DIY question - using a transformer as an inductor

[zenguitar]

I've been looking at some guitar circuits (like the Gibson Varitone and LS6) and Wah pedals that use inductors. And it's not easy to source small inductors from 1/2 a Henry through 1.5H to 7H, but a lot of what I've read suggests using signal transformers like this as an inductor.

Of course, regular readers will know that I am in need of help, but specifically what I am asking is whether there is a simple method of calculating the inductance of the primary and secondary of a transformer like this. Everything thing I've found so far starts fine but then descends into gibberish... ie. 'You count the number of turns and wibbly wobbly bibbly bobbly bob, obviously'.

Maybe Hugh, Dave, Folderol, or one of the other smart guys here could help.

Andy

[dmills]

Strap a small cap across it and find the resonant frequency by injecting the output of a signal generator through a few K ohms of resistor and watching the voltage across the transformer with a scope or high impedance audio millivolt meter, once you have found the resonant peak then apply

f = 1/(2 * Pi * sqrt(L*C)) with your known values for f and C to find the inductance, easy.

Moderators: Can we PLEASE have LaTex support on this forum, $$\math {2^2}&& works on the OU forums and is useful, it would be nice here as well!

Regards, Dan.

[Folderol]

That's probably the only practical way to do it, but not a lot of help if you don't have a box full knocking around, and are looking up catalogues.

Might be worth contacting somewhere like Oxford Electrical Products explaining the situation and telling them what sort of inductance you need.

P.S.
In theory you could calculate it from the DC resistance and the impedance, but there is a slight problem. Impedance only really applies to a single frequency and they never say what frequency the impedance is measured at!

[grab]

The easy answer is "find someone with an LCR meter". If you were anywhere near me, I could lend you mine, but you're a bit far for that. (Own trumpet: I won it for "letter of the month" in EPE with a quick and accurate method of rescaling 0-1023 to 0-999, which you often want for reporting 10-bit ADC measurements on PICs and Atmels.)

Also be aware that it's not just primary and secondary inductances but also the effect of the transformerness (to avoid wibbly words), which will change the overall inductance depending on how the secondary is connected.

As Dan says, if you have a sig gen producing a sine wave and a meter that can do true RMS (i.e. not fixed at 50Hz like many multimeters that do AC), you can first work out the DC resistance, then put the sig gen on and work out the total impedance at some frequency. A quick bit of maths after that tells you the inductance. Incidentally, PC soundcard inputs and outputs will be good enough for producing a sine wave and metering to do this. Maths to be added later, although any decent electronics book should give you the sums for a potential divider where one side is an inductor or capacitor.

[ef37a]

http://uk.rs-online.com/web/p/products/2508972543/?searchTerm=inductor+1H&relevancy-data=636F3D322664663D5926696E3D4931384E44656661756C74266C753D656E266D6D3D6D61746368616C6C7061727469616C26706D3D5E285C772B5B2D5C2E5C732F5D292B285C772B293F2426706F3D3926736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D45524943267573743D696E647563746F722031482673633D592677633D4E4F4E4526
They have a vast range. Yes, you can use a traff but as peeps have said you need the kit to measure it and depending upon the turns ratio it will be lossy.
Also Google "gyrators"

Dave.

[zenguitar]

Many thanks guys, that link is a good candidate dave, thanks.

Folderol is on the money, I don't have the equipment or skills to make the measurements as described. But amongst my adventures online I did see reference to calculating from the DC resistance, impedance, and number of turns. And that's what I was hoping to find.

Andy

[Folderol]

Hey guys, it's OK to call me Will
... especially as it's my name

see v
. . . . v
. . . . v
. . . . v
. . . . v

[zenguitar]

OK Will

And thanks everyone for your help. I've been trawling t'interweb for assorted clues and struck lucky.

Someone who uses a number of Xicon 42TL series transformers has measured the inductance of several models in the series. And if anyone stumbles across this thread in the future and would find this useful, here they are...

42TL022 1.5K Ohms .56 Henry
42TL021 4K Ohms 1.5 Henry
42TL018 7K Ohms 2.6 Henry
42TL019 10K Ohms 3.75 Henry
42TL025 17K Ohms 6.4 Henry
42TL017 20K Ohms 7.5 Henry

All are available from Mouser UK and at far more attractive prices than the Kent Armstrong tone choke which is a very useful 1H but a scary £26.51.

An online LRC calculator will let me pick the capacitor and resistor values to get the right frequencies for each setting.

Andy

[Howdy Doody Time]

Nice to see resolution. All's Will that ends Will.

[Folderol]

Nice to see resolution. All's Will that ends Will.

{ahem}

[wjmwpg]

OK Will

And thanks everyone for your help. I've been trawling t'interweb for assorted clues and struck lucky.

Someone who uses a number of Xicon 42TL series transformers has measured the inductance of several models in the series. And if anyone stumbles across this thread in the future and would find this useful, here they are...

42TL022 1.5K Ohms .56 Henry
42TL021 4K Ohms 1.5 Henry
42TL018 7K Ohms 2.6 Henry
42TL019 10K Ohms 3.75 Henry
42TL025 17K Ohms 6.4 Henry
42TL017 20K Ohms 7.5 Henry

All are available from Mouser UK and at far more attractive prices than the Kent Armstrong tone choke which is a very useful 1H but a scary £26.51.

An online LRC calculator will let me pick the capacitor and resistor values to get the right frequencies for each setting.

Andy

Andy, a decade has past but I just found this post and it made my day - thanks!

Long-shot question - would you happen to know if these measurements were made using only the primary as an inductor, or running through the primary and secondary in series?

[zenguitar]

Hi wjmwpg,

Glad you found this vintage topic useful.

A decade is a LONG time !! So it isn't easy to recall, but my gut instinct is that the figures are for using the Primary as an inductor. However, it's difficult to be sure. Since my original post I've switched from Windows PCs to Apple Macs so I can't even track down the post I was quoting.

Andy

[BigRedX]

Hi wjmwpg,

Glad you found this vintage topic useful.

A decade is a LONG time !! So it isn't easy to recall, but my gut instinct is that the figures are for using the Primary as an inductor. However, it's difficult to be sure. Since my original post I've switched from Windows PCs to Apple Macs so I can't even track down the post I was quoting.

Andy

OoI did you ever use this to produce the relevant inductors?

One of the problems I've come across is that nearly all the schematics for Gibson guitars and basses with varitone controls available on the internet give part numbers rather than values for the inductors which doesn't help if you are trying to build your own circuit.

[zenguitar]

I ordered parts but never got around to building a circuit. I've had a trawl through my old data.

Here's more detail on the inductance of the various transformers.


And I found this old schematic with inductor values if that helps.


Andy

[wjmwpg]

Extremely helpful! Thanks Andy!