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My central heating controller stopped working. It's essentially an AA battery powered device with a thermistor, some buttons, and an LCD screen, which lets me program desired temperatures for various day/time periods. When the controller decides heat is required, it enables its relay which powers on the boiler for heating.

When I went to turn up the temperature temporarily today, I saw that the screen no longer came on. Trying various newly charged batteries didn't help. The controller did also die a year ago while in the middle of using it, but it worked again after taking it apart, rebuilding, and replacing the batteries. I believe the unit has consistently complained for maybe three years about a low battery, even with new batteries.

The PCB surfaces look pretty much pristine (AFAICT), aside from this one component, which I'm unsure of its purpose and whether it's intact. It looks a bit like a capacitor that has leaked — but it's labelled as "L3", which apparently can denote an inductor or bead (I'm not familiar with them). The body has "CTC" written on it a few times, but I didn't notice any other markings. Other capacitor-like components on the board are labelled as "Cx".

Photos I took a year ago of this component also look identical to now, so I don't know if this is the component at fault (though I suppose debugging the board further is a separate topic).

What is this component, and is this normal? Are there any meaningful measurements I can make with a multimeter to test its functionality?

PCB shot 1 PCB shot 2 PCB shot 3

Chris
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    That looks like it could be epoxy to hold the inductor in place to avoid whining sounds during pulsed or AC operation. Inductors generally have solid or air cores and don't have a liquid material which could leak. Is the inductor surrounded by circuitry (e.g. a switching regulator) that would suggest that it could be exposed to such AC/pulsed operation that would justify affixing it this way to avoid whining sounds? – nanofarad Nov 15 '21 at 20:34
  • The glue is damaged, but still okay. No sign of burn on the inductor, so it is okay. – jay Nov 15 '21 at 20:39
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    Just FYI, your "AA battery powered device with a thermistor, some buttons, and an LCD screen, which lets me program desired temperatures for various day/time periods. When the controller decides heat is required, it enables its relay which powers on the boiler for heating." is a thermostat. – brhans Nov 15 '21 at 20:50
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    Looks like one of [these](http://www.ctccoils.com/eng/products/CHIntroduction.htm) by [C.T.C Coils](http://www.ctccoils.com/eng/index.htm) with glue on it to make it stop whining. It looks OK, as far as you can tell from a photo. – ocrdu Nov 15 '21 at 20:50
  • I don't think that part is bad, however those soldered-in batteries or supercap look suspicious to me. – Spehro Pefhany Nov 15 '21 at 21:04
  • Thanks folks. As it wasn't labelled as a capacitor, I did wonder if that substance was glue. But thanks for the identification and that it appears as expected :) – Chris Nov 15 '21 at 21:30
  • @nanofarad Thanks for the info! I really don't know enough about this to give a good answer; there are a bunch of small diodes, capacitors, resistors, transistors and a tiny IC on the opposite side behind L3. I don't know what pulsed operation is, I'm afraid. Power-wise the thing appears to do relatively little — it sets/resets a 3V relay now and then, and I would (very naïvely) guess that the AC current that it's switching is isolated from the rest of the circuit due to the relay? – Chris Nov 15 '21 at 21:39
  • @SpehroPefhany It seems to be labelled as a 0.047F capacitor. On basic inspection it appears normal, to me. Is there anything in particular that would flag it as suspicious? – Chris Nov 15 '21 at 21:42
  • You seem to have a degenerative power-related issue and such things tend to wear out. Also my opinion is influenced by a lack of other visible components. A SMPS chip with a chunk out of it would certainly change my mind. – Spehro Pefhany Nov 15 '21 at 21:58
  • @SpehroPefhany Ah, I see. Thank you for the insight. I'll test it, as rdtsc suggests below :) – Chris Nov 15 '21 at 22:33
  • @SpehroPefhany That supercap seems to be working fine as I responded below; but a degenerative power-related issue sounds very credible. I was able to bring the device back to life simply by using more powerful batteries. I'd always been using rechargeable batteries, but despite using various newly charged batteries, supplying ~2.8V, none of them would work. Inserting new, non-rechargeable batteries supplying ~3.2V brought the thing back to life (though conceivably it may fail again in future even at these voltages). Thanks for your help. – Chris Nov 16 '21 at 15:57

2 Answers2

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L3 is an inductor and it is normal to have glue on it. Most likely not damaged and problem is elsewhere. Multimeter on continuity mode should read approximately 0 ohms over it.

Justme
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You said it's AA battery powered, but is that also a stack of coin batteries on this board? If so, check/replace them, as well. They look like they are held in place with a spring clip.

Graphician
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  • That was my initial thought when I saw it, but it's apparently a non-removable capacitor — it's labelled as C30 and seems to be marked with the capacitance 0.047F. – Chris Nov 15 '21 at 21:44
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    If it is a (47,000µF supercap), a voltage measurement should show some DC voltage across it, perhaps 2.5 Volts. – rdtsc Nov 15 '21 at 22:07
  • @rdtsc Spot on, thanks! The AA batteries were supplying ~2.8V, and I measured ~2.6V across that capacitor. In any case, it turns out that getting more powerful batteries (non-rechargeable, supplying ~3.2V) brought the whole thing back to life… – Chris Nov 16 '21 at 15:50