Why does SMPS take 20 minutes to power up? - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 02:46:03 am
content: I got a sewing machine to repair, which is said to have a faulty power supply. However, after watching a YouTube Video online and trying the suggested trick, it works all fine. The only thing is, I needed to turn on the power switch (no response at first whatsoever) and leave it there for some time (it's said to be up to one hour) before it powers up. In my case, it took 20 minutes before the machine turned on without any problems. Next time (according to the video, as long as you don't leave it idle for long), it will turn on immediately without any problems.It's the rubbish SMPS type, not the traditional linear transformer thingie.What do you think is likely the problem that makes an SMPS work but takes a long time before waking up. Have you seen this kind of behaviour of SMPS before? I've attached photos of both sides of the board to get an idea of its design.Practically, since now I know the trick, I don't need to fix it. But it's always good to bring it back to the way it should be. With a second thought, it is possible that the PS has no problem but there is some problem in the main board of the machine. Unfortunately, now I have no way to ascertain if this is the case, because without leaving the machine idle for I-don't-know how long it takes, I'm unable to reproduce the problem!

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 10, 2023, 03:23:01 am
content: Quote from: max.wwwang on September 10, 2023, 02:46:03 amWell, you could check the DC voltages coming out of the power supply. If they are all there instantly at turn-on, then the problem is likely elsewhere in the machine.

author: xavier60
author_rank: Super Contributor
timestamp: September 10, 2023, 03:30:16 am
content: Also, carefully listen for any faint ticking from the PSU while it's in the fault state.

author: amyk
author_rank: Super Contributor
timestamp: September 10, 2023, 03:30:23 am
content: Almost certainly bad caps. Their ESR is far too high when cold, but once they've warmed up they are just barely OK to work.

author: xavier60
author_rank: Super Contributor
timestamp: September 10, 2023, 03:34:18 am
content: Yes, likely the control ICs Vcc capacitor.

author: fmashockie
author_rank: Frequent Contributor
timestamp: September 10, 2023, 04:10:07 am
content: You've got the SMPS board out of the unit. If you can, power it up to mains (use an isolation transformer if you have one for safety purposes) and check the DC output voltages. The AC/DC converter (likely IC1) could be going thru an auto-restart cycle (hiccuping) due to some kind of fault on the board. It might eventually resolve itself as the board warms up like amyk said.Also, if I am correct in describing the issue, there will come a time when it doesn't turn on at all. Ever. And you will need to fix it.

author: asis
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:16:50 am
content: Hi,It’s trite, this ECAP has degraded.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 10, 2023, 07:17:10 am
content: Replace the electrolytic capacitors. Start with the ones working at high frequency and low voltage. The ones after the mains bridge are usually OK. The smaller ones dry out, or leak, or whatever they do that they increase their ESR. The electrolytics placed near hot components tend to fail first. Higher temperatures damage them faster.However, for damaged capacitors the ESR lowers with temperature increase, and that might explain why the SMPS eventually starts after 20 minutes.Don't bother much measuring each electrolytic capacitor, just replace all the smaller ones.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:37:59 am
content: Quote from: xavier60 on September 10, 2023, 03:30:16 amWhere would this ticking be coming from? Are there relays in the circuit? I don't think so. Keen to know. Thank you.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:39:08 am
content: Quote from: Kim Christensen on September 10, 2023, 03:23:01 amYes, I can check this to find out if the problem happens again next time.

author: wraper
author_rank: Unknown
timestamp: September 10, 2023, 07:42:36 am
content: Quote from: max.wwwang on September 10, 2023, 02:46:03 amUntil a year later this trick will no longer work. I'd say most likely it's a small electrolytic capacitor nearby transformer, with less probability capacitors on the output. But it's better to first measure actual output voltage to confirm if it's actually a PSU issue to begin with.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:44:35 am
content: Quote from: asis on September 10, 2023, 07:16:50 amGood to hear; that's promising! And thanks for pinpointing the ECAP. Its values are 25V 100uF.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:53:56 am
content: Quote from: xavier60 on September 10, 2023, 03:34:18 amThere are two 8-pin ICs on the board, one is UC3842B (bottom right in the photo, near the AC input), which is a "HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER". Another I'm not sure, its markings are:ST(logo) CHN063ACK2X651Which IC do you think have the faulty Vcc capacitor, and why? Thank you.

author: xavier60
author_rank: Super Contributor
timestamp: September 10, 2023, 08:01:06 am
content: Quote from: max.wwwang on September 10, 2023, 07:37:59 amThe control IC, likely UC3842, is supposed to become active only when the Vcc capacitor, as pointed out, has charged to 16V from the HVDC via the resistor next to it. If there is a fault that prevents sustained operation of the control IC, it will stop and then try again when the Vcc capacitor charges to 16V again. The repeated restarts usually cause ticking from the transformer.It's possible for the Vcc capacitor itself to be the cause.

author: magic
author_rank: Super Contributor
timestamp: September 10, 2023, 08:15:07 am
content: The capacitor near UC3842 would be my first suspect too.My first thought was that perhaps it fails to reach 16V at all and hence the controller never starts, until it the cap has been powered for a very long time.But I suppose if capacitance is too low and/or ESR too high the capacitor could also fail to hold charge delivered from the AUX winding and cause PSU restarts.Another common reason for continuous PSU restarts, not related to the capacitor, is overload or short circuit on the output.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 08:23:11 am
content: The capacitor ESR change when warming up theory makes sense. The strange thing here is that even when the culprit (supposedly) capacitor has completely cooled down, when I plug it in and turn it on it works immediately without hesitation. According to the video I mentioned, as long as you don't leave it idle for very long (I take it as, say, more than a year), and keep regular use of the machine, then it works just fine. Since I got this machine only yesterday, so I don't have experience yet as to how long idle time will trigger this fault. All that I have experienced so far is, since the 20 minutes of 'warming up' for the first time, it behaves as if it had no problems at all!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 10, 2023, 08:27:07 am
content: Quote from: magic on September 10, 2023, 08:15:07 amBasically what you are saying here is that what looks like the process of the PSU 'warming up' is simply its repeated restart due to a failure somewhere. Is that correct?Since the machine is now working (supposedly with the warmed-up capacitor) the way it should, can we reasonably rule out the possibility of overload of short circuit of the user side of the PSU -- that is the main board of the machine?Thanks.

author: magic
author_rank: Super Contributor
timestamp: September 10, 2023, 09:16:10 am
content: Either continuous restarting or not starting at all. At any rate, it's clear that there is no major damage if everything works normally after those 20 minutes.It is said that old electrolytic capacitors can sometimes be "fixed", or at least improved, simply by charging them. It's called "capacitor re-forming" and it appears to apply to capacitors with damaged oxide layer and hence reduced breakdown voltage and increased leakage current. The flow of leakage current triggers an electrochemical reaction which consumes electrolyte (if there is anything left to consume) and rebuilds the oxide layer, until the cap stops leaking and holds its rated voltage again.This could explain why waiting gets the machines back to operation. But if the problem repeats, it is obviously better to locate and replace the failing capacitor. I would also consider replacing other similar capacitors on this PCB.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2023, 01:16:35 am
content: Quote from: max.wwwang on September 10, 2023, 07:53:56 amAs others have said, the SMPS PWM controller IC.These often fail in older gear and usually they are charged via a resistive dropper chain from HV DC then when the UVlockout point is exceeded the IC kicks off for its VCC to be sustained by the flyback winding on the transformer via halfwave rectification.If the VCC cap is not in good order (leaky and/or diminished capacitance) the IC supply will fail to meet UVlockout and the IC will not start.It's a good idea to replace these caps with a Low ESR equivalent to better withstand the charge pulses from the flyback winding.Whenever you have successive starts kick any DUT into life start checking caps, properly checking them too not just in circuit.To better understand how SMPS work download and study the controller IC datasheetandthe often included typical application schematic as so often it's closely copied by the SMPS manufacturer.SMPS ticking symptoms are another matter where normally the overcurrent sense is kicking in and out and the noise originates from transformer magnetizing currents building to overcurrent limits then failing, rinse and repeat.This may stem from dud output caps or a short/excessive load further from the supply.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 11, 2023, 07:32:29 am
content: Quote from: tautech on September 11, 2023, 01:16:35 amThanks. Will have a close look at the datasheet and application notes. I watched a couple of videos of Dave not very long ago and I think understood the basics of SMPS, but unfortunately most of it is forgotten!

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2023, 07:45:26 am
content: Just remember UVlockout must be surpassed for SMPS to start.They must have a startup VCC usually via a dropper network from HV DC and these dropper resistors go open too.The VCC cap must be sound for the IC to start.Normal failure points are in no order:HV DC dropper resistor/sVCC capFlyback diodeNot lots to check but if it's a tick fault then you can normally assume the primary side is okay but while in there always check the IC VCC cap, you'll be surprised how often you find sick ones about to give problems.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 11, 2023, 07:19:30 pm
content: Quote from: tautech on September 11, 2023, 07:45:26 amThat really covers everything. Thank you very much. Will have a poking around with my homemade isolation transformer, DMM, and scope, and take this opportunity to refresh my knowledge on this subject. Have a great spring day sir.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2023, 07:40:00 pm
content: Consider powering just the SMPS IC with an external LV supply to be able to check it's functionality without having HV DC to contend with otherwise a differential probe is required as these things are generally at elevated voltages.Safest approach is checking the passives and diodes when unpowered and then power up with a dim bulb tester to better contain any magic smoke.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 12, 2023, 04:38:13 am
content: Quote from: tautech on September 11, 2023, 07:40:00 pmGreat point (safety first, in terms of both personal safety and safety of DUT). With regard to LV, a few more questions:What range of voltage would be good for the test/diagnosis purposes?Would a stepped down low AC input from the AC plug achieve the same thing? I believe so. That would be easier.

author: magic
author_rank: Super Contributor
timestamp: September 12, 2023, 05:00:03 am
content: Quote from: max.wwwang on September 11, 2023, 07:19:30 pmThis can be as dangerous to the PSU as it is to yourself. If you really must play with a switcher and see how it works, use some other junk that you don't care about.If this PSU ever acts up again, simply measure VCC voltage to UC3842 with an isolated DMM and replace that stupid capacitor if it's less than 16V or whatever is the UVLO turn-on threshold specified in the datasheet.Or simply replace the stupid capacitor right away and forget about it...Again, other junk can be used for experiments if you want to understand how things work.

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2023, 05:12:47 am
content: Quote from: max.wwwang on September 12, 2023, 04:38:13 amLV DC supply directly to SMPS controller IC VCC at levels that exceed their UVlockout.Consult datasheet.In some cases SMPS will run from LV AC however this is often at levels that still present a risk for the technician and his/her equipment, eg 90VAC.Remember you still need sufficient HV DC (rectified mains) via the resistive dropper chain to surpass VCC UVlockout for SMPS to kick into life.

author: asis
author_rank: Frequent Contributor
timestamp: September 13, 2023, 04:32:20 am
content: Hi,Not all components are indexed.If you provide the data, I will make changes.

author: picburner
author_rank: Frequent Contributor
timestamp: September 13, 2023, 05:49:32 am
content: One R16 terminal was connected to the wrong negative.

author: asis
author_rank: Frequent Contributor
timestamp: September 13, 2023, 06:31:39 am
content: Thank you.Corrected.What voltages does the PSU produce?

author: MathWizard
author_rank: Super Contributor
timestamp: September 13, 2023, 07:04:36 am
content: Do you have an oscilloscope ? I'd solder on a few wires around that chip, and the secondary outputs, and it should be easy to see what voltages don't get to or stay at what they should be. And with a modern digital scope, it's easy to single capture the start-up, and see any rail that slow to rise, or shut off again/reset.I'm guessing that chip gets is startup power from some large value resistors, maybe some diodes too, right before or after the mains rectifier. If all that is ok, then once it's running, there's usually a winding on the transformer, that helps reset the transformer's magnetic's, and they rectify that winding voltage and use it as the low voltage Vcc for everything on the primary side.Personally, I would map out the circuit and put it in LTSpice. Even if the whole circuit can't run in the sim, it's easy to check some parts, and see what voltages should be on the real thing.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 16, 2023, 06:31:22 am
content: Quote from: asis on September 13, 2023, 04:32:20 amThank you. That's great! How did you do that? Just from the photos I took?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 16, 2023, 06:47:46 am
content: Quote from: magic on September 12, 2023, 05:00:03 amGood points. Safer to fiddle with a junk one. I measured the voltage across the Capacitor when the PS is working. I got 11.5V or so. Datasheet gives Von 16V Voff 10V. It seems to be about to fail again (close to 10V). But how can it start with only 11.5V?When I get a new replacement capacitor, I will replace this stupid guy. The sentiment is felt.

author: mzzj
author_rank: Super Contributor
timestamp: September 16, 2023, 07:02:13 am
content: Quote from: max.wwwang on September 10, 2023, 02:46:03 amLucky for you this is so common problem with this type of SMPS that anyone who has repaired more than 1 SMPS is able to tell you right away what to look for. (as you can see on this thread from replies)If left this way the UC3842 tends to short out requiring bigger repair in future.Replacing the UC3842 vcc cap probably cures the problem but for long-term repair also I'd recommend replacing other small electrolytic capacitors on the board.Depending on your luck these are also about to fail and if not replaced in time can wreack havoc later on.(Probably good for another round of drying out the UC3842 VCC cap but since replacing them all is only couple of bucks this is probably wrong place to save.)Largest electrolytic cap on the primary side needs rarely ever replacement.

author: xavier60
author_rank: Super Contributor
timestamp: September 16, 2023, 07:41:51 am
content: Quote from: max.wwwang on September 16, 2023, 06:47:46 amThe UC3842 needs 16V to start. It will stay active so long as VCC doesn't fall below the UVLO threshold of 10V.

author: tautech
author_rank: Super Contributor
timestamp: September 16, 2023, 07:44:51 am
content: Quote from: mzzj on September 16, 2023, 07:02:13 amYup.However we should distinguish between primary and secondary side failures of which only primary side has been discussed in detail.These are similarly simple, open diodes, dud caps, dud optos and other simple faults in the feedback circuit.

author: magic
author_rank: Super Contributor
timestamp: September 16, 2023, 08:33:17 am
content: Quote from: max.wwwang on September 16, 2023, 06:47:46 amIt doesn't start with 11.5V.Initially the chip is idle and consumes little power. A big resistor slowly charges this capacitor from the high voltage rail. When (and if) voltage here reaches 16V, the chip starts working and quickly discharges the capacitor. In order to keep things working, there is an auxiliary winding on the transformer and a small diode which provide power to the chip. This winding may produce only 12V or thereabouts, whatever is required to keep the chip going.

author: asis
author_rank: Frequent Contributor
timestamp: September 16, 2023, 08:39:51 am
content: Hi,@max.wwwangYes, of course, everything was done from a photo (great experience in “reverse”).That’s why I ask you to clarify the index and values of the components that are marked in brown or (?).It would be nice to hold the product in your hands, but this is impossible to do across the ocean.This post proposes a technique - how to determine the response thresholds of optocouplers by connecting an adjustable power source to the output of the test subject and smoothly by changing the voltage, determine these switching thresholds of optotransistors (pin-3,4) which, in fact, will show what voltages the PSU should produce.https://www.eevblog.com/forum/repair/switching-power-supply-smps-transformer-low-output/It was also said there that measurements regarding ECAP are best carried out with a regular pointer tester, and since I am an old-fashioned person, all these ESRs are in digital values - they don't say much.The arrow is a living organism!- unsolder the ECAP- discharge it- look at its value- select the ohmmeter limit starting from X1- connect to ECAP pins- change polarityWatch the needle react for a few seconds with each measurement.Do this several times.If there is a leak in ECAP you will see it.I don't rule out that this can be done using DMM, but I don't know how to interpret these "jumping numbers".

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 17, 2023, 12:11:46 am
content: Quote from: xavier60 on September 16, 2023, 07:41:51 amThat's exactly what I thought, so this looks strange to me.

author: xavier60
author_rank: Super Contributor
timestamp: September 17, 2023, 12:32:31 am
content: Quote from: max.wwwang on September 17, 2023, 12:11:46 amIf there is12V11.5V on the VCC pin and the PSU is working, it's because the VCC has already reached 16V, the UC3842 has started, then VCC has dropped back to12V11.5V. No problem so long as VCC doesn't fall below 10V.After the PSU has started, VCC for the UC3842 is supplied by Dx from the transformer as shown in asis's schematic.The12V11.5V may or may not be the normal UC3842's VCC voltage for this PSU.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 17, 2023, 08:36:10 am
content: Quote from: xavier60 on September 17, 2023, 12:32:31 amYes, this explains. Thanks!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:36:30 am
content: Quote from: asis on September 16, 2023, 08:39:51 amSorry for my tardiness. I've added some indices and values. Let me know if you need close-up photos of any part.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:58:34 am
content: So, I'm looking to buy the replacement low ESR ECAPs from Digi-key for these (along with a micro switch for another job, and perhaps more to add up to an order that qualifies free shipping):100uF 25V470uF 16V1000uF 10VI'm new to Digi-key and find it complex to get the right parts in the basket. With time, I can figure it out. But I guess experienced people may be able to easily pinpoint the recommended brand and parts?

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2023, 10:01:13 am
content: Just use a common brand such as: Panasonic, Rubicon, Cornell-Dubilier, Nichicon, Kemet, Nippon Chemiconm, Würth etc. Avoid dodgy Chinese ones such as Capxon, Suntan or unbranded.Be chareful to check the foot print, diameter and height.It's fine to use a higher voltage rating, but that often makes it physically larger.

author: asis
author_rank: Frequent Contributor
timestamp: September 21, 2023, 10:15:52 am
content: Hi,There are still red spots.But, white ones also appeared.....Thank you.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 22, 2023, 02:57:41 am
content: Quote from: asis on September 21, 2023, 10:15:52 amMore added.Apparently the circuit is transparent to you. Can you please add some annotations as of what the functions of certain components, or parts of the circuit, are? That'll be super helpful to people like me.

author: eurgenca
author_rank: Regular Contributor
timestamp: September 22, 2023, 05:53:05 am
content: Sorry I can't go out of my skin, but do you replace that stupid cap?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 22, 2023, 06:37:32 am
content: Quote from: eurgenca on September 22, 2023, 05:53:05 amNot yetbut surely that's the plan. I need to order the new caps to make this happen (now still in my Digikey 'cart'). (Trying to fill my cart with some Digikey 'hot-buys' to make the bill big enough for free shipping. It does not make sense to buy a few caps and pay nearly $30 for shipping.) I think I definitely have salvaged caps that probably to the job, but I'm not going to use them here.In the meantime, before the stupid caps are replaced, I guarantee you I will not plug it to the power socket.

author: asis
author_rank: Frequent Contributor
timestamp: September 22, 2023, 10:01:36 am
content: I would like to get the result of stable operation of PSU.It is easier to “read” a circuit diagram than to describe it in words.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2023, 04:51:11 pm
content: Quote from: max.wwwang on September 22, 2023, 06:37:32 amAre there really no capacitors available locally in Australia?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 23, 2023, 07:47:03 am
content: Quote from: asis on September 22, 2023, 10:01:36 amThanks. What do you mean with "the result of stable operation of PSU"? Without waiting for new parts, I have replaced the Vcc CAP with a used one (salvaged capacitor) -- under too much pressure.It works (no surprise), just like it worked with the bad-ish capacitor. I think as long as it worked, there is probably no difference in terms of behaviour now with the replaced capacitor?The values of some parts I'm unable to figure out, for example SMT capacitors, some resistors due to space, the rectifier bridge too close other parts, etc.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 23, 2023, 07:53:03 am
content: Quote from: magic on September 22, 2023, 04:51:11 pmI believe there are plenty of capacitors anywhere in Australia. But here, the beautiful and other-worldly Middle Earth, there might be just I don't know. There is a local electronics parts store however the brand I've never heard of (MaxCap or something). I've replaced the "stupid" capacitor with a salvaged, but good-looking one of identical specs for now.

author: eurgenca
author_rank: Regular Contributor
timestamp: September 23, 2023, 01:51:28 pm
content: So:1. You are so lucky that pull off bad cap again. If that's so, you definitely need to buy a jackpot ticket.2. The problem is elsewhere...

author: tautech
author_rank: Super Contributor
timestamp: September 23, 2023, 07:38:32 pm
content: Quote from: max.wwwang on September 23, 2023, 07:53:03 amValue is not important, 10, 22, 33 or 50uF Low ESR will work. It's a IC VCC rail bulk storage cap, nothing more.