Transformer questions

[Welchs101]

For some time now i have posted questions on this forum and gotten really great answers. So i thought i would post a few questions regarding transformers in hopes that i can learn a few things.

Enclosed you will find a picture of a transformer. This transformer is located on the inverter board of an LCD monitor.

Questions:
1-How to tell which side is the primary and secondary side of this transformer.
2-Why are there so many leads on each side?
3-Why not only two per side?
4-I know a multimeter can not make ALL the necessary tests to determine if a transformer is bad. But what type of tests can you do with a multimeter to check a transformer.

Based on the answers to questions above i will probably have a few additional questions.

[PlainBill]

Re: Transformer questions

For some time now i have posted questions on this forum and gotten really great answers. So i thought i would post a few questions regarding transformers in hopes that i can learn a few things.

Enclosed you will find a picture of a transformer. This transformer is located on the inverter board of an LCD monitor.

Questions:
1-How to tell which side is the primary and secondary side of this transformer.
2-Why are there so many leads on each side?
3-Why not only two per side?
4-I know a multimeter can not make ALL the necessary tests to determine if a transformer is bad. But what type of tests can you do with a multimeter to check a transformer.

Based on the answers to questions above i will probably have a few additional questions.

1. The right side is the primary, the left is the secondary. There are multiple clues that lead to this deduction. It is an inverter transformer. The right side has driver transistors; the left doesn't. The right side shows obvious signs of overheating, the left doesn't. Additional clues could be obtained by checking the resistance of the windings.

Note that these clues depend on knowing the application. If someone is handed an isolation transformer (two windings of the same gauge of wire and approximately the same number of turns) it is difficult to determine which is the primary and which is the secondary.

2. The circuit requires three sets of primary windings (well, 1 primary and two feedback). I am guessing that there are two secondaries, or there are two terminals for each end of the winding.

3. Because that wouldn't work in this design. 'Form follows Function'.

4. About all you can do is measure resistance. This is useful when you have multiple transformers, but only one is likely to be bad.

PlainBill

[retiredcaps]

Re: Transformer questions

This video might help.

http://www.youtube.com/watch?v=eNuGWBPRGKA

Part 2 shows what a bad transformer look like.

Questions:
1-How to tell which side is the primary and secondary side of this transformer.
2-Why are there so many leads on each side?
3-Why not only two per side?
4-I know a multimeter can not make ALL the necessary tests to determine if a transformer is bad. But what type of tests can you do with a multimeter to check a transformer.

[Welchs101]

Re: Transformer questions

Ok. Some good info thanks.

Questions:
1-Should ALL the leads from the primary side be isolated from the secondary side? ie....if i measure resistance from ANY primary side to ANY secondary side should i read an open?

2-If i measure the resistance on the primary side to any of the leads on the same primary side........what resistance value should i read.

3-Similar question as in #2......but for the secondary side. If i measure the resistance on the secondary side to any of the leads on the secondary side......what resistance should i read..

[PlainBill]

Re: Transformer questions

Ok. Some good info thanks.

Questions:
1-Should ALL the leads from the primary side be isolated from the secondary side? ie....if i measure resistance from ANY primary side to ANY secondary side should i read an open?

2-If i measure the resistance on the primary side to any of the leads on the same primary side........what resistance value should i read.

3-Similar question as in #2......but for the secondary side. If i measure the resistance on the secondary side to any of the leads on the secondary side......what resistance should i read..

1. Not necessarily. Often a transformer secondary will have a center tap. This makes a convenient way to monitor the current through the CCFLs. By bringing the center tap to the primary end of the transformer it becomes easier to lay out the PC board, and isolates the high voltage from the control circuitry.

2. It will vary with the design of the transformer. My expectation would be it should be under 100, possibly under 10 ohms.

3. I would expect it to be about 1000 ohms - say over 500 and under 2K.

Again, the value isn't predictable. Comparing to a known good transformer is some help. As an example, I had an inverter that had 12 transformers, each with dual secondaries. The secondaries varied from 950 to 980 ohms. Except one that read over 1.2K. With that number of samples, it was easy to determine the bad secondary. If I had only the one, I would be unable to say which winding was bad.

PlainBill

[Welchs101]

Re: Transformer questions

Thanks for the info.