Transformers as inductors

[rankot]

I need an 1.5H inductor for a project I'm working on, but I can't buy anything like that here. What I have is a handful of 600:600 phone isolation transformers, and their inductance (primary and secondary are the same) is 700mH. So I got an idea to use them in series with 100mH inductances that I can buy here, just like shown on this picture:



What I want to ask is this: will this work, and do I need to use two transformers, or I can use primary and secondary of only one connected in series?

[antonis]

If you could seperate windings (take them apart not to form transformer anymore), it might work..

[GibsonGM]

If you could seperate windings (take them apart not to form transformer anymore), it might work..

Yes...what Antonis sees is that the fields which will be formed in one transformer coil will interfere with its other coil (literally, "transformer action"), and affect its operation.  The coils are so close together (often wound right on top of each other) that these interactions have consequences. 

Some people have used ONE coil of a transformer as an inductor, and just don't use the second coil.

[rankot]

I thought so. I will try with right schematics in that case - two transformers in series and one additional inductor of 100mH.

[anotherjim]

Hope this isn't too much of a high-jack.
Is there a trick to be had with the unused winding. Fitting some RC network to it so it has a resonance with some effect on the signal in the used winding?

[duck_arse]

I thort a coil of wire was a coil of wire. if you get the phase of the coils right, connecting the end of one coil to the start of the other coil gives you ..... more coil. and more coil equals more inductance. surely.

[amptramp]

Inductance goes up with the square of the number of turns.  If you have two equal voltage windings on a transformer, connecting them in series aiding (rather than cancelling) will give you four times the inductance.

[EBK]

Inductance goes up with the square of the number of turns.  If you have two equal voltage windings on a transformer, connecting them in series aiding (rather than cancelling) will give you four times the inductance.

Interesting.  Some more details:
https://electronics.stackexchange.com/questions/33860/how-do-i-use-a-transformer-as-an-inductor

You might also consider experimenting with leakage inductance by shorting the unused coils.

[GibsonGM]

I thort a coil of wire was a coil of wire. if you get the phase of the coils right, connecting the end of one coil to the start of the other coil gives you ..... more coil. and more coil equals more inductance. surely.

What about coupling between the coils, tho?  Or maybe we could call it interference, if we like.   "Real" inductors are small, resistor-like objects that would tend to be placed away from each other if more than 1 is in use. 

If you used bigger, real coils...like in a radio..they must be mounted at right angles and as far away from each other as possible to avoid interaction.  By "interaction", I'm assuming weird inductance changes due to fluctuating fields cutting across each other and such....or maybe not, if the transformer is small? 

I wouldn't hesitate to at least TRY it, there is no harm in that, for sure.   I'd like to know if it's a no-go or if it's fine...

[EBK]

If you used bigger, real coils...

[GibsonGM]

^ Did you get that from Small Bear? 

[PRR]

> What about coupling between the coils, tho?

If coils are far apart, coupling is like 0.01, then 1H+1H= 2H.

If coils are on the same iron core, coupling is like 0.99, then 1H+1H= 4H (nearly). Unless you get them out of phase, making about 0.001H.

Leakage inductance on iron cores will be near 0.001. Shorting another winding makes most of the inductance vanish.

Do not overlook small power transformers as ad-hoc audio inductors.

[GibsonGM]

> What about coupling between the coils, tho?

If coils are far apart, coupling is like 0.01, then 1H+1H= 2H.

If coils are on the same iron core, coupling is like 0.99, then 1H+1H= 4H (nearly). Unless you get them out of phase, making about 0.001H.

Leakage inductance on iron cores will be near 0.001. Shorting another winding makes most of the inductance vanish.

Do not overlook small power transformers as ad-hoc audio inductors.

I again wonder how you manage to simply keep data in your head like this, Paul, LOL!

Ok, shorting another winding makes perfect sense...but what about the interactions of trying to use a transformer set up coil to coil in series?   Could you talk on that more, if you have more info?  I do not understand "1H + 1H = 4H".    Likely, you'd have wildly different inductances between primary and secondary, not 'same same'...so, how does one "play" with these things?  Using one side is second nature....CAN we use BOTH primary and secondary in series, as the OP asked, and does that make sense?  Or is it nuts?

Damn transformers.

[EBK]

CAN we use BOTH primary and secondary in series, as the OP asked, and does that make sense?  Or is it nuts?

Your answers are: yes, yes, and yes. 

[anotherjim]

I do not understand "1H + 1H = 4H".

Apart from the fact that mutually coupled coils must interact, I'm not sure why the increase in H is so much more than x2.
I'm thinking that if coil A induces coil B, and coil B also induces coil A, then it's like having 4 coils of the mean average inductance  of A & B. It's a mirror trick - 2 originals and 2 reflections.
I know how the value is arrived at. Sum of the square root of each windings H, then square the total.

[greaser_au]

We're talking about a 1:1 transformer which has (close to) identical windings for primary and secondary, so what we have is two identical coils wound on the same former over the same core.

If we connect the start of one to the finish of the other we have  a single centre-tapped coil.

If we then ignore the centre tap, we have a single coil that has twice the turns of the primary (or secondary) alone - and at this point it's no longer a transformer - it's a 'solenoid' (beware, Roger Ramjet!!)

The formula for inductance of a solenoid is:  L=(N^2).u0.ur.A/l   (inductance = number of turns squared x absolute core permeability x area / length).   

By 'daisychaining'  windings, we only change the number of turns, and everything else is the same so can be ignored, so L=(N^2)k

So if Npri = Nsec, then Ntotal  = 2N   so Ltotal = 4.Lpri

So if either winding is 1H, both windings in series is 4H,  so  PRR has shown that1H+1H =4H   

david (magically transported back to 1991 and Analog Electronics II ...)

[duck_arse]

well, it's better he say it than I say it. at least you know it makes sense then.

what's the inductance of half a split humbucker? what's the inductance of the same when humbucking? [they are wound equal coils aren't they? are they split at half as well?]

[Phoenix]

what's the inductance of half a split humbucker?

Well, a humbucker has the two windings on two separate bobbins with their own "cores" (slugs/screws/magnets/whatever), so the inductance just adds, no need to play with the square of the sum of the square-roots of the windings like if they were on the same core. So if a humbucker is 2H, split will be ~1H.

what's the inductance of the same when humbucking? [they are wound equal coils aren't they? are they split at half as well?]

Typical humbuckers might be anywhere between about 3-9H.
They're not always wound with equal coils - one trick for better high frequency response (in addition to scatter-winding), is to wind fewer turns on one of the coils for unequal cancellation. Also, at least for traditional Gibson style pickups, there's the screw bobbin, and the slug bobbin, which have different core mass, different permeability, etc, so they won't have equal inductance even if wound with the same number of turns.

[duck_arse]

I'm going broke from paying all this dumb money.

[R.G.]

Sorry to be late on this one. Yes, you can use transformers as inductors. A transformer is an inductor, a special case.

When you use only one winding, all the others open circuit, then the inductance you see is proportional to the square of the number of turns in that winding. Since number of turns is what gives you the voltage, the inductance you see on a different winding can be estimated as the inductance of the first winding times the square of the voltage ratios.

If you use more than one winding, as noted, turns is turns, and you get an inductance proportional to the square of the new total of turns, which can be estimated as the total of voltages on the conjoined windings squared.

If you short one winding and measure the inductance on another winding, you measure the leakage inductance as seen on the winding you're measuring.

Using the primary of a 1K:8 small audio transformer for a wah inductor has been a trick that's floated around the pedal world for a long time, back in the 90s. Started because back then there were not a thousand cottage shops selling wah inductors, nor 0.5H inductors available from vendors.

One other issue you run into using transformers as filter inductors is that the inductance of the windings is not well controlled by the manufacturers for audio and power transformers. The maker is just trying to ensure that there is ENOUGH inductance for it to meet the audio or power specs, not that it has the correct inductance +/- some tolerance. So be prepared to measure and allow for the actual inductance, and if you're going to manufacture/sell stuff with these, for the variation of inductance from unit to unit, which can and will be large.