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Flyback transformer testing and replacement

gkmaia

Jan 5, 2019
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Can anyone help me diagnose this transformer or suggest a replacement?

Primary coil takes 5v and the feedback coil takes another 5v. I cannot get any output voltage so I assume it is faulty.

But it outputs to a 3v power regulator and a 1.5v power regulator so I suspect at the same 5v range.

As you can see by the photos. I get no output no matter what.

It has printed on it 3305 n181 and part number is S1673305 and unfortunately I cannot source a datasheet or the part.

IMG_2713.jpg IMG_6771.jpg IMG_9851.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Transformers require AC. Connecting them to a DC source won't tell you much.

It actually looks like a transformer (I thought it may have been a DC to DC converter). It is possible it might have an integrated rectifier, but I don't see any evidence of that.

Did you pull it out of a circuit?

The easiest test is to measure the windings with a multimeter. They should not be open circuit.

The next most likely fault is a shorted turn, but that is more difficult to diagnose.
 

gkmaia

Jan 5, 2019
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When T1 was on the circuit. I was getting:
- 0v on the left side of D6
- DMMV +5v (as it comes from the main board)
- DMMP low = 0v high = +5v
- IC4 (REG LINEAR 3V 250MA SOT89-3) output was 0v
- IC5 (REG LINEAR 1.5V 200MA SOT89-3) output was 0v
- Q9 & Q10 have been tested and both read 0.6v on the right direction

Off circuit coils from 1 to 5 gives 3ohms and coil from 2 to 4 gives 4.8ohms
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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And between 7 and 9?

Transformers are quite rugged components. They are not the thing I would first suspect unless there was physical damage or an awful smell coming from them.

Was there an AC voltage across any of the windings?

Is D6 shorted?
 

gkmaia

Jan 5, 2019
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Attached are the reading I get from D6 when tested off-board on diode mode on my multimeter.
If I am not reading wrong the datasheet says 0.35 volts forward voltage.
Seems to be fine, right?
 

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  • d6 datasheet.png
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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That seems fine, but it's a very odd diode to use in that application.

Have you checked the continuity between pins 7 and 9 of the transformer?

If it's OK, put the transformer back in circuit and see if there's an AC voltage on the primary windings when the device is powered up. My guess is that there is not.
 

gkmaia

Jan 5, 2019
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Yes I did it is:

input 1-5 gives 3ohms
input 2-4 gives 5ohms
output 6-8 gives 10ohms (not 7 & 9)
There is no continuity between any of the 3 coils

I will have to wait a bit to do that as the main board is waiting for a few parts I removed for replacement.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, it might be a good time to check that none of the semiconductors on the primary side have failed short circuit (that's the most likely failure mode). Open circuit isn't good either :)
 

gkmaia

Jan 5, 2019
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I have removed Q10 and tested as a diode with my DMM. These are the results:
CB = 0.6v
EB = 0v
 

(*steve*)

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If certainly be looking for a replacement for that. How is Q9?
 

gkmaia

Jan 5, 2019
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Thanks steve!
Q9 results. BC=0.6v and BE=0.6v
Now I am struggling to replace Q10 as it is obsolete. Originally a 2SC2812L7-TB. I have attached the closest datasheet ive found.

Thought about replacing by those. Would you be able to let me know if I choose the right replacement bellow? If not, why they are not the best ones.

I thought about those:
https://www.digikey.co.nz/product-d...-and-storage/TBC847BLM/TBC847BLMCT-ND/6198789
or
https://www.digikey.co.nz/product-d...nd-storage/TMBT3904LM/TMBT3904LMCT-ND/5403487
 

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(*steve*)

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Yeah, the BC847 looks fine, but the 2N3904 has a lower gain than the 2SC2812, so I wouldn't choose that one.
 

gkmaia

Jan 5, 2019
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Thanks Steve.
I should never get something with lower hFE. But how far above I can go. For example the original was 200 hFE, can I use one with 800 hFE for example? Or that will be then too much gain?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Higher is rarely a problem.

The gain of transistors is extremely variable and circuits should not be designed to operate with a certain gain. Having said that, a certain amount of gain will always be required. Choosing a replacement that might have a gain lower than the specified minimum for the replaced part is potentially a problem.
 

gkmaia

Jan 5, 2019
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Thanks steve! I am calling my transformer a "flyback transformer" and I believe that is not right.
As you have seen my schematic, what would the right technical name for my transformer?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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This circuit is a flyback converter, and therefore the transformer is a flyback transformer.

Some definitions for flyback transformer equate it with a LOPT (a line output transformer). A LOPT is a use of a flyback transformer (once, probably, the most common use), but not the only use, so this is a false equivalence.

If you don't want to be controversial, just call it a transformer :)
 

gkmaia

Jan 5, 2019
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Thanks for all the answers! I think I understand it better now.

I am now trying to understand how the current flows on the circuit and what would be the expected voltages on A and B considering I have a couple of regulators that would take up to 15v. I imagine my B should be something around 10V ac right and A around +5v DC?

Screen Shot 2019-01-08 at 11.47.23 AM.png
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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A few notes on your markup. Firstly, current only flows one way in each of the main windings. Q9 energises the transformer by allowing current to increase in the primary winding. When this current is interrupted, the collapsing magnetic field causes current to flow in the secondary winding (because it is blocked in the primary winding). Consequentially, neither waveform looks like a sine wave.

Secondly, I don't see any feedback which will determine The voltage of the rectified DC on the secondary side. It has to be greater than the regulator voltage (plus overhead) and not exceed the capacitor's voltage rating (or the max input voltage of the regulator -- whatever is less). It also may be important to limit power dissipation in the linear regulator. If this is the case, something closer to the lower limit is preferable.

As for the voltage regulators (and their output voltages), have you googled for their datasheets?
 

gkmaia

Jan 5, 2019
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I have them attached now. Power dissipation is 400mW. But as I have very limited knowledge I will need to research about the pwoer dissipation issue.

YDA sends current to VDA and that makes sense to me.

If my VSS current flow is correct YHA seems to be sending it back to the transformer's coil. Why is that?

Also do you think the input sine I defined of 10v AC is correct?
 

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gkmaia

Jan 5, 2019
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As an update to where things are

Transformer
A sine @ 15khz- 5v on pins 1-5 outputs 8.2v on pins 6-8
A sine @ 15khz- 5v on pins 2-4 outputs 8.2v on pins 6-8

Other components
Q10 was faulty on B-E
Q9 is ok
D6 (schottky) forward bias 0.3 is ok
ZD1 was ok, but replaced anyway.
All capacitors and inductors have been replaced. Some were leaking.
Traces and board are fine

Got to put it together and test.
 
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