# How multiple winding transformer work?

#### Toni Rodri

Joined Dec 3, 2023
19
Hey
How multiple winding transformer work
for example if I have 1 winding in the primary and 2 windings in the secondary (like the topology in the picture).
What Happen if I connect the port (5-6) to ac voltage? what I will see at port (3-4) and (2-1)?
There is a leakage to port (3-4) if I put sine wave at port 5-6?
and what happend if I put sine wave at port (1-2)?
(I assume that the ratio is 1:1:1)
Thank you very much

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#### ronsimpson

Joined Oct 7, 2019
2,894
Assuming 1:1:1.
If you put an AC voltage across any winding, all other windings will have the same voltage across them. It does not matter which winding you drive.

#### BobTPH

Joined Jun 5, 2013
8,662
Since your 3 coils are 1:1:1, you can drive any of the 3, and the voltage on the other two will be the same as the input.

#### BobTPH

Joined Jun 5, 2013
8,662
Man, I shoulda hit POST faster!

#### MisterBill2

Joined Jan 23, 2018
17,800
Power transformers often have multiple secondaries. The first thing to know is that the total power out will be a small bit less than the power in. and the thing to avoid is putting power in to more than one winding, usually, there are exceptions.

#### MrChips

Joined Oct 2, 2009
30,476
Power transformers are designed to convert voltages at 50/60Hz LINE frequency. They are not as efficient at other frequencies.
Pay attention to which side is primary and which is secondary.

#### Tonyr1084

Joined Sep 24, 2015
7,777
For a 1:1 ratio (I know you said 1:1:1) you have an isolation transformer. All you're doing is isolating the AC voltage from the AC source. You might use such on a circuit you don't want grounded.

The transformer you show is typical of one that has options. Suppose your transformer primary is designed to operate at 120VAC and each secondary is designed to output 12VAC each. You could parallel the secondaries (pins 4 & 6 and pins 3 & 5) to have 12 volts but twice the amperage. Or you could connect in series (pins 4 & 5) for an output of 24VAC but not at a higher amperage.

CAUTION: Putting a higher voltage on a coil designed for a lower voltage could result in extreme overheating and possible fire. The windings of a transformer typically use different gauge wires because they're meant for different voltage(x)current (VA which stands for voltage times amperage) (In a DC circuit you call it W - or Watts). Either way VA or W both refer to "Power". Wire has resistance. The longer the wire the more resistance. The smaller the gauge the more resistance. If you take a low voltage winding designed for high amperage and you put high voltage on it (say a 12V secondary) your output on the primary will be (in theory) 1200VAC. However, and here's the CAUTION part: The low number of turns and heavy gauge wire will draw a whole lot of current in the secondary; a case that was never intended. You'll pop the breaker or blow the fuse. If no such protection exists then you will burn out the transformer very quickly. There may be sparks, smoke and possibly a lot of excitement (a.k.a FIRE!)

While in theory you can do that - in practical applications you would never drive a coil at a higher voltage than it's designed for. Remember Ohm's law: Current ( I ) is equal to Voltage ( V ) over Resistance ( R ). A 12V coil rated for 10 amps will have a resistance of 1.2Ω. If you change that voltage to 120V, at 1.2Ω (the resistance isn't going to change) ((( and for those who know this for a fact - coils are not referred to in resistance but rather in reactance ))) if you just look at the resistance the wire is going to present, at 1.2Ω and 120V your current will be 100 amps. Likely FAR MORE than the coil is designed for.

Yes, I spoke incorrectly about the resistance of a coil - sort of - magnetics have an interplay with the "resistance", which is why it's called "Reactance" and not resistance.

In short of this rather lengthy editorial, yes, a 1:1:1 ratio will result in the same voltage on all coils. It's highly unlikely you'll find a transformer with a 1:1:1 ratio. That would have to be a specialty wound transformer for a highly specific purpose. One I can't think of.

#### crutschow

Joined Mar 14, 2008
34,047
An AC voltage applied to any winding, will generate magnetic flux in the transformer core directly proportional to the voltage and inversely proportional to frequency (up to the point where the core magnetics saturates).
This will generate a voltage on any other windings wound on the core, proportional to the turns ratio of those windings compared to the winding being driven.

#### Tonyr1084

Joined Sep 24, 2015
7,777
This is a FeroResonant Transformer. It's used for AC voltage stabilization. 120VAC in and 120VAC out. It's also an isolation transformer. The extra coil with the capacitor is the resonant part of the transformer. The primary voltage can vary greatly, such as 109VAC to 135VAC and its output will remain nearly constant 120VAC.

This is a multi-tap transformer out of a stereo I scrapped years ago. To this date I've not found a use for it. Yet I still hang onto it. I don't know the amperage or wattage of each set of windings. Across A & C is 65V. Across D & H is 32.6V as measured by multimeter. Notice I & J - their output is 5.1V. Likely it's for a logic part of the stereo circuit. Chances are it's going to be rather low amperage. Pin B is a center tap for the 65V output. That's likely what drove the amplifier. Output values at E & G are possibly the stereo pre-amp portion with F as a center tap providing 12.1V from E to F and F to G. The rest of the possibilities I can't imagine what they may have been for. Maybe for the fluorescent display screen - but I don't know that for a fact.

#### Tonyr1084

Joined Sep 24, 2015
7,777

#### MrAl

Joined Jun 17, 2014
11,268
Hey
How multiple winding transformer work
for example if I have 1 winding in the primary and 2 windings in the secondary (like the topology in the picture).
What Happen if I connect the port (5-6) to ac voltage? what I will see at port (3-4) and (2-1)?
There is a leakage to port (3-4) if I put sine wave at port 5-6?
and what happend if I put sine wave at port (1-2)?
(I assume that the ratio is 1:1:1)
Thank you very much
Hello,

As others have said, if you put a voltage on one winding it will (theoretically in simplest terms) appear on the other windings too.

However, many transformers have a primary and secondary, where the primary takes a higher voltage than the secondary puts out. For example, 120vac in and 12vac out. So many transformers are not 1:1 or 1:1:1 they are a lot different. If you tried to put 120vac in the secondary side of that 12vac output transformer, the winding could vaporize if the circuit breaker or fuse does not blow open first. It's a dangerous thing to do.
You might be able to input 12vac to the secondary and get around 120vac out from the primary, but it may not work that great, it depends on the transformer.

There is also electrical current to consider. With that 1:1:1 transformer assuming it can take 120vac input on the primary, the two secondaries could put out 120vac too, but the current may be limited for those two. Then it depends on the wire gauge used on the primary and secondary. You'd have to try to find that out too.

#### k1ng 1337

Joined Sep 11, 2020
910
You'll learn a lot by winding transformers and taking measurements. I recommend starting with isolated low voltage DC-DC converter circuits because it's the same principle.

Verbal descriptions are pretty much useless to me in this area because everyone describes ideal transformers. Inductive coupling was the hardest thing for me to wrap my head around when I started learning electronics because it varies so much from the ideal scenario.

#### MisterBill2

Joined Jan 23, 2018
17,800
A more detailed answer to the question in post #1 is simply that a voltage applied to any one winding will appear on all of the other windings in proportion to the turns ratio. ALL COMMON transformers are bi-directional, signals will travel in either direction equally well.
Some very special transformers are a bit different, and they are much more complex. That includes "flux-gate" devices, used in some compass systems.

#### MrAl

Joined Jun 17, 2014
11,268
You'll learn a lot by winding transformers and taking measurements. I recommend starting with isolated low voltage DC-DC converter circuits because it's the same principle.

Verbal descriptions are pretty much useless to me in this area because everyone describes ideal transformers. Inductive coupling was the hardest thing for me to wrap my head around when I started learning electronics because it varies so much from the ideal scenario.
Hi,

I could post a couple models if that would help. They include the leakage inductance so it's a bit more practical. This isn't the models you might find in LT Spice, these are models where you can set the various inductances to find out how it changes the entire circuit.

This goes for anyone else also interested in this. It's a bit more difficult to do but not exceedingly so.

Joined Jul 18, 2013
28,513
How multiple winding transformer work
for example if I have 1 winding in the primary and 2 windings in the secondary (like the topology in the picture).
What Happen if I connect the port (5-6) to ac voltage? what I will see at port (3-4) and (2-1)?
There is a leakage to port (3-4) if I put sine wave at port 5-6?
and what happend if I put sine wave at port (1-2)?
But powering a secondary as use as a primary the device suffers loss of efficiency, This is why primary's are typically always wound first, with secondary's wound over the primary.

#### k1ng 1337

Joined Sep 11, 2020
910
Hi,

I could post a couple models if that would help. They include the leakage inductance so it's a bit more practical. This isn't the models you might find in LT Spice, these are models where you can set the various inductances to find out how it changes the entire circuit.

This goes for anyone else also interested in this. It's a bit more difficult to do but not exceedingly so.
I'm interested.

#### crutschow

Joined Mar 14, 2008
34,047
But powering a secondary as use as a primary the device suffers loss of efficiency, This is why primary's are typically always wound first, with secondary's wound over the primary.
Also I the secondary is normally wound with a few more turns than the theoretical to allow for winding resistance so that the rated voltage is achieved at rated load current.
This would mean a 1:1 isolation transformer would have a lower voltage if the primary and secondary were reversed.

#### MrAl

Joined Jun 17, 2014
11,268
I'm interested.
Hi,

Oh ok great. I'll try to get back here a little later with some drawings.
It's nice to see other people interested in this stuff.
Oh in the mean time, one of them is the two port T model which you can also find on the web easily.

#### MisterBill2

Joined Jan 23, 2018
17,800
Transformers typically are used in one of two classes, either for Power, where they serve to change voltage levels and provide isolation, or Signal, where they transform variable voltages to other amplitudes and impedence levels and provide isolation as well.
Power transformers are mostly optimized for efficiency , while signal transformers are usually designed for minimum signal distortion. And in most instances minimizing the cost is also a factor in the design,
Those goals are often in competition, and so in many cases signal transformers do not serve well as power transformers, and many power transformers serve poorly in audio applications.
The result is that transformer design is more complex than the basic formulas imply.