How To Repair A Switch Mode Power Supply Without Blowing It Up! Part 2
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- Опубліковано 10 лип 2024
- Sometimes you can replace the faulty components in a switch mode PSU only to plug it in and watch your shiny new parts self destruct. I recently have this problem with a power supply that blew up even though it was connected to a current limiter (light bulb). Find out why this happens and how to successfully repair this type of fault
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Thank you
Richard
Always a great pleasure following your dive into the design of electronics
Teşekkürler . sizin sayenizde yeni bilgiler öğreniyorum. bu kadar kaliteli ve öğretici bir kanalın hak ettiği değeri görmesi en büyük temennim.🙏
You do you Richard. I enjoy and have learned a lot from you. If i don't have time the watch the whole video i just pause and start it back up later. Personally the more details in a video are a good thing. For example if you edit out the soft start to shorten video it might lead me down the wrong rabbit hole. As an electrician i cat find and fix any electrical faults in a building easily but i need all the help i can get when I'm looking at circuit boards. Keep up the good work!😃
Cheers! Been eagerly awaiting your follow-up. I'm thankful for all the information you put out.
Some of it is new to me, and some of it is not. But all of it is useful to go over again.
PS/ Assumptions are normal and part of the process. It's what we call an educated guess, and it's educated because of your experience.
I love your patience, me, I would be filling the swear jar by now.😁
Im exhausted, but happy😊.
I love the updates day by day and the fact that you include your 'mistakes' is brilliant as they reinforce the correct thinking as that makes better sense or logic. 👍
This seems to me to be a very complicated circuit and fault finding is tricky. But like a sleep on it overnight, a bit of distance helps and even if I dont understand all of it 1st time around, a look at a simpler circuit now makes more sense and is more familiar, so thanks for that.
One stupid question … why can't that resistor by the mosfets be measured correctly in circuit with the meter probes either side of it ?
This was a really interesting one, good explanations, thanks.
Another great vid 👍
you very good tnx man
Making good progress. Personally, I wouldn't ponder for too long as to why the cap doesn't explode. I'm looking at those two series resistors effectively bypassing the top cap and thinking OMG (for a no-load situation, in my opinion).
One does have to wonder what they were thinking when the designed this one...
1)What is the year of mfg. of the 3525 chip ....
the older ones, the analog blocks were combined with R-T-Logic ( tech. limitations of that time)...I suggest changing over to the "A or B " versions...
The 1st gen. , older ones
(RTL) may be having multiple pulse suppression..but certainly not a memory latch or pulse steering FF before the NOR logic.
so the side that was turned off after a fault ,the same one may be again turned on after the fault is cleared...,such repeated occurrences can lead to a flux buildup causing core saturation and eventual mosfet failures..this may occur unknowingly quickly or even over a couple of days.....
...the core may remain partially magnetised (magnetic retentivity)even with no power..
Despite Being a voltage mode control chip there is no volt-sec. balance ckt...even though there's a " dc" blocking cap...
(V-Sec balancing can be added externally with some R,C, & 1 NOR gate hardware. as part of a feedback loop from the chips two outputs to the shutdown pin. etc.)
2) the diagram shows two different logic/control blocks on pg 2 compared to that of pg.4/5 ..(test setup) ....if the internal of the chip is as shown on pg. 4/5 ..then that is reason of chip limitations for possible failures.
3)Go for chip replacement with new gen. "A /AN /B suffix versions.of recent manufacture...
For more clarity on the chip improvements over different versions .. just look up the internal block diagrams for comparison..of some of these...
3524 vs 3524A/B/D..
3525 vs 3525A/AN
3526 vs 3526A/B ....3823...3846 etc...
I can say with confidence that I would not have been able to figure this one out.
Hi! Richard hope you doing well. I got this question where I can find no answer to. I would like to know how does a pc monitor switch on automatically when your pc boots up? Like how does work and maybe can you do the same with a tv when using it as a monitor to switch on automatically when your pc boots up.
If you just go watch without focusing on it you will not understand the issue on this video, the meat was on the last part hahaha nice 😂
Hi Richard. Loving these vids, learning so much. I have a beginners question on generic SMPSs. Why isn't the start up mechanism used to power the PWM chip continuously used so that the secondary winding connection and associated circuitry can be done away with?
I don't know if this is correct as don't design these things but i think it is for two reasons. One, the circuit powering the PWM controller from the HT+ would dissipate (waste) a lot of heat if it was designed to provide enough current to run the IC and associated circuitry continuously. Two, designing it this way means the PSU will either not start, or will shut down automatically, if there is some fault condition (such as short circuit on the output) that prevents the Aux winding from providing the power to keep the PWM running, without having extra circuitry to monitor and check for various possible faulty conditions and then disable the PWM if something occurs. Some designs also monitor the voltage on the Aux winding to regulate the PSU output voltage which does away with having an opto isolator and TL431 circuit.
@@LearnElectronicsRepair Ah yes, of course. I guess in order to lower the HT voltage to that of the chip their is probably a high value resistor forming a voltage divider that would burn up watts continuously and your comments in the use of the aux winding makes perfect sense. Thank you. 👍
@@johnhawkes7681 To make a complete explanation of this I should mention that the PWM IC also has a property called hysteresis. This means it will not start running (and draw current from the supply) until the voltage reaches a certain level, let's say 15V. Once it starts it will continue to run until the supply voltage drops to a lower level, for example 8V. At that point it stops drawing current and then, depending on the type of IC, it will either stay shut down until the power supply is disconnected from the mains and reconnected, or it will wait until the voltage supply capacitor charges back up to 15V then try again. This is what causes some power supplies to make a repeating 'chirping'sound when they can't power up due to some fault condition
@@LearnElectronicsRepair Thanks Richard. I seem to remember you explaining this in depth in a previous video so I will seek that out and have another watch. Fascinating stuff.
Cliffhanger ending! I think the chip is melted.
I don't believe the evidence in front of us tells us that is the case. Something (and it looks like the supply to the PWM chip) starts to draw current when the soft start circuit activates. It then continues to do that until the lower bulk capacitor discharges to the point that the chips own supply voltage drops to around 8V. It clearly then stops drawing current as the voltage across the capacitor does not drop any further. While some PWM chips will then stop drawing current until the supply voltage increases to the point they can restart and try again (let's say 15V-20V) other chips will give up and hold the supply voltage down to the lower threshold 8V-9V until mains power is interrupted. That is what appears to be happening here and if the chip was fried I don't see how it could behave like that. Therefore I would say this chip is OK (will test in the next part of course) and there is either some problem in the circuitry where the PWM drives the output mosfets (looks like four transistors and some associated components) or all is OK and the replacement mosfets were fake. That's where the current evidence takes me at least, could be wrong of course.
Weird I had posted a reply to another comment here and that post never showed up. Let see if this shows up.
It does
Gee Rich, what you're asking amounts to prior restraint.
please edit your vids sir and make them shorter, we don't all have the time and i like your vids.
To be honest if you don't have the time to watch the video then you probably don't have the time to do this sort of repair work. However I'm always open to constructive criticism, let me know which parts of this video you think were not important to the learning experience and should have been discarded during the edit
@@LearnElectronicsRepairI feel honored just to get a response from you, thanks.
That's what the bar at the bottom of the screen is for, it gives you the choice of how much you watch. *( I say with a smile )* 😁
You can play the video two times faster at YT.
This saves your time 🤷♂️🤷♂️. ... it's magic ....💨💨
@@LearnElectronicsRepair agree, must be the degreg... deterior... generation used to get everything on the fly by delivery service, ready to consume and never faced and solved any problems.
Such kind of people are excellent in discussing not to be resposible for anything. Welcome to the Zombiapokalypse 😉