Contributor
timestamp: November 15, 2021, 11:38:49 pm
content: It's an idea, but it's difficult.You'll need to expand your circuit with a capacitor across R1+L2, which is the inter-winding capacitance, and it can be quite large. Your 6.4 kohm is not unrealistic, the Stratocaster was around that, the inductance will also be high (lots of windings), but I'm not sure if it will go into the H range.Your CX seems very low. IIRC, the tone control caps were in the 47...220 nF range (I'm unsure here, it was a long time ago).

author: dazz
author_rank: Frequent Contributor
timestamp: November 16, 2021, 02:01:14 am
content: Quote from: Benta on November 15, 2021, 11:38:49 pmThanks, Benta. The inter-winding capacitance wouldn't affect the filter performance at the resonant frequency, right? I'm guessing it would just let the highest frequencies through.I run a quick test using a horribly cheap pickup that measured 2k6 ohms, and a 470nF cap. Interestingly, the filter worked as if there was no inductance at all. It behaved as a simple RC filter. Weird.ETA: Well, of course it didn't resonate, the cap is too large. Doh!

author: Benta
author_rank: Super Contributor
timestamp: November 16, 2021, 08:07:37 pm
content: Quote from: dazz on November 16, 2021, 02:01:14 amIt certainly will. It will create a second resonance point somewhere, which will skew things. But both resonance points will be heavily damped by the winding resistance, making it difficult to identify a peak in the response. Leo Fender experimented for years before he had the bright sound of the Stratocaster pickups nailed.It might be better to do a frequency sweep of the pickup alone first. You know the resistance, and this would then give you the relationship between induction and inter-winding capacitance (a damped resonance peak). Then take it from there and think about the second-stage measurement to determine the inductance.EDIT:OK, dug out my old Stratocaster bible. Inductance on early Strat pickups was around 2.2 H (rising later on to ~2.5 H), resistance 6.0...6.1 kohms. Q ~2.2. This is only valid for the classic single-coil types. The Lace pickups are higher (~4 H). I've no info on interwinding capacitance.So your figures are absolutely ballpark.

author: dazz
author_rank: Frequent Contributor
timestamp: November 16, 2021, 08:59:04 pm
content: I've done some tests with the cheap 2K4 ohm pickup and I got pretty consistent results at some 800mH using a number of different caps. I'll need to get an LCR meter to check for accuracy.

author: t1d
author_rank: Super Contributor
timestamp: November 19, 2021, 02:34:31 pm
content: Quote from: dazz on November 15, 2021, 06:28:03 pmNot my area of direct knowledge, but...IIRC, this is a known method for measuring inductance with a scope. At some point, I have done it. There was lots of info on the web as to how to do it.I would think that a "humbucking" pickup might not work well with this method, because, IIRC, the two coils are in series, but wound in opposing directions. I'm not certain, though.