Small Transformer Making Procedure - for Low Power PCBs

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Introduction: Small Transformer Making Procedure - for Low Power PCBs

Winding house-keeping power-supply transformer
(manual procedure)

Please find pdf version here.


Introduction:
This document illustrates how to wind the transformer used to obtain house-keeping power supply voltages on typical small circuits. Such transformers have been used on PCBs .

This transformer is used in a blocking-oscillator configuration to generate the relevant low voltages required to run chips on a typical PCB design. Power levels up to a couple of watts can be supplied by this transformer. The input and output sections/windings are completely isolated with respect to each other. In this particular example the transformer output generates a +5V, +9V, -9V and another isolated (with respect to the these three supplies) power supply of +5V. Typical currents drawn from the supply: +5V@500mA, +9V@100mA, -9V@100mA, +5V@100mA. So the total power provided is approximately 5Watts.

Material required:
The following materials are required to wind this transformer:
1) EE 25x7 Ferrite core
2) Bobbin (horizontal : implying the core lies horizontally) for this core
3) Wire: 35 SWG (Standard Wire Gauge) enameled wire, or dual coated wire
4) Cello-tape (10-12mm width) or Taco Tape
5) PVC insulating tape for SMPS design (5mm wide roll)
6) Teflon tape – used by plumbers – available at any hardware store
7) Fevikwik or any equivalent cyano-acrilite adhesive
8) Glue stick pieces – for securing the winding in certain steps
9) Tools: Thin triangular file, knife, soldering iron/solder wire.


Ferrite Core and Bobbin for use in housekeeping supply

Material required: Tapes and Glue

Preliminaries:
The following diagram illustrates the winding details of the transformer. The most important thing to follow is the dot-convention. The diagram along-side shows the winding details for our house-keeping transformer. The view is from the top. That is as you see it from top of the PCB. The pins are on the other side of this view. All the three primary side windings (mains, 230V side) are connected to pins on one side of the bobbin, and all the low voltage secondary side windings are on the other side. This provides for an extra degree of safety to isolate the primary and secondary side of the transformer windings with respect to PCB connections and tracks.

Between each winding, electrical tapes of various kinds are applied, to provide adequate insulation, and prevent shorting of turns. Summary of the number of turns in each winding:
1) NP : 100T of 35 SWG wire on layer 1 - 2 or 35, between pins 8 (dot) and 9
2) NP2: 5T of 35 SWG wire on layer 3, between pins 9 and 10 (dot)
3) NP1: 5T of 35 SWG wire on layer 3, between pins 6 (dot) and 7
4) NS1: 8T of 35 SWG wire on layer 3, between pins 4(dot) and 5
5) NS2: 4T of 35 SWG wire on layer 3, between pins 2 and 3(dot)
6) NS3: 8T of 35 SWG wire on layer 3, between pins 1 and 2(dot)
Dot Convention:
The most important thing about these transformer designs is to strictly adhere to the dot-convention. Dots indicate phase relationship (same or opposite polarity) of different windings. Two windings that have dots on one of their terminals will have the same voltage polarity with respect to the other terminal, and any time.

Physically, this means that both windings, say starting from the ‘dot’ end, will have the same sense of turning around the core (say clockwise for both or anticlockwise for both). Both turn the same way, and this is the most important thing to remember in dot conventions. If they turn the opposite way, then the phase difference will be 180 degrees (i.e. of opposite polarity).

To avoid confusion during winding, it is best to mark small dots on the plastic bobbin using the hot tip of your soldering iron. It is also good to mark pin 1 with a ‘1’ mark, so that all the other pins may be located unambiguously (follow the same convention used in IC pin numbering). In the images below the dots are marked. I did not get to mark the ‘1’ at this stage, but did it at the end. It is better to mark at the start itself.


Construction:
The following procedure illustrates the step-by-step construction of this transformer. Most of the steps are indicated with images, so that there is no confusion about the steps.

Step1: Wind NP
Start NP, the primary side winding. We start by removing insulation from the end of the enameled wire (35 SWG), say 15mm and then tinning that end. Then solder it to pin8 of the bobbin.

This winding will terminate on pin9 (dot). In the steps we followed, we started the winding sense going counter clockwise (from our view in this photograph).




The first few turns: This shows how NP is wound. The wire must be tightly wound (without stretching or bending and kinking of any kind) around the bobbin. Any gap between the turns must be avoided. Try to keep the gaps minimized by pushing the turns towards the side the winding star

Dimensionally the bobbin can take up to 56-57 turns. Our target is to put 55 turns on this first layer.

Once the first layer is done, continue to wind on layer two, i.e. above layer 1, until the winding reaches the starting end once again. Typically, if the winding is done with care (remember we are doing this winding manually), one should be able to accommodate 55 turns on each of the layers. The two layers together will therefore provide the full 110 turns which are required for NP.

The photograph here shows the two layers of NP done. The wire terminates on pin9 (dot), which is not yet soldered.

This wire will continue to make the next winding to pin10. But first the surface of this winding must be insulated with tapes. Notice a few small errors during the winding led to a little anomaly close to the center of the winding.
Step2: Insulate NP
 
The Teflon tape being soft, makes a nice insulating layer, just above NP. The surface becomes smooth when the softer and fragile Teflon tape insulation is enhanced with cello-tape for mechanical strength. The other windings are may be put on top of this insulating layer. Now that NP is secured well in the bobbin, one can remove insulation from the enameled wire and solder it to pin9 (dot) of the bobbin.

Step 3: Wind and insulate NP2
The NP2 winding consists of only 5 turns of wire (35 SWG). For winding NP2, pin10 is the dot end. So, when starting from pin9, one winds the wire going in the direction of pin8 (i.e. counter-clockwise, as was done with NP). The winding terminates on pin10, which is the dot end.

It is important to make sure that the PVC tape insulation is also provided between the return wire (going to pin10 and the already wound 5 turns of wires of NP2, starting from pin9. This process will be true for all such windings.
Step 4: Wind and insulate NP1
 
Now all three primary side windings are done (pins 6-10). As the photograph shows, these windings are all well insulated from any other winding that may be created for the primary side (pins 1-5). In this application, the secondary (low voltage) windings have very few turns, and they could all get accommodated on the remaining half of the layer3 space. These secondary windings must also be well insulated with respect to each other.


Step5: Wind NS2
Notice that pin3 is dot end with respect to pin2 for winding NS2.

The important thing about this transformer is that the dot convention may be followed separately for all the primary windings or all the secondary windings. Between all the secondary windings, NS1, NS2 and NS3, the dot convention must be followed.

So here, we actually have the winding’s direction sense reversed as compared to the primary. Here we have starting at dot end go counter-clockwise (remember the bobbin is now seen from the other side!).
Step6: Wind and insulate NS3
NS3 was wound again making sure that the cross-over of its end going to pin1 is insulated, using PVC tape, from its 8 turn winding below it.

Cover this winding with PVC tape so that the final winding NS1 can be done on top of it.

Pin 2 is dot end, for this winding between pin1 and pin2. 8 turns, closely spaced, and wound closer to the secondary side of the bobbin, ensures decent functional properties.



Step 7: Wind and insulate NS1
 

A final look at the dots on primary and secondary sides:

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    My detailed notes on how to do winding of this transformer, dot convention for transformers

    ---------------------------------------------------

    NP

    ---

    To be wound: #1

    Winding type: Primary winding

    Pins : Pin 9 (dot end, winding end pin), Pin 8 (winding start pin)

    Forms layer: 1 - 2

    No. of turns: 110 T 35 SWG

    Winding direction: Also known as winding sense. Start your transformer winding procedure from doing NP winding. First, remove enamled wire insulation (around 15 mm approx.), tin (a soldering concept) its end. Then solder it to pin8 of the bobbin. This winding will terminate on pin9 (its dot end, worrying about what is dot end? Don't worry. There is nothing so hard and special about dot convention; see my NP winding note #3 for detail about this first glance looking arcane concept to an elctronics newcomer like me, just ensure you maked the dots on each pin as indicated in the pictures of this article and only concentrate on your primary winding, then use dot convention to derive corresponding secondary winding direction. Corresponding means the secondary winding in front of paricular primary winding). Start from pin 8, wind in counter clock wise direction (i.e. going towards pin 7, while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side). The wire terminates on pin9 (dot end), but we will not solder this end right now. First we will do our NS winding electrical isulation using teflon and cello tapes. Why we need do insulation first before soldering final end? Because it provides ease of soldeing, by securing NS winding on bobbin by insultaing it from other next windings to be done over it and providing extra stength through the use if cello tape. So, we do insulatation of NP winding now. Insulate winding with teflon tape. The Teflon tape being soft, makes a nice insulating layer, just above NP. The surface becomes smooth when the softer and fragile. Then, teflon tape insulation is enhanced with a cello-tape insulation which provides extra mechanical strength. The other windings (NP1, NP2, NS1, NS2 and NS3) goes on on top of this insulating layer formed by tapes (top most we will have cello tape at this stage). Now that NP is secured well in the bobbin, one can remove insulation from the enameled wire and solder it to pin9 (dot) of the bobbin. NS primary winding is completed.

    Insulation: Teflon tape then above it cello tape (for extra mechanical stength).

    Inductance value: 19 mH

    Notes:

    1. About layer and required number of turns. Dimensionally the bobbin can take up to 56-57 turns. Our target is to put 55 turns on this first layer. Once the first layer is done, continue to wind on layer two, i.e. above layer 1, until the winding reaches the starting end once again. Typically, if the winding is done with care (remember we are doing this winding manually), one should be able to accommodate 55 turns on each of the layers. The two layers together will therefore provide the full 110 turns which are required for NP.

    2. The wire must be tightly wound (without stretching or bending and kinking of any kind) around the bobbin. Any gap between the turns must be avoided. Try to keep the gaps minimized by pushing the turns towards the side the winding starts.

    3. Dot convention and its use. Dot convention simply tells voltage (current) relationship of primary and corresponding secondary winding. From this dot convention based information, if we initially know our primary winding direction (also known as winding sense), we can derived our corresponding secodary winding sense during transformer design and construction. That's it! Nothing hard, nothing special!

    So, what is Dot Conention in transformer: We use the "Dot Convention" to indicate polarity on transformer windings. The polarity has to do with the direction that the winding goes around the core. The "Dot" is placed on the "+" end of the winding. If the primary winding and the secondary winding are in the same direction, i.e., wound the same direction around the core (in this case we see dots on same side of primary and its corresponding seconday winding in the transformer diagram), then they are "in phase" - the voltage sine waves will have no phase shift (i.e. when primary side voltage has positive values, induced corresponding secondary side voltage also has positive values. Similary, when primary side voltage has negative values, induced corresponding secondary side voltage also has negative values) If the secondary, for instance, is wound in the opposite direction, then the polarity will be opposite, and the Voltage sine will be exactly 180 degrees out of phase (i.e. when primary side voltage is positive, secondary side voltage is going to be negative and vice versa).

    So, how to use Dot Convention to determine secondary winding direction / sense? Let's take case of our first NS primary winding. From NP winding direction guidelines (see above NP winding direction) we know, NP winding is in CCW (counter-clockwise) direction. So, as dot convention mentioned by the author of this article, we see NP primary winding has pin 9 as its dot end and its corresponding NS2 secondary winding has pin 3 as its dot end. We see here dots on the dot ends for primary and secondary windings are placed on the opposite direction (i.e. they are not on same side). So, we conclude from above learned concept of Dot Convention in transformer that, the primary winding (in this case NP winding) and the corresponding secondary winding (in this case NS2 winding) are going to be in opposite direction (i.e. not in the same direction). Since, in our case of NP winding construction case, we started from pin 8, wound it in counter clock wise direction (i.e. going towards pin 7, while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side). Following dot convention, then for NS2 winding we do its winding in opposite direction to NP winding as follows: Assuming bobbin direction keeping same that we have used while winding NP (i.e. while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side), we start from pin 2 (we can also start from pin 3, as author of this article suggest, it does not matter, only matters is that we should do our winding say for NS2 in correct direction. Directions are CW and CCW.), then for NS2 winding start winding in clockwise direction while moving towards pin 1, go beneath bobbin, come up and so on. Remember, you are winding in clockwise direction (while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side). Going from start side and moving towards farther side of the bobbin, complete 4 turns of 35 SWG wire. At last, remove enamle of the final end of the wire (around 15 mm), tin the end and solder other end to pin 3 (in case our start pin is pin 3, then final pin will be pin 2) of NS 2 winding. Insulate NS2 winding which forms layer 3 with PVC tape (blue). NS2 winding is complete.

    For convenience, before starting first NP winding itself, we should mark this transformer's dot convention dots based dots on the bobbin, using a soldeing iron tip. Please note again, that choice of our start and end pin of winding does not matter. Only matters is the correct winding direction. So, if we go through suggestion by the author of this article and choose pin 3 as our start pin, then we should be assuming that we have placed our bobbin such that pins 1 - 5 are away from us and pins 10 - 6 are towards our side.

    4. Why there are multiple (not single pair of) primary and secondary windings in this transformer design? This is because this single transformer design is capabale for providing us three different voltage outputs: +5 V @ 500 mA, +9 V @ 100 mA and +5 V @ 100 mA.

    NP2

    ----

    To be wound: #2

    Winding type: Primary

    Pins : Pin 10 (dot end, winding end pin), Pin 9 (winding start pin)

    Forms layer: 3

    No. of turns: 5 T 35 SWG

    Winding direction: Counter-clockwise (CCW). Similar to NP winding direction. Start from pin 9, wind in counter clock wise direction (i.e. going towards pin 8, while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side). The wire terminates and soldered finally to pin 10 (dot end). Insulate this NP2 winding that forms layer 3 with PVC tape (blue). NS2 winding is completed.

    Insulation: PVC tape.

    Inductance value: 29 micro H

    NP1

    ---

    To be wound: #3

    Winding type: Primary winding

    Pins : Pin 6 (dot end, winding end pin), Pin 7 (winding start pin)

    Forms layer: 3

    No. of turns: 5 T 35 SWG

    Winding direction: Counter-clockwise (CCW). Similar to NP winding direction. Start from pin 7, wind in counter clockwise direction (i.e. going towards pin 6, while bobbin is placed such that pins 6-10 are away from you and pins 5 -1 are towards your side). The wire terminates and soldered (like guided while doing first NP winding) finally to pin 6 (dot end). Insulate NP1 winding that forms layer 3 with PVC tape (blue), similary as we have done with NP2 winding. NP1 winding is completed.

    Insulation: PVC tape.

    Inductance value: 29 micro H

    NS2

    ----

    To be wound: #4

    Winding type: Secondary winding

    Pins : Pin 3 (dot end, winding end pin), Pin 2 (winding start pin)

    Forms layer: 3

    No. of turns: 4 T 35 SWG

    Winding direction: Clockwise (CW). How to wind? Follow dot convention in transformer used to determine secondary windings direction (should I wind CW / CCW?). NS2 winding procedure is explained in detail in note #3 of NP winding.

    Insulation: PVC tape.

    Inductance value: 15 micro H.

    Notes:

    1. Between all the secondary windings, NS1, NS2 and NS3, the dot convention must be followed. See NP winding note #3 for detail explanation on dot convention used in transformer.

    2. Please, note as already said in NP winding note #3, start and end pin selection while doing winding does not matter. Only matters is the correct winding direction. So, if we go through suggestion by the author of this article and choose pin 3 as our start pin, then we should be assuming that you have placed our bobbin such that pins 1 - 5 are away from you and pins 10 - 6 are towards your side. Then seeing the bobbin in the placement, our NS2 winding direction will be counter-clockwise (CCW). Choosing start pin and end pin of winding just depends on the ease the user while doing winding during transformer construction. Also, start pin, end pin choice of a given winding has no relationship with dot convention concept used for transformer.

    3. After doing and completing each secondary winding NS2, NS3 and NS1 (given in order in which winding to be wound first), it has to be insulated with PVC tape (blue) so as to insulate it from next new winding to be done.

    NS3

    ---

    To be wound: #5

    Winding type: Secondary winding

    Pins : Pin 2 (dot end, winding end pin), Pin 1 (winding start pin)

    Forms layer: 3

    No. of turns: 8 T 35 SWG

    Insulation: PVC tape.

    Winding direction: Clockwise (CW). How to wind? Winding procedure exactly same as NS2. Start from pin 1, goining beneath the bobbin moving towards pin 2, wind in clocwise direction. For detail see similar NS2 winding procedure in NP winding note #3. Finally insulate with PVC tape.

    Inductance value: 88 micro H.

    NS1

    ---

    To be wound: #6

    Winding type: Secondary winding

    Pins : Pin 4 (dot end, winding end pin), Pin 5 (winding start pin)

    Forms layer: 3

    No. of turns: 8 T 35 SWG

    Insulation: PVC tape.

    Winding direction: Clockwise (CW). How to wind? Winding procedure exactly same as NS2. Start from pin 5, goining on top surface of the bobbin moving towards pin 4, wind in clocwise direction. For detail see similar NS2 winding procedure in NP winding note #3. Finally insulate complete winding area of the bobbin with PVC tape, as it is final winding.

    Inductance value: 88 micro H.

    I'm an electronic neophyte. These notes are evolved during my SMPS power supply design learning and while reading this great beginner friendly article as part of it. Though, author of this article has done the great work in providing method of simple transfomer design, but during reading I found the need for detail eloboration for few parts, specially realted to winding procedure for each windings of the transformer; which I think core gist of transformer construction. Hence, this effort. Please, let if something is missed or need correction for betterment.

    Thanks for reading.

    HSKB1.jpg

    ​How to calculate winding with the ratio between the bobbin diameter,number of turn and voltage ?

    For example 220V primary and I have a small bobbin with 3cm diameter and Np = 45T....how many winding do I need for it ?

    Can you provide me the addresses of suppliers India for good quality ferrite cores and bobbins.

    very good information .I am electronics service engineer practicing like this works.i will try to rewind my defective smps transformer of tv using ur techniques.thank u very much...

    sir, i'm interesting to make a transformer for this project

    http://redcircuits.com/Page124.htm

    it needs a small transformer with 230v input, 12v output 100 to 150 mA. How much turn main and secondary do i need? it said that i have to reverse connected the transformer

    Regards
    Prasetyo