Capacitive dropper power supply no output - Page 1
Forum section: electronics / repair

author: Dannyx
author_rank: Regular Contributor
timestamp: February 18, 2025, 06:31:28 pm
content: Good day everyone. A friend of mine asked me to take a look at his coffee machine which no longer works. It's a Beko brand, but he found the exact same chassis under the Black&Decker brand, so that's irrelevant.I took the little board out of the machine to take some measurements.TL;DR The only voltage I get on it is a measly 2.8v on the two electrolytic caps in the corner. Even though it's not near what I'm expecting it to be, apparently that's just enough to wake the little microchip on the board, because if I power the board from mains and press the only button on the machine, the "power" LED starts blinking. Of course, nothing else happens, since the rest of the machine is not connected, plus we can tell right off the bat the relay itself requires 24v to pull in, or at least close to that, so that's not going to work either.Trouble is, I can't seem to find the reason why it's only giving me 2.8v - all the components check out fine, with the exception of the main cap itself (C12 on the top). When I measured the original one, it was way under the 0.82uf rating printed on it, so the blue one you see in the image is the one I replaced it with. STILL no change in voltage.I traced out some of the most important traces in the image and also uploaded the unmodified one, so hopefully it's of some assistance.It's a typical capacitive-dropper power supply:-mains comes in through resistor F3 (47ohm)-the other "leg" of the mains input is the YELLOW trace-mains goes through cap C12 (0.82uF)-reaches the ORANGE trace and goes through anode of D13 (and cathode of D11 to YELLOW - "neutral" leg).-D13 passes it to the PINK trace. I'm expecting this to be our 24v "regulated" output......but it's not - as described, it's very low. It's also here where I'm not entirely sure how this circuit operates. I was expecting the zener diode D14 to "regulate" the "incoming" DC - after all, its cathode IS connected to the PINK trace like normal, but the anode is NOT connected directly to the YELLOW trace "GND" like I was expecting, so I'm not sure how it operates.-instead, the anode of D14 goes to the RED trace-through trial and error and using basic electronics knowledge, I worked out the component E2 in the top-right is a TL431 - it's got two of its legs conveniently marked R and A ('ref' and 'anode'). The "cathode" leg is connected via the small ferrite bead to the MCU itself, so that's where I'm expecting to see something close to 3.3v or 5v. That ferrite bead proved very handy, because in order to protect the MCU from me injecting voltage and messing around, I simply pulled one leg, thus isolating the MCU section from the power supply circuit. This also ensured it's not the IC itself dragging down the supply rail. The IC itself is probably fine, because if I power it with my bench supply through the wire link that passes through the ferrite bead, it responds to the power button and blinks the LED as described.I replaced the TL431, but still no luck. The original one turned out to be functional, since I tested it on the bench.I also tested D14 off the board and it turns out it started conducting at 18v, so it's an 18v diode - slightly lower than I was expecting, given the rating of the relay, but I'm not even getting that, so the diode is probably fine.As a last ditch effort, I replaced the "first" electrolytic cap on the pink trace.At this point I don't know what I should try next and can't figure out why I'm not getting anything on the pink trace. The way the zener diode is wired also stumps me. I'm hoping someone can take a look at it and maybe recognize the setup and set me straight. It's really annoying because it's one of the most basic power supplies, with only a handful of parts, yet it's proving really stubborn.

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2025, 06:36:00 pm
content: It's usually the dropper cap going low value, check C12. Then it's a shunt regulator, the zener also can fail.Why is the lead lifted on the ferrite bead?edit: I think it's a 24VDC relay coil

author: madires
author_rank: Super Contributor
timestamp: February 18, 2025, 06:44:19 pm
content: If the dropper cap circuitry is fine maybe some component draws excessive current, e.g. an MLCC to ground.

author: inse
author_rank: Super Contributor
timestamp: February 18, 2025, 07:34:13 pm
content: You can try to carefully feed some voltage into the green electrolytic cap with a lab power supply.If something is drawing excessive current, you will notice…

author: Dannyx
author_rank: Regular Contributor
timestamp: February 18, 2025, 07:46:31 pm
content: Quote from: floobydust on February 18, 2025, 06:36:00 pmYes, I did mention all this, including why the ferrite bead is off the board. I know it's a long post, so people probably give up reading half-way through, but still, all the info is thereQuote from: inse on February 18, 2025, 07:34:13 pmThat cap is the one I replaced. Your suggestion is what I tried first, just to confirm the MCU still fires up and it does. There's no direct short or excess current draw anywhere apparent, unless I'm missing something.

author: fzabkar
author_rank: Super Contributor
timestamp: February 18, 2025, 09:12:57 pm
content: https://images.chipyun.com/pdf/C883402_DA6E15596C52F41AE3DE0B77519D5C7A.pdfThe MC96F8208S microcontroller is rated for 1.8V ~ 5.5V.Vdd = pin 12Vss = pin 13ISTM that the TL431 is setting the Vdd supply to 3.6V.Vdd = (20K + 9.1K) / 20K x 2.5V = 3.6375VThe Zener then elevates the pink trace to 3.6V + 18V = 21.6V. That's the coil voltage for the relay.I would suspect D11, D13 and the two electros.You could calculate the current draw by measuring the voltage drop across F3.The relay's Pick-up voltage (Max) is 24V x 75% = 18V. The coil power is 0.36W, so coil current is 15mA.https://www.sanyourelay.ca/public/products/pdf/SRD.pdf

author: Smile2016
author_rank: Regular Contributor
timestamp: February 18, 2025, 09:33:39 pm
content: Quote from: Dannyx on February 18, 2025, 06:31:28 pmtypo ? 470 Ohms or measured?check the 2x 220k smd resistors R56+R57

author: Dannyx
author_rank: Regular Contributor
timestamp: February 19, 2025, 06:21:43 am
content: Quote from: fzabkar on February 18, 2025, 09:12:57 pmHow does this part work ? This is what I was having trouble understanding, since I'm more used to seeing a zener going straight to GND, so that's why I couldn't work out how it operates in THIS configuration.D11 and D13 are "M4" diodes, for the record. They read fine on my DMM, but you never know.Quote from: Smile2016 on February 18, 2025, 09:33:39 pmYes, that was a typo - it's 47ohms, so I corrected that.R56-57 are fine, though I don't know what they do, since I haven't traced that part.

author: fzabkar
author_rank: Super Contributor
timestamp: February 19, 2025, 07:05:24 am
content: Quote from: Dannyx on February 19, 2025, 06:21:43 amIt's just a Zener-regulated supply sitting on top of a TL431-regulated supply. It would be like a 12V battery sitting on top of a 3V battery.

author: Dannyx
author_rank: Regular Contributor
timestamp: February 19, 2025, 09:00:23 am
content: Here's where I think I made a screw-up: when I replaced the original TL431, I failed to take note of the orientation of the 'ref' pin. Since the 'ref' pin is on the left, this is probably a TL432. I forgot to check this and probably used a TL431, which of course was not going to work, since the 'ref' and 'k' pins are swapped. I figured it out through trial and error, so here's what I did: first, to be sure I get the pinout right, I just ditched the SOT package for now and soldered on a regular TO-92 on there. This worked out far better, since I could clearly read the label and figure out what part it really is, so I could orient it the right way. To confirm this, I soldered a 1k resistor to the "PINK" trace and fed it from my power supply (since there's no series resistor to that trace otherwise). This also seemed to work: as I increased the input voltage, the TL431 was now working as it should and kept the rail at a constant 3.6v, which is correct.I measured it on mains as well and the same thing happens, though the pink rail is still stuck at 3.6v.....that is until I put the board back in the machine and it suddenly started working ! WTHIt makes sense to a degree: with all the sensors and switches now connected to it, the MCU was now satisfied and actually started the "program" and activated the relay to the heater.....but still, there was no voltage going to that relay, so how can it do it ? I THINK it has something to do with that brown trace which comes from the MCU pin 1 is connected to the RED trace via that 20k resistor: initially it seemed like V3 is an NPN which pulls the anode of D14 to GND when the MCU tells it to, but pin 1 of the MCU is also connected to the 20k resistor, so that can't work either. I guess it works now, but it would be great to learn how it works as well

author: fskozrit
author_rank: Newbie
timestamp: February 20, 2025, 07:21:09 pm
content: There is desoldered point on relay coil . Why ?

author: Dannyx
author_rank: Regular Contributor
timestamp: February 22, 2025, 09:56:29 am
content: Image was taken when I was still troubleshooting and at one point I separated the relay from the board, both to check it with my power supply to make sure it actually works and pulls in and also to make sure it's not the one causing the power rail to sag. Like I said, it appears to be working now with the botch-soldered TL431 in place, but I'm waiting to get a proper TL432in SOT23 package, so it doesn't look ugly. Still not entirely sure HOW exactly it operates, to be honest

author: Psi
author_rank: Super Contributor
timestamp: February 22, 2025, 10:39:22 am
content: It's important to understand that capacitive droppers are current powersupplies, not voltage.You have some amount of current to play with based on the size of the mains rated series cap.You can short it with a Zener to get any voltage you'd like at that current.All the current you have available from the mains cap is burnt off as heat somewhere, weather you use it or not. It's either burnt off by your load or burnt of in the Zener when you're not using it.

author: Peabody
author_rank: Super Contributor
timestamp: February 22, 2025, 03:11:55 pm
content: I've been able to repair two dropper/zener supplies that had died - two Disney coffee warmers. In both cases the series dropper cap was fine and the zener was fine. But the big electrolytic smoothing cap on the output had gone bad. They still measured reasonable capacitance, but ESR had gone off the charts. Anyway, replacing them is what worked. It's not clear why those caps would have failed while the dropping caps didn't. The output caps never saw more than 5V, while the dropping caps would have been exposed to whatever spikes came in on the mains. But of course they were not electrolytic, so maybe that explains it.Big Clive did a video on these supplies, and said that problem would very likely be the series cap. But that has not been my experience.