the mystery of inductances not adding?

[dai h.]

I bought an LCR meter which allowed me to take some measurements on some small transistor radio type transformers (apparently manufacturers do not typically provide the L readings) which I want to try for pickup simulation (wah output to buffer to pickup simulator using the transformer winding as inductor interfacting experiment). Something I don't understand is that center tapped windings read less than half of the whole winding (which seems strange because my understanding was that inductors add like resistors, ex: 1mH+1mH=2mH, etc.). One of the transformers for example measure about 1.1H from each end (same basic DCR from ea. end to CT--125ohms, 124ohms) to CT but across the whole winding the L reading is about 4.4H(about 250ohms) and not 2.2H(1.1H+1.1H). This seems to be true for the couple other dinky transistor radio type transformers I have as well. Additionally, series connecting primary and secondary also seems to give a big increase in inductance (the aforementioned transformer primary measures about 20.4H--series connecting P+S gives about 42.7H and not 4.4H+20.4H). What on earth is causing this (non-linear?) change in L?

[Steve Conner]

Inductance is proportional to the square of the number of turns. So between one end and a centre tap, you'll see one-quarter of the inductance of the full winding, which is what your results show.

[dai h.]

ahhh, BIG THANKS for the BIG HINT!

didn't quite understand (I am quite a saddo at math..err..maths?), so I googled a bit and found this:

The value of an inductance varies in proportion to the number of turns squared. If a coil was of one turn its value might be one unit. Having two turns the value would be four units while three turns would produce nine units although the length of the coil also enters into the equation.

http://www.electronics-tutorials.com...inductance.htm

So between one end and a centre tap, you'll see one-quarter of the inductance of the full winding, which is what your results show.

makes sense now!

(hmm... so I gather this means if you wanted to switch between 1L and 1/2L, a CT would not work as the switching point.

http://www.muzique.com/images/pickup2.gif

original article:

http://www.muzique.com/lab/pickups.htm
)

[Sock Puppet]

(hmm... so I gather this means if you wanted to switch between 1L and 1/2L, a CT would not work as the switching point.

That's right, you'd need the tap at 1/(sqrt 2) = .7071 of the number of turns.

Or of course two separate inductors.

S.

Incidentally, I borrowed a Peak atlas LCR40 last week for some theremin coils and like it a lot...
Maybe enough to fork out £70 for my own but I'm still on the fence, don't know if I'd use it enough.

[dai h.]

thanks SP. Mine was kind of expensive at about the equiv. of £144 (for me anyway not Bill Gates). It looks like the ones with the yellow on the outsides:

http://www.escorttw.com/escort/digit..._%20meters.htm

The specs look like ELC-133A but ELC133A seems to be more expensive, so maybe mine is not as accurate or something. Not quite sure exactly how good the meter is or isn't but it is nice to be able to conveniently read L.

re: 1/2L, it appeared 3/4 of the winding was close, so I think I can use a 1:1 ratio with CT, then connect P and S in series and use the 3/4 point to get about 1/2L. After comparing the readings on a couple of the Xfrmers I've got, it appears a 1k to 1kCT might be about right, then maybe (for hum-cancelling purposes) 600 to 600CT (P+S in series) in series, humbucking connected, tapped at 3/4 point for approx 1/2L.

I also saw some info about software LCR readings using your PC sound card, so maybe you might want to look into that if cost is an issue. I found the info after I bought the meter so it was too late (plus apparently you needed an input which my Onkyo sound card doesn't have--although sound quality is pretty good). I didn't dig long enough to find a download (only an out of date Russian one plus one with a free trial period but google advised to stay away) but maybe with more digging you can find one.

here's some of the bits of info:

about "building an LMS Impedance Bridge (build a unique PC sound card impedance bridge)"

http://www.arrl.org/qex/2005/Steber.pdf

copy of the Russian DIY site info from the wayback machine (program not avail.) :

http://web.archive.org/web/200208100....publi.ro/ady/

[Sock Puppet]

Interesting post, thanks.

Cost isn't an issue there unless I weigh it against 70 quids worth of beer, unless my interests change (as they do...) it's just that I don't think it would get much use unlike my multimeters which get dragged out every day.

I'm mostly a hobbyist that worked in the electronics field for enough years to "Spoil a good hobby" and I've forgotten more than I know - seriously.

I borrowed the meter from a radio amateur who lives not half a mile from me and still in fact have it - hasn't been used since I wound the coils I wanted last week but I'll have to return it as it's something that does get used a lot at his place.

WRT software (audio, not LCR related) have you seen Visual Analyser?

http://www.sillanumsoft.org/

Freebie multifunction instrument that unfortunately doesn't work with my EMU soundcard, not that I'd want to risk damaging the said card in any way but the blank screens look good.

I do miss the facilities at my old job though, parts, test gear, mentors etc but having said that the souped up net gives conditional access to gurus (read consultants-for-free it you're polite and a tad coarse) like Enzo, R.G. Keen, Mark Hammer et al. - priceless!

Got to go, Simpsons is on.

S.

[dai h.]

wow that's pretty cool. I've got a couple of signal generator as well as an O-scope program from the internet but I've never seen one with all that before. Thanks for the link!

oh yeah, w/regards to the cost there were some less expensive ones but I had some concerns about ending up with something having trouble reading L with high R (gtr. pickups, specifically) which was apparently a problem with some meters when I asked about them in another thread. Then there was also another model basically the same but with slightly less measurement frequencies and functions for about £120, so my logic was that I might as well spend the extra £24 if I'm already going to spend at least £120. Don't know if that was the wisest choice, but it's done so I'll just have to try to get the most out of it.