multiple winding transformer

Multiple Winding Transformers

Thus far we have looked at transformers which have one single primary winding and one single secondary winding.

But the beauty of transformers is that they allow us to have more than just one winding in either the primary or secondary side. Transformers which have more than one winding are known commonly as Multiple Winding Transformers.

The principal of operation of a multiple winding transformer is no different from that of an ordinary transformer. Primary and secondary voltages, currents and turns ratios are all calculated the same, the difference this time is that we need to pay special attention to the voltage polarities of each coil winding, the dot convention marking the positive (or negative) polarity of the winding, when we connect them together.

Multiple winding transformers, also known as a multi-coil, or multi-winding transformer, contain more than one primary or more than one secondary coil, hence their name, on a common laminated core. They can be either a single-phase transformer or a three-phase transformer, (multi-winding, multi-phase transformer) the operation is the same.

Multiple Winding Transformers can also be used to provide either a step-up, a step-down, or a combination of both between the various windings. In fact a multiple winding transformers can have several secondary windings on the same core with each one providing a different voltage or current level output.

As transformers operate on the principal of mutual induction, each individual winding of a multiple winding transformer supports the same number of volts per turn, therefore the volt-ampere product in each winding is the same, that is NP/NS = VP/VS with any turns ratio between the individual coil windings being relative to the primary supply.

In electronic circuits, one transformer is often used to supply a variety of lower voltage levels for different components in the electronic circuitry. A typical application of multiple winding transformers is in power supplies and triac switching converters. So a transformer may have a number of different secondary windings, each of which is electrically isolated from the others, just as it is electrically isolated from the primary. Then each of the secondary coils will produce a voltage that is proportional to its number of coil turns for example.

Multiple Winding Transformer

multiple winding transformer


Above shows an example of a typical “multiple winding transformer” which has a number of different secondary windings supplying various voltage levels. The primary windings can be used individually or connected together to operate the transformer from a higher supply voltages.

The secondary windings can be connected together in various configurations producing a higher voltage or current supply. It must be noted that connecting together in parallel transformer windings is only possible if the two windings are electrically identical. That is their current and voltage ratings are the same.

Dual Voltage Transformers

There are a number or multiple winding transformers available which have two primary windings of identical voltage and current ratings and two secondary windings also with identical voltage and current ratings. These transformers are designed so that they can be used in a variety of applications with the windings connected together in either a series or parallel combinations for higher primary voltages or secondary currents. These types of multiple winding transformers are more commonly called Dual Voltage Transformers as shown.

Dual Primary & Dual Secondary Transformer.

dual voltage transformer


Here the transformer has two primary windings and two secondary windings, four in total. The connections to the primary or secondary windings must be made correctly with dual voltage transformers. If connected improperly, it is possible to create a dead short that will usually destroy the transformer when it is energized.

We said previously that dual voltage transformers can be connected to operate from power supplies of different voltage levels, hence their name “dual voltage transformers”. Then for example, lets say that the primary winding could have a voltage rating of 240/120V on the primary and 12/24V on the secondary. To achieve this, each of the two primary windings is, therefore, rated at 120V, and each secondary winding is rated at 12V. The transformer must be connected so that each primary winding receives the proper voltage. Consider the circuit below.

Series Connected Secondary Transformer.

series connected dual voltage transformer


Here in this example, the two 120V rated primary windings are connected together in series across a 240V supply as the two windings are identical, half the supply voltage, namely 120V, is dropped across each winding and the same primary current flows through both. The two secondary windings rated at 12V, 2.5A each are connected in series with the secondary terminal voltage being the sum of the two individual winding voltages giving 24 Volts.

As the two windings are connected in series, the same amount of current flows through each winding, then the secondary current is the same at 2.5 Amps. So for a series connected secondary, the output in our example above is rated at 24 Volts, 2.5 Amps. Consider the parallel connected transformer below.

Parallel Connected Secondary Transformer.

parallel connected dual voltage transformer


Here we have kept the two primary windings the same but the two secondary windings are now connected in a parallel combination. As before, the two secondary windings are rated at 12V, 2.5A each, therefore the secondary terminal voltage will be the same at 12 Volts but the current adds. Then for a parallel connected secondary, the output in our example above is rated at 12 Volts, 5.0 Amps.

Of course different dual voltage transformers will produce different amounts of secondary voltage and current but the principal is the same. Secondary windings must be correctly connected together to produce the required voltage or current output.

Dot orientation is used on the windings to indicate the terminals that have the same phase relationship. For example connecting two secondary windings together in opposite dot-orientation will cause the two magnetic flux’s to cancel each other out resulting in zero output.

Another type of dual voltage transformer which has only one secondary winding that is “tapped” at its electrical center point is called the Center-tap Transformer.

Center Tapped Transformers

A center-tap transformer is designed to provide two separate secondary voltages, VA and VB with a common connection. This type of transformer configuration produces a two-phase, 3-wire supply.The secondary voltages are the same and proportional to the supply voltage, VP, therefore power in each winding is the same. The voltages produced across each of the secondary winding is determined by the turns ratio as shown.

The Center-tap Transformer

center tap transformer


Above shows a typical center-tap transformer. The tapping point is in the exact center of the secondary winding providing a common connection for two equal but opposite secondary voltages. With the center-tap grounded, the output VA will be positive in nature with respect to the ground, while the voltage at the other secondary, VB will be negative and opposite in nature, that is they are 180o electrical degrees out-of-phase with each other.

However, there is one disadvantage of using an ungrounded center tapped transformer and that is it can produce unbalanced voltages in the two secondary windings due to unsymmetrical currents flowing in the common third connection because of unbalanced loads.

We can also produce a center-tap transformer using the dual voltage transformer from above. By connecting the secondary windings in series, we can use the center link as the tap as shown. If the output from each secondary is V, the total output voltage for the secondary winding will be equal to 2V as shown.

Center-tap Transformer using a Dual Voltage Transformer

dual voltage center tap


Multiple Winding Transformers have many uses in electrical and electronic circuits. They can be used to supply different secondary voltages to different loads. Have their windings connected together in series or parallel combinations to provide higher voltages or currents, or have their secondary windings connected together in series to produce a center tapped transformer.

In the next tutorial about Transformers we will look at how Autotransformers work and see that they have only one main primary winding and no separate secondary winding.


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  • G
    George william

    how many output voltage can take in secondary side of HT transformer and how ?
    ( I need multiple secondary voltage in HT transformer is it possible? or not )

  • R

    Please furnish the period of year and date and first time framing of IS 4540 standards for two secondary winding transformers.

  • T

    this article was very helpful to me especially Center-tap Transformer using a Dual Voltage Transformer. thank you for the information was not sure if it was possible to do that with transformers.

  • B

    I have a question about tapped transformers: what if the tap is NOT precisely at the center? Lets say the tap is about 25% of the way through the secondary, and you put 100 VAC on the primary. You do get 75 VAC on the longer winding and 25 VAC on the shorter winding? And are both signals still 180-degrees out of phase?

    • Wayne Storr

      The tapping can be at any point on the primary or secondary windings and not just the center. The result is the same.

      • B
        Bret M

        Thanks, Wayne. I’m wondering specifically about how a tapped secondary works in a 1920s Neutrodyne radio receiver. An antiphase signal from the tapped secondary is fed back through a tiny value condenser back to the previous tube’s grid. Sometimes the tap is right in the center, sometimes its about 3/4 of the way through the winding.

        But is the signal on the tap phase-inverted? I thought the tap is always neutral and to get an opposite phase signal you’d need to use the bottom of the tapped winding… I’m not an EE, and 90 year old radio circuits might not be your wheel house, but just thought I’d ask…

        • Wayne Storr

          Not necessarily, the neutral can be the bottom of the winding, the tapping point(s), or the top, its where you want it, but voltages and phase angles are referenced from the neutral wherever it is.

          • B

            OK, I think I get it. Whether the tap is “neutral” or not depends on whether it’s used as a common/ground. And depending on which tap/winding lead is grounded, that determines the phase angles and voltages of the other windings. The “center tapped” transformer discussed above is written assuming that the tap is grounded. Thanks for the input!

          • Wayne Storr

            Exactly, 🙂

  • A
    Ajine Emmanuel

    It is really help me in connecting of my small transformer I am so happy the way I do it

  • Y

    I would like to ask why the diode which is presented in the electronic circuits is reversed biased, cause of saying the the cathode presenting the bar and anode represents the arrow. simply the current passes when the positive terminal of the source is connected with the P type-material (Anode) which is positive, and the negative terminal to the N type-material (cathode) which is negative, and said the current in this case will flow. Note: the current direction here is conventional flow.

  • i

    What is reason low voltage coil inner side of high voltage coil in power transforms

  • R

    You say “It must be noted that connecting together transformer windings is only possible if the two windings are electrically identical.” This is only true for parallel connections. I could connect a 12V secondary in series with a 4 volt one to get 16V (or 8 if connected anti-phase). The current will be limited to that of the lowest rated winding.

  • h

    I have a DV 4295 transformer (salveged from an unknown purpose PCB) with multiple winding on the secondary side (2×17.5V 2×1.5VA). I could not find the datasheet for it and there is no indication on it’s housing about the dot orientation. I would like to connect the secondaries in parallel to have available cca. 3VA. Are there any method to determine (or measure) the dot orientation of secondary windings?

    • Wayne Storr

      Single-phase transformers can be connected in parallel but they must have the same secondary terminal voltages and their polarities must be correct to ensure that the current outputs of the transformers will be in proportion to their kVA capacity. When transformers are supplied by different manufacturers or have different secondary windings with no dot orientation it will therefore not be known whether they have additive or subtractive polarities when their secondaries are connected together. If the dot orientation are in the same direction the voltages will be subtractive and if the are opposite the voltages will be addictive.

      There are a couple of ways do a transformer polarity test. Assume Primary terminals are P1 and P2 and the Secondary terminals are S1-1 and S1-2 for the first, and S2-1 and S2-2 for the second, (two windings). You do not state primary voltage, so we will assume 120v for now.

      To test one secondary at a time with NO LOAD attached. Short P2 and S1-2 together. Connect a voltmeter across P1 and S1-1 and connect primary to supply voltage (120v in this case). If dot convention is same, that is correct then the voltmeter reads Vp minus Vs (120 – 17.5) subtractive. If the voltmeter reads Vp plus Vs (120 + 17.5), addictive then the windings are reversed. The beginning and end of the primary and secondary windings can then be marked as you want and then repeat for the second secondary winding.

      • h

        Thank you for the detailed answer. I have done the tests following your instructions and got the results similar to your description. (230VAC/50Hz Primary, 19VAC Secondary – both; with no-load. Results: 202VAC and 249VAC). I suppose, the dot position sould be marked by the position of voltmeter connection terminals, where the reading was 202VAC.
        Being a beginner in electronics at my age (56) the following additional questions have arised (in my mind): Would this metod of testing work for center tapped transformers, too? As far as I know, there are several methods of winding (single/multiple wires, same/opposite winding directions and the combination of those), so the testing methods probably should be different for each case. Could you please write an article about it, or suggest any link(s) for further studies?

  • m

    to add, it will look like this: (if image gets messed up i’ve marked where ‘enter’ aka carriage return should go, if you copy and paste into notepad.

    —– hydro -240— (enter)
    120v N 120v (enter)
    l l l (enter)
    wwwwwwwwww (enter)
    ====== x-fo ==== (enter)
    mmmmmmmm (enter)
    l l (enter)
    0v 120v (enter)

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