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Basic transformer use and power supply circuits

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im_in_asia_now

New Member
I've pulled apart some of my old 220 V Power Supplies used for home electronics (I moved here from Korea) and the larger transformers I've desoldered are kind of confusing not to mention intimidating.

What I've noticed in general is the use of smaller 4-pin transformers near the AC in, and larger, 7, 10, various different numbers of pinned transformers after the rectifier and closer to the DC out.

Ok, so the 4-pin transformers weren't too complicated to figure out. There's a short between the input and output pins, one for the signal, and the other for ground. When I used them in a circuit I built though, it seemed that there wasn't a noticeable step-up or step-down (like you would expect when going from 220VAC to 12 VDC).

So I've got how the 4-pin transformers at the top of the circuit work, but if they aren't step down, what is their purpose? Just for coupling? On the original circuit board they're labeled as "LF1" while the larger transformer is labeled as would be expected as T1. What does "LF" mean?

Then this is really something that's been bothering me because I haven't been able to find a lot of practical information on the internet about this. How in the world do you use a 7 or 10 pin transformer?

When I tested for shorts between the different pins, it just got more confusing. Could I get some help sorting through what I've observed so far:

10 pin transformer:

original circuitry takes output from transformer LF01. Output from LF01 connected to a GBP 206 bridge rectifier. Positive and negative outputs from GBP 206 rectifier are then connected to side 1 of the 10-pin step down transformer through additional resistors, conductors, and some are connected through more complicated circuits.

Side 1
pin #....shorts to pin# ....originally connected to

1.........2......................first a diode then an IC circuit fed from neg output of rectifier
2.........1......................neg output of rectifier
3.........4......................pos output of rectifier
4.........3......................connected to the IC circuit

Side 2
(continuing counter-clockwise)

5.........7,8
6.........9,10
7.................................goes through some components to -VDC output
8
9.................................goes through some components to +VDC output
10

There are no shorts in the transformer from side one to side two. The two sides are connected through additional circuitry.

On the side of pin 1 and pin 2 theres a circuit involving a few capacitors and a resistor going to a circuit on the side of pins 8-10. On the side of pin 3 and pin 4 is the IC circuit with input from neg side of the bridge rectifier, connected to a circuit on the side of pins 5-7 through a resistor, capacitor, Q01, U01 and U02.

Ok, no schematics and a lot of information to look through, no useful numbers on the large transformers... Are there any datasheets I could look at for reference for general 7, 10 or however many pin transformers? Are these large transformers even reusable or are there too many unknowns? Then the big question. Can step-down transformers such as these be used as step up transformers? Thanks for your time
 
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BeeBop

Active Member
Hi im_in_asia_now,

welcome to the forum.

So where are you now; you say you moved here from Korea. Where is here? Where did you live in Korea? I lived in Seongnam for a year and in Suwon for four years (I know, bad luck on four!) I wish now that I had stayed.:D

I think what you are trying to describe as power supplies are the converters which are so popular there for running 120V off of 220V and vice-versa. Am I correct?

A photo of the transformers would sure help, as would a cct. diagram if you could sketch it out a bit...

I'd guess the LF could mean line filter? only guessing though...
As for the larger transformers with multiple taps or windings, that is exactly what they are. There could be several secondary windings each with their own set of pins...
One way to find out would be to power the thing with the recommended primary voltage and measure the secondaries with a voltmeter. I won't even wager a guess as to how much current they can source... again a photo would probably get you more help.

Also, I would not recommend double posting on the same topic as it can lead to confusion and you may not get the quality of help you get in a single thread.

EDIT: I see you tried to delete it. It may help to just put the link to this one in your other post...
 
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im_in_asia_now

New Member
Well, I'm actually in the states working on a degree in EE. The name iminasianow just kind of sticks as one of those things I can remember because I used it for 5 1/2 years. I stayed in Pyeongtaek working in radio/comsec repair for the army for 3 years, then another 2 years between Pyeongtaek and a little town in Paju teaching English (barf).

These are from power supplies going from 220 VAC to 12 VDC, 5 VDC or who knows what voltage DC because I've salvaged most of the components already.

Here's the 10-pin transformer mentioned above. If you click on it so it's magnified you can see where I've marked the shorts between pins:

10-pin.jpg

Another 7-pin that I desoldered with shorts marked:

7-pin.jpg

And a somewhat intact power supply circuit with a similar layout- One 4-pin transformer near AC in and a larger transformer with more pins after rectification. On the original power supplies I desoldered the 7 and 10 pin from, the 4-pin near AC in is labeled as LF01. This is actually the main board from a 110 VAC in CRT monitor:

LF1 T1.jpg

As I've already salvaged most of the components off of the original circuits, I can't test them out and see how they operate and can't provide schematics.

What I was hoping though, was that from looking at which pins are shorted together, something might be figured out as to how they're supposed to be connected in a circuit.

Looking at the 10-pin it's kind of hard for me to imagine how this operates. With 4 pins on one side and pairs of 2 shorted together, and with 6 pins on the other side and groups of 3 shorted together, I was hoping somebody might know how this works. You would think just one side is primary and the other side is secondary, but there are supposed to be shorts between the primary and secondary, right?
 

Sceadwian

Banned
Why can't you test them, you don't have any way of feeding it a 1V sine wave at 1khz from an audio card or MP3 player?
 

im_in_asia_now

New Member
I meant I can't test the original circuit. I have tried testing the transformer, but frankly I don't know where the input goes in and the output comes out. Like I said, there should be shorts connecting the primary and secondary sides, right? But there isn't anything straightforward like that.
 

Sceadwian

Banned
Shorts connecting the primary and secondary? What are you talking about? Those are not auto-transformers the primaries and the secondaries are not electrically connected. Sorry seems as strange question coming from someone studying for an EE degree.
 

im_in_asia_now

New Member
I'm just looking for some information on the internet and there seems to be a lack of practical information on the net for transformers. Just going off of what I've seen with inductors and 4-pin transformers, the two sides are shorted together. Slap me around all you want, but nobody's taught me much about transformers in my general education classes.

This isn't a post about whether or not there are shorts between sides of the transformer. I'd like to know how to use these transformers in a circuit.
 
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BeeBop

Active Member
I'm just looking for some information on the internet and there seems to be a lack of practical information on the net for transformers. Just going off of what I've seen with inductors and 4-pin transformers, the two sides are shorted together. Slap me around all you want, but nobody's taught me much about transformers in my general education classes.

This isn't a post about whether or not there are shorts between sides of the transformer. I'd like to know how to use these transformers in a circuit.
Hi,
The reason for the lack of information is that these are custom wound on ferrite cores; the power supply is a switch mode. You may get some use out of them, however I'm skeptical. I think you would have to analyze the circuit to figure them out as they are, however some people do rewind them, and in that regard they get the ferrite core and perhaps a winding or two they can use...

I studied martial arts for a while in Songtan, which is very near Pyeongtaek. Your fly-boys would be pretty low when they passed over the do-jang - an exciting sound at first...:p

The JSA was exciting, and I don't think I'd have been in HanGuk if you guys weren't there...:D

EDIT: the black one with the four white spools, I think, is a line filter. I seem to recall seeing one used that way.

I also would recommend unwrapping one of those cores and having a good look at what you see. Some notes and drawings will increase your understanding.
 
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Sceadwian

Banned
A transformer is nothing more than two separate coils of wire, wound on the same form. Even two simple coils of wire brought together in free air will act as a transformer, the electricity is transmitted in the varying current from the primary inducing changing magnetic field which produce a voltage in the secondary. Bathroom electric toothbrushes are nothing more than transformers with physically separate primary and secondary coils used to transfer power for chargin the hermetically sealed battery pack. I used to think it was magic, and to some degree I still do because all the power is being transferred via magnetic fields understanding how the core material (there are hundreds up hundreds of different types for different uses) act under these induced magnetic fields is an entire field of study unto itself.

The first thing you do is what you've done, determine what pins electrically connect to what other pins. The one that doesn't connect to anything may either be a very high resistance or simply there for physical support. With multiple coils like you have there are a myriad number of ways you can connect them, all of them are almost never used all at once. Determining the thickness of the wire for each coil and the DC resistance will help. Apply an AC waveform to one coil and you can measure the voltage at the others to determine the turns ratio.
 
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BeeBop

Active Member
A transformer is nothing more than two separate coils of wire, wound on the same form. Even two simple coils of wire brought together in free air will act as a transformer, the electricity is transmitted in the varying current from the primary inducing changing magnetic field which produce a voltage in the secondary. Bathroom electric toothbrushes are nothing more than transformers with physically separate primary and secondary coils used to transfer power for chargin the hermetically sealed battery pack. I used to think it was magic, and to some degree I still do because all the power is being transferred via magnetic fields understanding how the core material (there are hundreds up hundreds of different types for different uses) act under these induced magnetic fields is an entire field of study unto itself.

The first thing you do is what you've done, determine what pins electrically connect to what other pins. The one that doesn't connect to anything may either be a very high resistance or simply there for physical support. With multiple coils like you have there are a myriad number of ways you can connect them, all of them are almost never used all at once. Determining the thickness of the wire for each coil and the DC resistance will help. Apply an AC waveform to one coil and you can measure the voltage at the others to determine the turns ratio.
Pretty much agree with you there, but I think these transformers expect a high frequency signal, and you should be clear about what you mean by an AC waveform... :)
I wouldn't try a very high voltage at first... certainly not line voltage from the house.. :)
I'm guessing that the dc resistance of some of those windings could be making them appear as shorts, at least on the one where the adjacent pins are shown as s/c, but I'm not an expert in switching supplies...

im_in_asia_now how did you measure them as shorts?
 
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Sceadwian

Banned
beebop, you feed that transformer a 1khz input to any of those coils I'll guarantee you'll get an output that will show the approximate turns ratio and phase of the other coils, as long as the induced current was relatively decent and safe for the coil, maybe a small audio amplifier, and a PC sound card with an atenuator for input. 1khz is a common test frequency for reactive loads and a PC sound card through and audio amp could easily produce 60hz with a slight output current, at least enough to test it.
 
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im_in_asia_now

New Member
Great, thanks. The LF symbol next to the Line Filter makes perfect sense and searching the web for a line filter here's what I get:

LF.jpg

That looks exactly like what I'm seeing at the AC in for all of these power supply circuits.

@Sceadwian it looks like these multi-taps will take some time for me to test out. I'm actually using a low-voltage ~.25 hertz spiked wave alternating between pos and neg. There's nothing wrong with testing it out with low frequency, is there?

@BeeBop I first measured them with the short-setting on my DMM. For the 10-pin I've double checked and I'm getting .4 ohms for the aforementioned pins on the secondary side and .6 ohms for those on the primary side. There are opens between all other pins.

As a side note, on one of the line filters I'm getting 1.4 ohms between primary and secondary for both sets of pins.
 
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Sceadwian

Banned
If you're reading ANY level of conductivity under a few 10-100's of megs between the primary and secondary the transformer is dead. Crack it open strip the wire and re-use the form, those looks like CCFL transformers, the core material is probably good up into the high audio range.

The way you drew the circles on those diagrams doesn't seem too weird though, the first one looks like two center tapped coils on the one side with six connectors, and two separate coils on the 4 wire side.

The second 7 pin image looks like a primary with a feedback coil (the 4 pin side) the feedback coil is the closer two pins, and a single output at the other side with the third leg on the other end being not-connected and just there for package support.

How are you determining what is the primary and what is the secondary, because there is no method to do that, transformers are inherently bidirectional, either side whichever one you call the primary IS the primary.
 
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im_in_asia_now

New Member
Ok, I double checked the original circuit where the LF was used and I had the sides of the transformer mixed up. It's actually 1.4 ohms between the 2 pins on the primary, and 1.4 ohms between the 2 pins on secondary. Looking at a couple of circuits I traced from AC in to the LF. I get the primary of the line filter connected to neutral and secondary connected to phase. Maybe I should take some pictures tomorrow to make this less vague. Is the line filter's only use here to isolate the circuit from the house's AC power?

About primary and secondary, I'm just referring to which side was used as primary or secondary in the original circuit.
 
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im_in_asia_now

New Member
To give a clearer idea of the circuits I've been talking about, I've attached some photos.

Here's a clearer picture of how the line filter is put in right after AC in, with lines drawn to show N (neutral) going to primary and L (phase?) going to secondary: (click to magnify)

CRT LF01.jpg

And what I found about Line Filters in Wiki:

"A line filter is the kind of electronic filter that is placed between an electronic equipment and a line external to it, to attenuate conducted radio frequencies -- RFI, a.k.a. electromagnetic interference (EMI) -- between the line and the equipment.

In particular, an AC Line Filter is used between the AC power line and the equipment"

And the circuit I took the 10 pin transformer out of, with LF near AC in, a bridge rectifier and then the connections to the 10 pin transformer: (click to magnify)

PS LF01 T01.jpg
 
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