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This appears to be a common fault with these units. If your heat link fails out of warranty it may be an easy fix to get it going again.
Here are some simple tests that can be done on an unpowered unit to check that the rest of the circuitry is still working.
Note that in the UK our boiler (furnace) control systems are usually 240V. In a way it might be better to adopt the American 24V system. Especially since electrical training standards have plummeted due to the activities of a financially motivated "voluntary electrical safety" company.
All equipment associated with a heating system should be fed from just one circuit. Adding on other sections powered from other circuits can energise an isolated/disconnected system randomly via the control line.
These units have been educational. I really like the use of two different electrolytic capacitors in parallel. As well as being shorter than a single high value capacitor, if one is later found to be prone to drying out, the other will keep the system running. Likewise the use of two diodes in series means that if one fails short circuit the other will keep the system running. The small extra cost of these extra components is massively dwarfed by the cost of a warranty return/repair. Especially when the unit has to be replaced by a technician.
Note that the primary suspect component for failing is quite tricky to remove due to having a heatsink pad underneath. It is much easier to remove with two soldering irons being used as improvised heated tweezers.
If you have doubts about desoldering and replacing the diode, then don't risk damaging the PCB. There are eBay sellers offering a fixed price repair service for these units.
I wonder what replacement component they are fitting in the warranty exchange units.
The original is an S3100. The one I used is a B360B - which is rated 3A and 60V. I can't guarantee that this is a perfect replacement. A diode rated 3A or higher and 100V or higher is probably a better option. I've since found out that the original package is called a Powerdi.
Once I've got and tested some others in I'll add the stock numbers here.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.

Recently i took a look at this nest and fixed it now. I want to point out these nest thermostats. These are actually called the heat link units and they power the remote thermostat, but they also receive a signal back that then controls relays which switches the mains power, so these units do potentially contain mains at 240. Volts in the uk.

Keep this in mind. If you open them, however, now i've got another dead one to look at um. I can check some things out. Is it the same fault? That's happened? Is it uh? Has it got the same arrangement of two different capacitors, i'm going to take a look inside and see that, but it turns out that there's a very simple test you can do to check if it is the 12 volt power supply, that's failed! If you loosen this screw in the bottom, it can be done while the unit is in the situ and you just loosen it and lift the lid up like this.

It reveals the electrical connections in a usb port. Now it's worth mention at this point in time that, if you're not used to working with mains electricity, you should turn the power off before taking the cover off and for an electrician. It'd be quite useful to probe in there and see if it was getting 240 in the first place. But, as i said, turn the power off to the heating, also worth mentioning that just to treat it as if it's live anyway, because with the new era of one week, electrical training and becoming electrician.

There are increasing numbers of instances of back feeds coming from new heating circuits to the existing heating. Let's not even go there, it's a bit of a cesspit, but as long as large privately when companies are making lots of money. I guess that's. Okay, now notice the little led indicators here that i shall shield from the light.

This is a micro, usb, cable connect to that fetching pink power bank and i'm going to plug it in to the usb port and it's going to feed into the low voltage circuitry. The leds are going to light, the structure is going to work, but the relays are not going to click because they need the 12 volts. That kind of shows the circuitry is kind of working. There is another test that can be done if we bring in a meter set to diode and continuity, then the 12 volt supply here that feeds the remote thermostat appears to be basically connected directly to the 12 volt bus in here.

So if we turn this on - and we put it around one way - normally we'd get a diode one way and the sign of a capacitor charging i'll show you in that one and this one we're just getting a dead short circuit in either way round. In those connections so that kind of suggests that 12 volt is dead, very intriguing, i wonder why this has happened. Let's pop this one open i'll, show you what you should see if the 12 volt is not dead for a start uh if you set it to voltage while this was powered, you should get 12 volts on this, but if it's not powered here's what you would Get if the diode and everything was intact, you would get a rising voltage round. This way, which indicates a capacitor is charging and round the other way.

You would get basically a diode voltage drop about 0.5, which is the diode i put in probably less. If it's the original schottky diode, so that's quite an interesting couple of tests you can do, but now, let's pop the back off this, let us open it up and see if it's exactly the same fault occurred before. If there's a suspicion, it is that diode. I shall pop that out and we'll see if it clears find a replacement.

Finder replacement turned out to be quite tricky. None of my usual uh proper, real, not ebay. Electronic suppliers had that exact diode. That's annoying so unscrewing the back here.

This is also an indication. It's it's made sense of another product why they have? Oh, this double sided, tape's quite handy. It means you can just stick that off uh. It's why they had a couple of diodes in series.

So here's the mains power supply section. There is the suspect diode, which ultimately i get the feeling this is going to be short circuit. Isn't it continuity on? Is it going to go beep? Oh yeah yeah. I reckon that died screwed right here, while i've got this open and before i change that.

I just want to take a wee peek in here, because the other unit had two different styles of capacitors and i wondered if one had been changed at some point or, if they'd put in two different types of capacitor, just as built in redundancy in case. One of them failed, which is quite a clever approach. Putting two diodes in series is also a clever approach, but they'll be wiser for that. I wonder what made it fail in the first place, this has a ribbon cable attached, but i'm just going to lift it gently at this end, without stressing that too much it has the same two different styles of diode, okay, capacitor should i say, that's quite interesting That is quite educational.

Now what i was saying there about the multiple diodes. I was perplexed when i took another product apart recently it was the uh revolution, air freshener, and on the three evolutionary fraction there was a series diode from the main supply and then two current limiting resistors for the power supply. But in the back of it there was so this diode here to start off with that went up to there and then it went through a surface mount diode. So two different styles of diode, the through hole and the surface mount, but in series, and the reason for that is if one of those went if there was just one that went short circuit, these resistors would then operate at full power and they'd be dissipating their Turn series they'd be dissipating roughly one watt, each which is still within their rating, but it would the unit would stop working and it would also potentially uh bullets operating these resistors at their limit.

It'd get quite warm inside and not in the bit. That does all the proper oil vaporizing stuff in these, but also because there are two diodes and series, if say, for instance, this surface one surface might want one surface mount one went faulty. This one will still work and it adds a layer of redundancy and having those two diodes in series. That's quite useful to know.

I shall keep that in my mind for future right. Tell you what i'm going to de-solder this and i'm going to actually get two soldier irons and get into it because uh it's better to tweeze this out, so i'll pause momentarily and i'll be back in one moment once i've removed that okay, let's try getting this Rogue diode off and i'm going to tweezer off with my old antex xs-25 solder iron. This is what they call a thermally balanced sojourn. It puts 25 watts on no temperature control.

It's all based on the size, the mass of the bit, what temperature it gets to. So i'm going to use that one side of this component because it's very difficult to solder. In fact it's got a huge heat sink pad underneath it on the other side, i'm going to use another soldering iron, i'm going to try and tweezer off. This may not go to plan i've already wetted the pads.

I know that the one on the left is just going to be so there it goes. Oh that wasn't too bad was it. Oh. I dropped it.

Oh no, it's kind of worked right, get off. There's that pesky pad underneath see that shiny area underneath that is annoying, do you know it's disappointing? It looks as though they've made an effort to use a decent diode, and it's just let them down such a shame. I don't blame the designer for this problem at all. I wonder if part of the problem is the fact that that 12 volts goes out to external circuitry.

I wonder if it's an esd thing, but i'm not really sure very hard to say right, i'm going to just go and grab another diode and then try soldering it on there. One moment please: okay, let's clean the excess soda off with a bit of a soda wick and some fresh flux, squirted onto that soda wick. So it's extra juicy because that helps soak it up and we'll just dab it on that pad to clean it off and scrub it a bit to clean those pads. That's looking pretty good! Now diodes, i ordered the close.

I could get three amp 60 volt diodes. This one is comically huge. This one is a ss 36, shame really memorable name, but it's actually just too big for this. This one is just grim: it's called pme g6030 ep, but it's just marked a b and is just a bit squirmly small for that, the one that won my choice, whether it's suitable or not, is the b360b, which does fit in fairly well, but is quite a Deep diode right, so what i'm going to do, i'm going to throw some soda onto here and then i'm going to reflow it and sit the diode onto that holding it with tweezers i'm going to make sure i put it on the right way around.

That would be really really good. I have got my big daddy, so darn here my antics, i'm just trying to actually not touch it with my get burnt off it right. Tell you what so i'm going to sit that diode up make sure that the little lines are going the right way. You can probably see this better than me yeah.

That looks like it's going the right way. If it didn't it, the power supply just wouldn't be happy again. Uh, it wouldn't harm it, but it would just not be happy it wouldn't actually boot up properly, so i'm going to reflow those while pressing down. I think i might just do this.

My fingers, to be honest, this is probably a bad idea, big clumsy beer that i am, but anyway, i shall attempt to. No the system see surface mount is a curse for people. My size right, tell you what mm-hmm right! That's a that's promise. I can see solder flowing right.

That's pretty good! That's pretty good you're going to have to excuse any clumsiness here! It's just because well big fingers! So i'm going to move that over there. So i can get the other side sword now i'll come back to this side after i've done the other side, because uh it will need refluid, i'm going to use the antex outside, because it's got a bit more oomph and there's that huge solar pad underneath. So i'm going to flow some sodium using the antex and it is a one of those components that is the lead folded round underneath, so it should theoretically flow and do a decent job in that heat, sink the heatsink pad underneath and then coming back to the Side and just going to refill this again and add a bit of fresh soda, so there's plenty of flux, and that will hopefully be it where i'm going to unplug this soda iron. Before i burn myself, i'm gon na let that cool down, i'm gon na hook up the mainstay, and then we can see if it works.

One moment please: okay, i've not plugged in yet. I thought we'd share the moment together, just in case it explodes. It always makes good entertainment some plug it. In now, relays click.

The unit is back working again. This is good right. I don't know if that's the ideal diode for that it's got a slightly lower voltage rate than the previous one, but it is supposedly exposed to a volts ish, but anyway it is the three amp. Although i kind of doubt it's uh, the size always makes me squirm these ten little three amp diodes.

However, the band of it i should have mentioned is pointing towards this processor type chip - it's not a processor, so i'm not sure that is, but that big chip the little band on it uh on the dial just pointing over. To that way, it also goes towards the uh, the biggest the hardest pad to soda. Look at all the flux around that should clean that option. Uh.

Yes, i should, but anyway, i'm going to put this back together and that will be it fixed, so that diode i used was the the one that fitted was a b360b uh. It's order number from cpc if it's even the right one, i'm not taking my ability for this because uh you know, i don't want to say everybody swap them for this um, but it's sc17585 uh. I was quite happy to be honest. It was an easy repair using the uh leading diode, the one that i just went onto this pin of the transformer and over to the large pad, but i'm not sure what current the external unit actually draws from this.

I don't think it's going to be a huge amount. I don't think the unit is actually going to be anything remotely near an amp total, but that is it so yeah, that's that's a fix. I guess i suppose i could give it back and they could try it and see how long it lasts. But that's strange so disappointing that a reasonably good looking design has been let down by by such a thing as a diode, and it does show you there is space the vicinity there.

They could have put two diodes in series, possibly and and actually alleviate that problem, because if one of them had gone short circuit, the other one would potentially have kept the party going until it went short circuit. But then that would have half the number returns. But there we go that, is it fixed?.

14 thoughts on “Fixing another nest heat link – with easy tests”
  1. Avataaar/Circle Created with python_avatars Sudds1888 says:

    I fail to see the point of all these network enabled heat controls, and I would be a big tech head.
    If youโ€™re maybe a person who is constantly travelling for work then yes it would be useful but in a busy household, normal zoned timeclock and stats controlling valves and the burner is fine in my opinion. ๐Ÿคท๐Ÿปโ€โ™‚๏ธ

  2. Avataaar/Circle Created with python_avatars ndupontnet says:

    Hi Clive, remembering the previous Nest, I wanted to ask you if you would fancy some post-mortem on an horrendous 4 ports USB adapter with a split PCB badly put back together with some resin and thick solder points. It ran for 6 years before going out with a bang, an exploded glass fuse, and a capacitor that melted through the top ABS cover. Just for entairtainement of couse ๐Ÿ™‚ If you're in, I can e-mail you some pictures of this epice contraption.

  3. Avataaar/Circle Created with python_avatars Ellis The DJ says:

    This diode is too big this diode is too small this diode is just right it goes to show that one tiny component knocks out the entire thing in this case a weak diode I fitted an Hive active heating over a year ago its still doing fine I have noticed that we are not topping up the gas meter as much

  4. Avataaar/Circle Created with python_avatars Tony Sheerness says:

    In mass production if there is a weakness in one component, it will affect thousands of gadgets nor machines. For instance if you have a model of a car with say near side window winder broken and go down to the scrap yard to get a replacement all that models of that car will have window winder missing as every one wants one.

  5. Avataaar/Circle Created with python_avatars Fred Flintstone says:

    why on the right hand pads does it seem that there are two seperate pads and the diode removed seemed to have 3 points?? on the data sheet it looks like one diode that can be used for two circuits seems strange set up

  6. Avataaar/Circle Created with python_avatars Flying0Dismount says:

    If the diode failed due to ESD, that could be the fact that they chose a Schottky device.. They tend to be much more sensitive to ESD than conventional rectifier diodes, so possibly an argument to just replace the diode with a regular 1N4000 series.. I once ruined almost an entire package of axial leaded Schottky diodes simply by taking them out of the package by hand, grabbing on the leads.. Originally thought I got a bad batch and called the supplier and he asked if I used ESD precautions before unpackaging.. It turned out that that particular diode really did not like ESD. Once in-circuit it was fine, but needed to be handled very carefully when installing.. This was the one and only component that I have ever had ESD issues with.. I remember the early days with CMOS 4000 series chips where everyone was initially paranoid about ESD, but never ever blew one of those (well ok, I may have killed one inverter out of a hex chip once, but it may also have been because I connected power to the output, but regardless, I had 5 other nverters to use on the chip, so I don't count that)..

  7. Avataaar/Circle Created with python_avatars Gordon Lawrence says:

    I have seen this before. When I was a project manager for a large electronics manufacturer we started getting a lot of boards back. Basically the manufacturer to save money has stopped individual testing of the diodes so about 200ppm were way below spec for reverse bias voltage. IE 1000V diodes that sort of worked at 200V or sometimes only 100V. When you are shifting 50,000 units a month and each board has 17 diodes in mains circuits that's over 2000 early life failures a year from that one fault alone.

  8. Avataaar/Circle Created with python_avatars Stoney_Eagle says:

    I've bought a Chinese IRAlan dimmable led ceiling fixture but it's in its lowest setting waaaay to bright and a watch your eyes is needed.
    It has warm and cold that is mixable with the remote but it only has 2 wires so it's definitely digital.
    I like to have the minimum brightness lower but I can't find any info on these things.
    The transformer outputs 18-28v 2.2A
    Can I just put a beefy resistor between it, or do I kill the thing?
    I do have solder skills but my knowledge in electronics is basic ๐Ÿ˜‰

  9. Avataaar/Circle Created with python_avatars Chris Horry says:

    Thank you for the tutorial on surface mount components, I just need to practice at them I think. That USB port is also used for pulling diagnostics and doing a factory reset if needed, I've had to do it on a few Nest thermostats over the years.

  10. Avataaar/Circle Created with python_avatars Simeon Walker says:

    Don't most boilers these days have a a low voltage data bus for stuff like this? Mine must be 15 years old and connects to the Tado box with just two low voltage wires for data and power.

  11. Avataaar/Circle Created with python_avatars fanplant says:

    What are the 240v heating circuits typically fused at in the UK? Over on this side of the pond typically the only line voltage stats are for resistive baseboard, one stat per room in many cases and they're dumb bimetallic and micro type switches.

  12. Avataaar/Circle Created with python_avatars Frankhe78 says:

    In basically all European countries, except Britain (out of the EU but still part of Europe), we use 24 VAC because it is just a little bit more safe and reliable. Disadvantage of 24 VAC is the relative high idle power consumption. There is alway a small 'traditional' transformer dissipating heat, usually somewhat around 4 Watt.

  13. Avataaar/Circle Created with python_avatars andy fletcher says:

    Probably don't want to replace the diode like-for-like as it appears to be intolerant of voltage transients and dies too easily. 1N4007 are pretty bombproof and would probably make good replacements. You can even get SMD versions of them.

  14. Avataaar/Circle Created with python_avatars Joey Strong says:

    Cool video big c. I'm wondering where you get your smd components from. I'm switching from through hole because I have enough th for the rest of my life. I know I could go to mouser or the like but I just want a variety of passives and suss out which one to use when the time comes. I have a bunch of caps and resistors on the way.

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