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Checking out a faulty thermostat sent by Roger for our exploration.
For simple power faults like this one has it does actually make sense to do repairs.
With very predictable failure modes these days you don't need much electronics knowledge or experience to attempt a repair. A really common failure mode of compact switch mode power supplies is the capacitor on the low voltage side. That's because the very high frequency it operates at results in an amplified electrical stress. When the capacitor starts to fail its impedance increases and that causes an avalanche effect with the gassing of the electrolyte causing a buildup of pressure that domes the top of the capacitor and forces the electrolyte out through either the rubber seal at the base or the domed end (which is designed to rupture) as vapour.
The main thing to note is that you must use a low ESR (Low Equivalent Series Resistance) capacitor to deal with the high frequency operation, it should be the same capacitance value and a voltage equal to or greater than the original, and you must ensure it gets put in with the correct polarity.
A full replacement thermostat would have cost around £55 while the replacement capacitor was available as a pack of five for £1.
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A faulty thermostat, a heatmaster thermostat. This comes in two modules: the back module has the power, switching circuitry and the power supply, and then it's got the electronic module with our memory backup battery on it and the node that came with this says. Please find enclosed one. Heatmiser smart thermostat for your investigatory powers.

It had been working fine, then suddenly the display went blank. I have no idea what's wrong with it, but i thought of a quick noise. Hence it's in bits. I suspect the problem lies in the mains power interface, which i managed to separate rather than the display itself.

I could have put it back together, but i don't think it would have helped to do so, and this has been uh rised out. So this module would normally be sitting down in here and when it's sat in there, this little cover clips in and then you actually swing them together. It makes contact with this little connector down here into faces of this one. So we can do a quick test on this.

How do we look at the circuitry? Let me zoom down on this and we'll take a little look together for a start. The first thing i looked for was a domed capacitor and well there's a dome capacitor. So this is a classical switch or power supply and a relay arrangement and the incoming supply comes in and it doesn't use a bridge rectifier. It uses two diodes in series to create a very crude dc supply, but why are they using two and series - and i thought well that's odd i've seen that before, but why are they doing that and then i saw the neutral as another diode against they've got Three diodes in series very strange they're, mostly to have redragged fire, but there must be some reason that the manufacturer, putting all these diodes in series, there's a little bit of damage at the edge because uh is it george, let's just double check the name roger.

Actually, it's roger synthesis uh had been leaving it out, it's very easy to damage circuit boards like that when uh they're in there but everything's intact. So we have the switch hold power supply based upon those crude diodes, a little filter, a smoothing capacitor and then the switch mode chip creating its own little supply. With this little chip here and then the output we've got a diode here, uh going to this capacitor and then we've got an auxiliary little 78 lo5 regulator which converts to 5 volts and i'm guessing. This is going to be 12 volts, because this is a 12 volt really and if we also look in the back, we can see there is a protection diode across the relay coil to protect this transistor and there's a resistor going to the transistor, which means that This module on this connect, we've got five volts, zero volts, the signal to the relay and then a couple of sensor inputs possible with their own ground.

Let's hook a mains connection to this and see if it goes, bang here is a cable, so there's the live and then normally open the relay it looks very robust, they've really gone to town with the isolation slots here so live is going in there make sure I put it into the right place, that would be extremely dramatic and it would make it a very short video if i put it into the wrong place. No earth connection and we've got two common neutrals here so and it goes to one of those and we'll plug it in to see if it goes bang. If it doesn't go bang, we shall probe it and see what voltages are present. So i'm plugging this in it's going to be live.

That means voltage just it hasn't gone. Bang, that's good! I mean there's plenty of time, yet let's bring in a meter and set it to the 20 volt range. This is this thing that i often see. Let's zoom out a little bit, i often see that i always use my meters on the 20 volt range in continuity.

It's true! It's almost that's almost exclusively! So, let's see if i can do this without shorting things out. So i've got a connection here and a connection here that should be about 12 volts, it's 5 volts, which wouldn't be enough. That's 5 volts theoretically against the voltage regulator, and it should be putting 5 volts out. It's not really managing much because um that power supply is way low.

That should be about 12 volts. So i'm going to guess that capacitor is faulty right. Tell you what i'm going to grab a capacitor and we'll swap the capacitor and we'll see what happens if we can find one, that's small enough! It's the curse of these power supplies that the little electrolytic capacitors - let's short, that capacitor just in case hold on there's the the mains capacitor. Is it dead? It's dead.

It's fine finger test, yeah, it's dead, uh! It's the curse of these power supplies that, because the uh output capacitor is run at very high frequency, it dissipates a lot of heat because it's seen current flowing in and out of it really really at a very high frequency. It just adds up to a lot of dissipation if the electrolyte fails, and it has to be a specific electrolyte for that it causes a gas to form inside which causes the doming and also damages the ability of it to act as a capacitor. So it tends to go a bit odd uh, so i shall pause momentarily. I shall go and get a capacitor uh i'll get the soldier iron on and we'll see.

If we can fix this one moment, please: okay, the soldier iron is up to temperature. Sadly, the only capacitor i've got in that rating, the 220 mega fart 50 volt bullets - 50 volt low esr, it's very big. I don't think it's going to fit. I mean it's no bigger than the main side electrolytic, but it's just going to be potentially a bit tricky to fit in here.

I do have a couple of 100 microfarad ones that could be put in parallel, but that's going to be a bit messy. I don't really care much of that. Maybe i should get the right components, but anyway, let's see if we can get this out and we'll start by zooming down for a start, and i shall flow some soda onto the existing pads of that capacitor, because the solder they'll have used is lead-free solder and We all know that lead-free solder is just terrible for these soldering. It benefits greatly from from some fresh juicy lead based soldier on it.

I could get the proper. I could get that low melting point, so that's used for stuff like this one day. I probably will this is also very hard to grip this component in the back. So can i use something to help grip this? Can i basically get a blue tack or something i don't know if this is going to work? I shall put a bit of julian eyelet style, blue tack on that component to squish it round it just to give me something to grip and then i'll, try and flow, both those sort of pads at once.

No that's just come off that has not worked. Let me just keep trying this again. Let me just make a dick of myself by getting nowhere and flow, both these sort of connections at once and jiggle that little component well i'll. Tell you what the the blue tack softening! That's! That's a start! It's it's! Starting to come out, it's actually reachable.

Now i can get my fingers on it, except now. It's slightly greasy because the blue tack or white texture says, since it isn't actually blue. Oh, you know what let's give myself a wee handle here. I'll use some captain tape.

This is very fruity, i'll, improvise and i'll keep making a dick in myself because uh this is what happens. Turn a camera on. Everything will go wrong. So, let's see, if i can get this tape onto this component in a very crude manner, it's almost there anyway.

It looks as though it's just wanting to come out. This is uh, something i've not really tried before. So if it doesn't work, it's because i haven't tried it before right. Let's try this.

It does feel better heat. The two soda pads to heat those plated through holes alternate back between the two of them until the capacitor pops out now comes the way to clear the solder out those holes that i could use the toothpick approach, which works quite well or i could try the Bang on the table approach, which makes quite a lot of noise, but also works pretty well, you heat the solder up and then you bang it and theoretically the inertia makes the solar fly out. Those holes are fairly clear. Now is my new big fat ugly capacitor going to fit in mmm tricky? I think it may well do make sure you're getting the right way around.

Oh, you know what you know. What i think that is gon na do just fine uh, make sure you check polarity before doing this, just because it helps putting the capacitor in the right way. Around is always a good thing. I shall also be checking it just relative to the adjacent circuitry as well.

The middle pin of the little regulator there is the negative pin on the 78l5. Oh, this looks good so far. I'm saying that it may never ever go back into the enclosure again, because i've just put a huge fat capacitor in on a plus note, the higher voltage rating and the higher mass of the capacitor means that it should do better with the ripple. Do we give this a check? Yes, we do where's the mains connection again and we'll power up uh, slight feelings of insecurity.

Yeah, that's bad! One always gets feelings of insecurity when one is doing electronic stuff, particularly involving mains voltage, it's quite normal for electricians to be. Shall we say pensive before powering something up, particularly if it's very big and expensive and variable frequency drives come to mind? There's a fantastic video floating around on the internet on facebook of a guy doing all his tests before turn the breaker on a variable frequency drive and the thing just detonates. So i've got two neutrals here coming together. So i'll use this one this time or maybe i won't because the terminal feels a bit chewed - is that just spinning it's just spinning that is a chewed terminal.

It may have been the subject of the wrong type of screwdriver. These things happen. I shall put into that terminal then right. Let's turn it on and see.

If it goes kaboom it may go cabin uh i'll, sweep all the metal bits out of the way. Since this is gon na wander all over the bench. Isn't it and it's powered up now, okay, bring in the meter set it to well. It is at 20 volt range.

My favorite range i'm expecting closer to uh 20 volts. Now i'm trying to do this without actually getting the wheel meter and we're gon na get anything here. I'm gon na get a connection. Even is this gon na need a new.

Oh, that's that is much better. 12 volts is much better. That means i should also get across this little dinky capacitor. I should get roughly 5 volts.

4.89 is perfectly acceptable for that right, i'm going to unplug. It now always remember to unplug it. You will get caught out that way. One day right tell you what let's put this little plate over just initially.

Is this even going to fit into that module now i'll find out later on, i guess and uh i shall sit it onto the connector, make sure it's the right position, because otherwise it's just going to blow everything up and then it would all end in sadness. Right is this going to power up the display? Is it going to do anything? I rather think it is it's displaying dashes as room temperature, because there is no thermostat sensor connected to it. I would guess: oh there, we go 22 degrees celsius luxurious for in here, but it has no external sensors set up. Oh the touch thing does work and it lights up.

I would say, to be honest: that's probably it fixed right. Well, that was a success. Now the question is uh: will it fit back into the original thing? So i shall just bridge that capacitor, because there is a space capacitor there there's a one that is prone to causing much pain and it's that electrolytic capacitor there. It's discharged.

That's fine finger test for confirmation. Yes, that's correct and we'll see if it fits back in here. It is the moment of truth drop. It in here observe the chew marks from uh roger's activity.

Have i got that name right again? Yes, i did get it right. It is roger, and is this going to click back in? Oh, you know: what does the circuit board does this just pack against the circuit board, or does it actually clip under, but it's not really sure mumble mumbo mumble uh talking to myself? Will this clip in at all is this how it goes in? Am i supposed to put it into that first? I think i might have to put that into that. First, hey! No, i think it just spaces like that yeah. This is all kind of just laughing around now.

Isn't it is that going to click down? I think the circuit board's gone down far enough. It's just uh yeah. That's it loud poppy clicking noises, that's going to sound great, and that is presumably oh, that does make together quite well. That's, presumably it so uh.

Yes, that is it fixed. So if you have one of these and that suddenly dies, this applies to so many things. The first suspect is these little capacitors, i mean: what's it even like inside, i don't think there'd be anything visible inside. Shall we open it up? Yes, we shall, let's just randomly open it up with random chemicals all over my fingers and see what it looks like inside there's a little seal at the bottom.

It might be completely dry when they get hot, they sometimes squish all the electrolyte out. Oh, it looks fairly dry inside. I think that's it. I think it's just basically it's vented it's the pressure's built up and it's basically squirted all its juice out and it's dry yeah that feels very dry and that's why the capacitor is dead.

Let's unravel it, let's go all the way with this, because we can where's a knife. Where is a knife? Where is my knife? I've misplaced, my knife, not to worry. I shall use this in a very crude knife-like manner to slit at that tape. That's not going to work is it.

This is unprofessional. Yes, it is it's. What you've come to expect in this channel unprofessionalism. So here is our file.

I've just stabbed myself in this sort of connection. Oh, it's a bit crumbly i mean, i suppose it's justified in being crumbly. Oh that's one of the sort of tab connections. Then it's the other part of the file yeah that is absolutely dry, just basically the wicking material for the electrolyte and separating the aluminum foils inside metal foils.

But there we go, the fix is done and that is more or less it. So if you have, one of these first suspect is look for those domed capacitors on top, because that's what happens they just the pressure, builds up. It's the sign that they're going to vent their electrolyte or the bubbles have formed and it's uh diminish the capacity and it's a very cheap and easy fix. But you have to use the low esr capacitors the ones suited to a very high frequency ripple current, because the switch would power.

Suppliers are really quite heavy on them, but there we go good fix.

11 thoughts on “Fixing a dead digital thermostat”
  1. Avataaar/Circle Created with python_avatars Paul Saunders says:

    There's a very common problem on vaillant boilers where the water temperature on the display fluctuates quite wildly. This is due to 2 electrolytics failing. You can always tell because if you take a hairdryer to them and warm them up, it fixes the problem until they cool back down obviously. It's because the voltage to the water temperature sensor has become unstable I believe. It's a very common problem with these boilers that use this particular control board which most of them do.

  2. Avataaar/Circle Created with python_avatars Joseph King says:

    Do you have a cold Clive? 🤧 Take care if you do, ok.
    I had a couple of Wireless Access Points that just stopped working after 8 or 9 years. Spent AUD$5.95 on 4 electrolytic caps and replaced the ones that looked like they had been eating too much of something and got them back working again. 👍 Most pleased and now I can give them to someone who may have use for a couple of 2 band WAPs.

  3. Avataaar/Circle Created with python_avatars Nigel Johnson says:

    Yesterday I spent some time attempting to de-solder a couple of transformers from one of those dimmable fluorescence light bulbs.
    My intention was to see if one of them could be used in a high voltage inverter circuit. The PCB was packed with a remarkable number of both leaded and surface mount components.
    Unfortunately all the components were held in with lead free solder that appeared to only melt well above 400C. Despite using all the tricks, solder sucker, hot air gun and two soldering irons, solder wick, I gave up after an hour.
    The transformers have five pins, which fitted near perfectly into the very well gold plated through holes. The solder had wicked up on the connections to make them look top and bottom soldered, and to add insult to injury the transformer pins had obviously been bent over during assemble, ensuring they would not drop out when all the pins were heated at once with the hot air gun. I suspect that the transformers were also glued to the PCB.
    Individually melting the solder on each pin was an exercise in futility, as the lead free solder re solidified as soon as the iron was removed from the joint. Because of the tight fit of the plated through holes, solder wick and the suction gun could not remove the solder on the sides of the connections.
    I tried diluting the solder with the lower melting point lead/tin but that did not work. I have to conclude that some products are just not made to be disassembled for salvaged. Did I mention how much I hate lead free solder.

  4. Avataaar/Circle Created with python_avatars Power Max says:

    dumb idea, what if you took a needle and injected it with some electrolyte? Can you recover dead electrolytic capacitors? Maybe try different fluids/electrolytes too! salt, lye, calcium chroide, Gatorade, coca cola, urine even, the ideas are endless!

  5. Avataaar/Circle Created with python_avatars Demef says:

    Electrolytics in switching power supplies are not your average run-of-the-mill capacitors, and for good reason. If you study how an off-line switcher like this works, you find that most of them are flybacks. That means the primary conducts current while the secondary is off, then on the other half of the cycle it reverses, where the primary is off and the secondary is off. The secondary current is either a triangular waveform (starts at zero, jumps to some large value in an instant, then ramps down to zero) or it's trapezoidal in shape.

    The key takeaway is that the AVERAGE of either of these current waveforms is the DC output current. If the DC output current is, say, 1A, it means the peak secondary current is quite high, maybe 4-5A. This current is dumped into the output filter capacitor, which is actually an ideal capacitor in series with the capacitor's Equivalent Series Resistance (ESR). A generic electrolytic has a very high ESR, several ohms; a switching capacitor has, by design, a much lower ESR, ideally in the milliohm region. The large flyback secondary current waveform hits this ESR, and as everyone knows, you then get I^2*R power loss in the ESR resistance, which produces heat. Heat is the mortal enemy of capacitors because it dries them out. This is why the usual failure mode in a flyback switcher is the output filter capacitor(s) — the capacitor simply dries out.

    For the budding EE out there, draw a triangular waveform that has a 50% duty cycle (just guessing that), and a peak current of Ipk. Draw in a horizontal line through this waveform and call it Iout. The equation is Iout = Ipk/2 * 0.5, the area under the triangle for the entire switcher period. As you can see, Ipk = 4*Iout. The same sort of stresses apply to the primary filter capacitor. Conclusion: flyback switchers stress the dickens out of their capacitors.

  6. Avataaar/Circle Created with python_avatars Uwe Polifka says:

    That was a quick fix. This year I learned more about capacitors than ever before. I think capacitors are most responsible for defective electronics at all.
    To make desoldering more easy I bought a cheap ADT03 "sucker" from my favorite Ali in china and I can recommend it. There are reviews on youtube.

  7. Avataaar/Circle Created with python_avatars carpespasm says:

    1) I'd never considered that other countries run their thermostats from the mains. In the US they run on ~24vac from a small transformer in the air handler usually.
    2) It sounds like it might be the norm to have an external temp sensors on UK thermostats instead of them being expected to be in the thermostat itself.

  8. Avataaar/Circle Created with python_avatars Gazr Gazr says:

    Auto flood light was just pulsing on and off, took to bits and it was the dropper cap knaffed, put a class x cap same capacitance value in place, magic, the existing one inside the pir sensor was just a cheap bog standard 400v 470 nf resin dipped type, voltage just dive bombed as the relay clicked in, so circuit was just a 1 second strobe lamp😎
    Ps how many pir flood lights and other things have been dumped for the sake of a capacitor fail.👍
    Gaz North Yorks.

  9. Avataaar/Circle Created with python_avatars L4rgo 117 says:

    HVAC systems in the US use heating or cooling unit (transformer) derived 24VAC and the thermostat has a bunch of wires that it relays that voltage between that then goes back to a control board to be interpreted or directly pulls in contacts. Only real systems I can think of that use a different system are vrf/vrv (variable refrigerant flow) mini split type systems which use modulated mains for signaling if I recall correctly. How do UK or European designed thermostats function differently? Obviously the machines they’re designed to work with are fundamentally different in some degree, could someone explain the difference? I’m picturing something akin to how an occupancy sensor works but I could be mistaken of course, it never occurred to me that Euro thermostats would work any differently

    Edit to say Technology Connections on YouTube has a good video on US thermostats if my explanation of US systems was unclear

  10. Avataaar/Circle Created with python_avatars kirkb4989 says:

    What a coincidence that you posted a broken thermostat as I had a thermostat "issue" last week. My Nest stopped turning on the furnace. After checking wires, running a new Common wire, and also restarting it, tests showed it COULD turn on the furnace in the equipment test mode, it just would not do it normally. I went through 3 days of a very cold house until I finally called Nest support. Turns out they had done a firmware update just that morning that apparently had caused it to THINK it was turning on the system but it really had not. A factory reset, and reinstall in the Nest app finally fixed it.

  11. Avataaar/Circle Created with python_avatars Ted McFadden says:

    Having been put through several traumatic events due to cheap components (especially capacitors) in power supplies, my own OCD would require every one of those caps be replaced. It's always fun to tell a customer why their $$$$$ audiovisual system doesn't work because of a $10 power brick I don't have a replacement for… 😅😳😁 👍️

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