Author Topic: Why 2 transformers? (pic)  (Read 4435 times)

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Offline billbyrd1945Topic starter

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Why 2 transformers? (pic)
« on: January 14, 2019, 12:42:13 am »
Been working on the wine cooler PSU for some months now. I've obviously made it a learning project having spent more already than it's worth. Loving every minute of it. I try to research rather than ask on forums. Not having any luck figuring this one out though. I understand the basics of how a transformer uses opposing coils to step up or step down a voltage.

What I don't understand is why I would need two transformers on one power supply. If someone can answer that one, I have a couple of follow up questions about how these particular transformers even work. They definitely don't work as I thought. Thanks for any help.
« Last Edit: January 14, 2019, 01:04:13 am by billbyrd1945 »
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #1 on: January 14, 2019, 01:37:45 am »
This is a switching mode power supply, so transformers here are not used like in a conventional linear power supplies.

It would be hard to tell why this design uses two transformers. It may be for feedback reasons, but that would be unusual way to do that.

You need to draw the schematic of the primary side. It will be obvious then. It is trivial to do with a single sided PCB.
« Last Edit: January 14, 2019, 01:57:45 am by ataradov »
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Online tautech

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Re: Why 2 transformers? (pic)
« Reply #2 on: January 14, 2019, 01:57:04 am »
Maybe to supply a standby power rail.
A bit of RE should make it obvious.
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Offline billbyrd1945Topic starter

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Re: Why 2 transformers? (pic)
« Reply #3 on: January 14, 2019, 02:03:30 am »
I don't know what RE means. But here's the schematic:
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #4 on: January 14, 2019, 02:07:42 am »
Smaller transformer is for driving bases of power transistors (primary side) from the secondary side where SMPS controller chip is located.
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #5 on: January 14, 2019, 02:16:03 am »
RE is reverse engineering. But if you already have the schematic, then there is obviously no need to do that.

Yes, TR1 is there to drive main switching transistors (that's what makes this a switching power supply). TR2 is a main transformer that actually handles the load.

IC1 takes feedback voltage from the unregulated output through D5 and adjusts switching frequency accordingly.

Unregulated voltage goes to power the fan, and 7808 provides linear regulation for the output voltage.
« Last Edit: January 14, 2019, 02:24:44 am by ataradov »
Alex
 

Offline billbyrd1945Topic starter

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Re: Why 2 transformers? (pic)
« Reply #6 on: January 14, 2019, 02:25:36 am »
Your answer helps in many ways (esp "why" it's a smps). I'm determined to learn to test every component that can be tested by a hobbyist. The only tests I've come up with for the two transformers is by way of multimeter pin-to-pin continuity. Is there a more sophisticated way to determine whether one or both of them is bad? I would order new ones to do side by side comparison, but these things don't seem to have any kind of number I could use for ordering. Thank you.
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #7 on: January 14, 2019, 02:28:32 am »
It is highly unlikely that you will be able to find those specific transformers. Depends on how much room you have in the target device, it may be easier to just find a 8 V power supply. Or 12 V supply and a secondary regulators.

Transformers are likely fine. In power supplies like this transistors are likely to die first.
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Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #8 on: January 14, 2019, 02:34:36 am »
IC1 takes feedback voltage from the unregulated output through D5 and adjusts switching frequency accordingly.
No it does not. TL494 or it's clone is powered though D5. Feedback is taken from resistive divider connected to the output.
Quote
adjusts switching frequency accordingly.
PWM duty cycle, not frequency.
 

Offline TrickyNekro

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Re: Why 2 transformers? (pic)
« Reply #9 on: January 14, 2019, 02:45:13 am »
The whole supply doesn´t seem to be mains isolated, just keep that in mind, might give you a tingle :-/O
edit: under fault conditions most probably though...

edit no: 2. Sorry... It is, my bad got a bit confused following lines!

IC1 takes feedback voltage from the unregulated output through D5 and adjusts switching frequency accordingly.
No it does not. TL494 or it's clone is powered though D5. Feedback is taken from resistive divider connected to the output.
Quote
adjusts switching frequency accordingly.
PWM duty cycle, not frequency.

Could be both, there are high efficiency supplies that do both.
« Last Edit: January 14, 2019, 02:47:57 am by TrickyNekro »
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Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #10 on: January 14, 2019, 02:46:20 am »
Transformers are unlikely to be faulty. And even if transformer is faulty, most likely is was damaged due other components failing. If windings have low resistance, they should be fine.
« Last Edit: January 14, 2019, 02:51:28 am by wraper »
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #11 on: January 14, 2019, 02:48:54 am »
The whole supply doesn´t seem to be mains isolated, just keep that in mind, might give you a tingle :-/O
edit: under fault conditions most probably though...

IC1 takes feedback voltage from the unregulated output through D5 and adjusts switching frequency accordingly.
No it does not. TL494 or it's clone is powered though D5. Feedback is taken from resistive divider connected to the output.
Quote
adjusts switching frequency accordingly.
PWM duty cycle, not frequency.

Could be both, there are high efficiency supplies that do both.
Can not. It's TL494. In any case, changing switching frequency by itself barely changes output voltage.
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #12 on: January 14, 2019, 02:50:11 am »
It is isolated. And I did misspeak, it is obviously changing the duty cycle.
Alex
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #13 on: January 14, 2019, 02:58:33 am »
It is isolated.
I'm not sure what you mean by that. TL494 is powered from secondary side through D5. Pin 12 is Vcc.
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #14 on: January 14, 2019, 02:59:48 am »
I'm not sure what you mean by that. TL494 is powered from secondary side through D5. Pin 12 is Vcc.
That's a comment about TrickyNekro's comment that this power supply is not mains-isolated.
Alex
 

Offline billbyrd1945Topic starter

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Re: Why 2 transformers? (pic)
« Reply #15 on: January 14, 2019, 03:04:06 am »
Are we all looking at the same schematic? I don't see a TL494.
 

Offline ataradov

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Re: Why 2 transformers? (pic)
« Reply #16 on: January 14, 2019, 03:04:41 am »
Are we all looking at the same schematic? I don't see a TL494.
IC1 is TL494.
Alex
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #17 on: January 14, 2019, 03:05:02 am »
I recalled a typical failure of such PSUs. C13 and C14 electrolytic caps loose their capacitance. Also one of the first things you should do is check T6 and T7. Of course after fuse was checked first.
 

Offline billbyrd1945Topic starter

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Re: Why 2 transformers? (pic)
« Reply #18 on: January 14, 2019, 03:07:30 am »
I recalled a typical failure of such PSUs. C13 and C14 electrolytic caps loose their capacitance. Also one of the first things you should do is check T6 and T7. Of course after fuse was checked first.

All replaced.
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #19 on: January 14, 2019, 03:08:57 am »
I recalled a typical failure of such PSUs. C13 and C14 electrolytic caps loose their capacitance. Also one of the first things you should do is check T6 and T7. Of course after fuse was checked first.

All replaced.
You should say such things first  :blah:. Why, were they faulty or just in case?
 

Offline wraper

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Re: Why 2 transformers? (pic)
« Reply #20 on: January 14, 2019, 03:10:32 am »
C16 film cap sometimes goes bad too.
 

Offline billbyrd1945Topic starter

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Re: Why 2 transformers? (pic)
« Reply #21 on: January 14, 2019, 03:11:06 am »
Sorry. I thought I stated earlier that most everything that I could find a part number for was replaced even when my primitive testing showed them to be good. Replaced every cap even though none of them APPEARED to be bad. I still don't know how you know IC1 is a TL494. But you're right. It is.
« Last Edit: January 14, 2019, 03:15:59 am by billbyrd1945 »
 

Offline Shock

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Re: Why 2 transformers? (pic)
« Reply #22 on: January 14, 2019, 10:34:47 am »
Is there a more sophisticated way to determine whether one or both of them is bad?

Unlikely to be them so I wouldn't go deeper than a resistance check. Normally you would use the  divide and conquer (aka half splitting or 50/50) technique by checking the output, feedback and switching.

If you suspect the transformer there are a few ways to check transformers aside from swapping out or putting the faulty into a known good board. Signal generator and oscilloscope, measuring live with oscilloscope with high voltage active differential probe, using ring tester and also few home brew inductive methods.
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Offline Shock

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Re: Why 2 transformers? (pic)
« Reply #23 on: January 14, 2019, 10:47:29 am »
Seen this video at all? Always pays to dive into google and lookup on the model and board part numbers. Especially consumer gear it's probably been fixed a hundred times already.

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Offline soldar

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Re: Why 2 transformers? (pic)
« Reply #24 on: January 14, 2019, 01:25:06 pm »
What I don't understand is why I would need two transformers on one power supply.
You have one transformer to isolate galvanicaly the output from the (mains) input. Then you need to have a loop back from the output to the input for the control. Most PSU's have the control circuitry on the mains side and use an optoisolator for the feedback loop. This one, instead, has the control part on the output side and the second transformer drives the switches while providing the necessary isolation.
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