Why 2 transformers? (pic) - Page 1
Forum section: electronics / beginners

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 12:42:13 am
content: 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.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 01:37:45 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: January 14, 2019, 01:57:04 am
content: Maybe to supply a standby power rail.A bit of RE should make it obvious.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 02:03:30 am
content: I don't know what RE means. But here's the schematic:

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 02:07:42 am
content: Smaller transformer is for driving bases of power transistors (primary side) from the secondary side where SMPS controller chip is located.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 02:16:03 am
content: 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.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 02:25:36 am
content: 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.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 02:28:32 am
content: 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.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 02:34:36 am
content: Quote from: ataradov on January 14, 2019, 02:16:03 amNo it does not. TL494 or it's clone is powered though D5. Feedback is taken from resistive divider connected to the output.QuotePWM duty cycle, not frequency.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: January 14, 2019, 02:45:13 am
content: The whole supply doesn´t seem to be mains isolated, just keep that in mind, might give you a tingleedit: under fault conditions most probably though...edit no: 2. Sorry... It is, my bad got a bit confused following lines!Quote from: wraper on January 14, 2019, 02:34:36 amCould be both, there are high efficiency supplies that do both.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 02:46:20 am
content: 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.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 02:48:54 am
content: Quote from: TrickyNekro on January 14, 2019, 02:45:13 amCan not. It's TL494. In any case, changing switching frequency by itself barely changes output voltage.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 02:50:11 am
content: It is isolated. And I did misspeak, it is obviously changing the duty cycle.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 02:58:33 am
content: Quote from: ataradov on January 14, 2019, 02:50:11 amI'm not sure what you mean by that. TL494 is powered from secondary side through D5. Pin 12 is Vcc.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 02:59:48 am
content: Quote from: wraper on January 14, 2019, 02:58:33 amThat's a comment about TrickyNekro's comment that this power supply is not mains-isolated.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 03:04:06 am
content: Are we all looking at the same schematic? I don't see a TL494.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2019, 03:04:41 am
content: Quote from: billbyrd1945 on January 14, 2019, 03:04:06 amIC1 is TL494.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 03:05:02 am
content: 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.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 03:07:30 am
content: Quote from: wraper on January 14, 2019, 03:05:02 amAll replaced.

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 03:08:57 am
content: Quote from: billbyrd1945 on January 14, 2019, 03:07:30 amYou should say such things first. Why, were they faulty or just in case?

author: wraper
author_rank: Unknown
timestamp: January 14, 2019, 03:10:32 am
content: C16 film cap sometimes goes bad too.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 03:11:06 am
content: 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.

author: Shock
author_rank: Super Contributor
timestamp: January 14, 2019, 10:34:47 am
content: Quote from: billbyrd1945 on January 14, 2019, 02:25:36 amUnlikely 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.

author: Shock
author_rank: Super Contributor
timestamp: January 14, 2019, 10:47:29 am
content: 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.

author: soldar
author_rank: Super Contributor
timestamp: January 14, 2019, 01:25:06 pm
content: Quote from: billbyrd1945 on January 14, 2019, 12:42:13 amYou 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 14, 2019, 02:36:42 pm
content: One problem with the analysis of this supply is thinking of it as a standard power supply to regulate the output. It functions as a variable power supply from about 8-12VDC and the output voltage to the peltier cooling device is controlled by the NTC sensor signal that goes through IC2 to IC1 that controls TR1. Watch the video and look at the two LEDs on the schematic. When the sensor is calling for heat both LEDs are on but when it gets to the set point the output drops to as low as about 8 volts and the green LED, that has a 7.5 volt zener in series with it, goes out.Once you start looking at this as a temperature controller rather than a standard SMPS it makes more sense.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 14, 2019, 02:50:36 pm
content: Quote from: Shock on January 14, 2019, 10:47:29 amAbsolutely correct.These types of supplies are very cheaply mass-produced by one of the larger far-east suppliers (Delta, LiteOn, and such) and are found literally everywhere.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 14, 2019, 05:03:38 pm
content: Watched it many times. I don't have enough experience to make much sense of it. I'll try again. I may not have the aptitude for electronics. But thanks for all the help.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 14, 2019, 06:21:40 pm
content: Thinking of the controller board as just a SMPS isn’t the way to understand the whole wine cooler. Instead of the board being a SMPS that senses the output voltage and feeds back a sample of this voltage to an IC that keeps the output voltage constant, it has a temperature sensor that feeds back a sample of the temperature (as a voltage) to an IC that varies the pulsed output voltage from about 8-12 volts to keep the cooler temperature constant.If the wine cooler is working you should eventually see the controller’s red and green LED flash slowly as the peltier effect cooler is fed power to cool the inside of the wine cooler. If the LEDs flash rapidly, just one LED is on, or both are out, there is a problem somewhere. I believe a common failure mode of any cooling device like this is the bank of thermocouples that make up the peltier effect cooler that are fed the D.C. power at a few amps starts getting weaker and the cooling gets less and less effective as the cooling unit draws less and less current.You haven’t said what the problem is and why you have been working on the wine cooler for a few months. Is it dead or is it just not cooling properly? If it isn’t cooling properly then if you monitor the D.C. voltage across the fairly heavy wires going to the cooling unit when you first power it up you should see the 12 to12.5 volts that it shows on the schematic.If everything is working, after some time the temperature should start to drop and eventually reach the set point. If you have 12 volts and it isn’t cooling as it should then it is probably the peltier effect device. One way to check the cooler is to disconnect it from the controller board and connect it to a D.C. bench supply. Increase the output voltage to 12 volts and see if the current drawn increases to 4 or 5 amps. If it does and the unit cools down, the cooler is o.k.. If it draws less than 4-5 amps at 12 volts then the peltier effect device probably has some open cells and needs to be replaced. There are inexpensive replacements on eBay and plenty of videos that show how to replace the cooler and it isn't that hard.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 15, 2019, 01:57:46 am
content: Your response is what I've been needing because it addresses the problem as a system. Unfortunately, a precise sequence of events may not be possible as I have poked around so much without keeping adequate notes. But, itall started one day when my wife reported a strange noise from the kitchen. I found the noise to be coming from thewine cooler door display which was flashing rapidly. I was at least smart enough not to order a new door. It proved to work normally when I connected it to a known power source.Then I took off the back panel and found twin power supplies connected to a common break out board. The voltage for the door emanates from the BOB, so I decided the problem had to be related to it. There's only on IC on the BOB, and my thinking was that it was far more likely to be the culprit than one of the caps, resistors, diodes, etc.I found the IC on ebay and waited several weeks. I installed it. No change. Then I started deep research into common failures of switch mode power supplies. That led to my replacing caps, then diodes, then transistors.I had violated all the rules of trouble shooting by replacing rather than testing. That's not the way I've spent my life as a person who generally succeeds in fixing most anything around the house. But I was out of my element. I simply didn't have any way of testing, didn't know how.So, I began asking one by one about this component and then that one. The idea was to learn how to test each component and then order parts based on those tests. I mostly got no help so resorted to replacement as a process of elimination.(At this point the only things I haven't replaced (on the two PSUs and the BOB) are four zener diodes, two for each PS. They are ordered.)After each attempt at repairing the boards, I'd hook it all up to find that it still wasn't right. The last attempt had the fans for one Peltier spinning slower that what I would expect while the other set of fans would pulse off and on. I could hear the faint sound of switching from both power supplies. I only just now learned that the switching sound comes from the power transistors.I did test both Peltiers with a 12v battery charger and both would heat/cool in a way that would seem normal based on what I understand about them.And that, sir, is my story. I hope I haven't left out anything important. With regard to finding success stories that I could benefit from-- not so much. I've found a few old posts about dealing with failed units, but the posters usually lose interest in the subject once they've found their problem (or have given up on it).As previously stated-- it ain't about the money. I've already spent more than the cooler cost. I'm just obsessed with seeing it through. After that I'm going to find myself a good electronics project with documentation so that I can go from start to finish without having to work blind.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 15, 2019, 04:37:15 am
content: It's getting late here but a quick thought or two. You know that the controller transistors/transformers shouldn't be 'singing' and that indicates that the switching frequency is too low, perhaps from an output overload of some sort. Where there are two controllers and two coolers it may be possible to locate which has a problem by only running either the top controller/cooler or the bottom controller/cooler by itself. I would guess that the fans running at the higher speed and pulsing on/off (which may correspond with the red/green LEDs) is the o.k. half and the slower set of fans indicate which half has the problem. If you can figure out which half is working properly then you could switch the slow fans/cooler to the working controller to see if the fans work properly on that controller. I suspect that the chip on the BOB is a dual opamp to send the same signal to both controllers.Of course you also realize trying to diagnose the problem remotely by text is like calling a doctor and saying: "What is this pain in my side?" then realizing you dialed the local deli by mistake anyway.My guesses may not be anywhere close.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: January 16, 2019, 12:37:44 am
content: I'll take your suggestions and see what I can come up with. One thing for sure-- Thanks for you time and interest!