How do I identify the primary and secondary windings on a transformer?

[mattpd]

I am a beginner and I am trying to repair some SMPS. I have a blue ring tester to test inductive components, but I am confused about which pins on the transformer I should be testing.

Of course the primary winding is on the primary side of the power supply board, and the secondary winding is on the secondary side. And if there are only 2 pins per side, then it's pretty straight forward.

I am confused by transformers that have multiple pins per side. How do I know which pins I should be testing? Is there a way to look at the board and determine the pinout?

[twbranch]

Re: How do I identify the primary and secondary windings on a transformer?

Grab the datasheet using the part number. If no part number...the primary comes from the input to the transformer (typically goes to wall adapter). The secondary will be coming from the Tx to components. I would have to see the application for the transformer to get a definitive answer.

Pictures would help.

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

Transformer can be used forward or back ward, lets say that we make one winding to be primary that will be fed with 120v (120 turns of wire), the other windings are winded so it will have 10v (10 turns of wire) on one winding and 20V (20 turns of wire) on another winding, so this transformer has 3 windings, if 120v is apllied to the 120V winding, the other two winding will be putting out 10v and 20v (based on turn ratio).
But if we make the 10V to be called primary, if you applied 10v to it, the 120v winidng will be putting out 120v, the 20v will be putiing out 20v.
When you use ringer test, if one of the winding has shorted turn, the ringer will be showing 1 or 2 rings instead of 5 or more rings. The purpose of using the ringer test if to tell if the transformer has shorted trun in it.

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

Budm,

When you say 3 windings, do you mean 1 primary (120v) and 2 secondary (10v and 20v)?

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

Yes, and you can make any one of the 3 winding to be the primary depends on what you want to feed the voltage with to the winding. If I decide to feed the 10V winding, then the other two will become my secondary side. Some transformers can have 5 windings or more.

[twbranch]

Re: How do I identify the primary and secondary windings on a transformer?

I like Budm explanation better. I can show but i have a hard time explaining.

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

Are their situations where more than one primary winding is used? I have some transformers that have 3 pins on the primary side. I assume that there are two windings being used and they both have one pin in common. Right?

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

Yes, you can have transformer wih two 120V winding, they are put them in parallel when it is used with 120V power line, or they can put them in series when thy want to use it oversea that runs on 230~240VAC power line.
Or they can have multi-taps primary that can be use with 100V (Japan), or 110, 120, 220V TAP.
http://www.decdun.me.uk/gc/trafo%20wiring.jpg
http://taweber.powweb.com/store/PTGPsch.jpg

[mariushm]

Re: How do I identify the primary and secondary windings on a transformer?

Ok, he's talking about SMPS transformers.

These are bit different than the regular power transformers.

Most have 2 pins on the side for input, and several pins on the other side for output, plus a big wire for the common (ground). Here's an example:



Some of these transformers also have a small loop of wires on the primary side for feedback to switching controllers.

For example, check this transformer out: http://www.digikey.com/product-detai...6-1-ND/2445744

If you save the datasheet to disk and load it you can see it has an auxiliary loop on the primary - the controller uses it to work properly.

If you have a random transformer and you're not sure which is the primary side, I'd suggest connecting a couple of pins from a side to a multimeter set on 250-750v AC then use a normal AA battery to send small pulses on the other side. Connect and release, and repeat, you're sort of making a square wave of AC voltage from the battery this way.

If the battery is on the primary side, you won't see much on the meter, as the 1.5v gets reduced down to almost nothing on the secondary.
If the battery is on the secondary side, the meter will see the voltage multiplied several times on the other side, so that's why the meter should be on a high setting 1.5v could go up to 100-500v (but low current)

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

But transformer still function the same way, I used low voltage side of the SMPS transformer as a step up transformer by driving the lower voltage winding instead. It depend on what you are doing with the transformer and call what ever winding for feeding as primary (feeding side).
The Blue ringer is used to find out if you have shorted turn and it does not care if it is transformer for SMPS or Linear power supply.

[mariushm]

Re: How do I identify the primary and secondary windings on a transformer?

They do work the same way, sort of.

SMPS transformers are meant to run at higher frequencies.

You can't get lots of amps of power on the secondary side by simply feeding 120/240v 50-60Hz on the primary side... which works with big power (or toroidal) transformers

For example http://www.digikey.com/product-detai...1281-ND/666167 - anyone can easily spot the difference in size and coil thickness and so on.

So yeah, you can feed a low AC voltage on one side and get something else on the other side, but you won't get the current unless you run it at high frequency.

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

Of cource you have to use correct frequency with the transformer you are work with, turn ratio is still applied, same reason you do not want to use transformer design for 60 Hz and run on 50Hz, you have to derated it. I think the asker do not have full understand witn the term Pirmary/secondary, it is simple, the one that is fed with power is the primary.

[tom66]

Re: How do I identify the primary and secondary windings on a transformer?

Just remember, you cannot put 115/230V AC on the secondary and get thousands out on the primary.

You will briefly manage it, but quickly destroy the transformer, due to the excessive secondary current.

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

You will have to match the voltage for that winding, of course. Just have to know what you are doing.
So the asker wants to konw "How do I identify the primary and secondary". Answer: the one that the pwer is being applied to to drive the transformer.

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

Yes, I am mainly talking about SMPS but to clear things up, I understand how to determine the primary and secondary SIDE of the transformers just by how they are situated on the board. What I don't understand is how to determine the primary winding when there are 3, 4, 5 or more pins on the primary side.

I now understand that some of the extra pins will be used for different voltages than my country. I assume these are the pins that are not even soldered to the board and I should not be concerned with them... But on the LCD TV power board I am working on now, the SMPT has 6 pins on the primary with 4 of them soldered in circuit. How do I determine which of the pins is the primary winding? When using the blue ring tester should I be testing more than one winding (two pins) on the primary side?

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

Not all the pins on the same side of the transformer are primary, they can be for the secondary also, they just run out of room to put more pin on the opposite side of the primary. If you can post the pictures of both sides of this power supply board then I can tell you which two pins are for the primary driving side. To use the ringer, you should disconnect all the pins from the circuit, otherwise you may get the wrong readings, from all the repair works I have been doing, I have yet to replace the SMPS transformer.
What is the problem you are having with. You can see some of the stuff I work on for fun here: http://s807.photobucket.com/albums/yy352/budm/

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

Ok, that explains a lot. I will try to take pictures tomorrow but in the mean time, when testing for shorts on the primary should I only be concerned about 2 pins (1 winding) and the rest of the pins are either for feedback purposes or they are secondary?

[budm]

Re: How do I identify the primary and secondary windings on a transformer?

You can use any pair of the winding, it only take one short turn to make the transformer bad, the ringing result will be low due to the shorted turn will damp down the ringing.

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

The problem I have with the TV is it is completely dead, with no LED or clicks. The problem was caused by a power surge. I know it is this board at fault because I have already replaced to board successfully. Right now I am mainly trying to become more proficient in using my tools so I am trying to check literally every component on the board until I find a fault.

[mattpd]

Re: How do I identify the primary and secondary windings on a transformer?

You can use any pair of the winding, it only take one short turn to make the transformer bad, the ringing result will be low due to the shorted turn will damp down the ringing.

You're saying that if there is a short ANYWHERE on the primary side, none of the windings on the primary side will test high on the ringer? And the same thing with the secondary side?