Transformers as inductors

[GibsonGM]

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.

Yeah, I also 'read that 1991 book', ha ha.  I am more curious what happens when it's NOT a 1:1 and we try to run the 2 coils in series...do they interact or not.    SEEMS that perhaps they do not. 

I'd like to 1) measure the H with an inductance meter if you put the coils in series, and then
2) see what result you get if you actually use the 2 coils run this way...test if the inductance stays where you think it will be.

[PRR]

As R.G. says, *square of turns count*.

So double the turns, 4X the inductance.

Consider the other factors. Copper resistance doubles. Maximum voltage doubles. Maximum current is half; due both to copper loss and iron saturation.

Any other answer violates natural law (no free lunch).

> what happens when it's NOT a 1:1 and we try to run the 2 coils in series...do they interact or not.    SEEMS that perhaps they do not.

1.01H and 1H is obviously the same deal, so near we do not care.

If we need an exact answer, we can work it out.

1H and 2H on the same core: obviously the 2H has 1.414 times the turns. So total we have 2.414 as many turns as the 1H winding. 2.414^2 is 5.8H.

> measure the H with an inductance meter

We have been deluding ourself that we can know the inductance of an iron-core winding. WE CAN'T, without massive measurement and graphs. It will vary with voltage/current and with frequency. So putting two coils series may, by splitting the test voltage, change the observed inductance up OR down depending which side of the permeability peak we were at. If you reach saturation the inductance mostly goes away. Inductance is max at DC (where it is meaningless) and for typical sheet iron is falling to ~~400Hz and falling faster above that (making a 1KHz inductance meter little use for much audio). Most sheet iron has gone useless (or too lossy) before the top of the audio band: most "iron core audio" transformers act as air-core by 20KHz.

When theory washes-out, just build it and try.

[rankot]

I am actually trying to make Acoustic 220 preamp, so I will try with 0.7H (which is inductance of small 600:600 transformer) instead of 1.5H, and I will change appropriate capacitor from 4u7 to 10u. I will report here about the problems, if any.

[rankot]

Just to report: this small transformer can be used as a substitute for this inductor, just had to add one 3k3 resistor after bass pot CW leg.

[blackieNYC]

What is it that is being said here?  Are we saying that the two coils of T1 connected as shown, and the coils of T2 and T3 connected as shown, are the same inductance, even though two coins are interacting thru a core and two coils are not?

[rankot]

I wasn't sure about that, so I've decided to use only one half of a transformer and adjust appropriate capacitor accordingly.

[duck_arse]

I think we are saying "yes", but in the real world, it might not work out exactly so. a coil of wire on a core produces an inductance. two coils of wire on the same core [and phased] creates a single coil of wire, on a core. two inductors in series still produces an inductance.

[rankot]

I didn't try to use both primary and secondary (they are the same, it is 600:600 transformer) because connecting them will not produce L*2, but actually it will work as a single winding with double number of windings.

[merlinb]

As well as what PRR said about iron being stupidly nonlinear, remember when using a transformer as an inductor that shunt capacitance across an unused winding is reflected back across the device. If you're using a lower-voltage winding as your inductor this can mean an unexpectedly large effective capacitance being reflected across it, which rather spoils its inductive properties and can ruin your cute LCR meter readings.

[rankot]

Thanks Merlin, but this seems too complicated for me to understand - English is not my primary language You mean that if I don't connect one winding, it will affect connected one? What shall I do - short unused winding?

[amptramp]

If you short the unused winding, this will be reflected across the core to a short on the connected winding.  There are losses which will reduce the effect from a dead short to something slightly less conductive but do not short the winding if you want to use the connected winding as an inductor.

[Rob Strand]

not that it has the correct inductance +/- some tolerance

Yes, and, for the reasons you gave,  the inductance may vary quite considerably with signal level.

Also, they are normally designed to handle a particular AC level (and frequency) without DC.
The transformers designed to have some DC will have a gap and so the inductance will vary less with level.

[PRR]

Leave unused windings NOT connected. This will have little effect.

If you short an unused winding, you have created an expensive imperfect short. A small resistor would be better, if that is what you wanted. (You don't.)

You can put *identical* windings in Parallel for *slightly* less series resistance. But most low-cost transformers, we can not be SURE two "same" windings have the exact same number of turns. If different, they fight, and tend to act together more like a short. Try it, but do not be surprised if it isn't good.

You can put two (or more) of any windings in series. If they are very different, the result won't be much different from the highest turn-count winding. If they are near-same, inductance can be 4X higher. Or can be near-zero, if they are wired out of phase. (Just try both ways.)

Really, pedal-size transformers, you can connect any which way and nothing will explode(*). We say "act like short" but a transformer is already a near-short to DC, so any connection is safe for your DC bias. It may act short for AC/audio, but for small circuits this just means "no sound"; make a note, put an X next to it, and try another connection.

If you are trying small power transformers as audio inductors, you should probably power-down while changing connections. A small power transformer, suddenly disconnected, might have enough spike-energy to punture small transistors.

(*)This is for small stuff. The power transformers at the end of my street could be connected 6 ways. The power circuit they are in, 3 of those possible connections would blow-out a lot of appliances in homes, 2 of them will start uncontrolled overheating and a BIG explosion of the transformers. (And I am not sure I have counted all the possibilities...) Even a fist-size power transformer with 115V+115V primaries, get it wrong and you burn a hole in your bench.

[rankot]

Thank you all for really great answers!