0
\$\begingroup\$

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

\$\endgroup\$
11
  • 1
    \$\begingroup\$ 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? \$\endgroup\$
    – nanofarad
    Nov 15, 2021 at 20:34
  • \$\begingroup\$ The glue is damaged, but still okay. No sign of burn on the inductor, so it is okay. \$\endgroup\$
    – jay
    Nov 15, 2021 at 20:39
  • 2
    \$\begingroup\$ 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. \$\endgroup\$
    – brhans
    Nov 15, 2021 at 20:50
  • 1
    \$\begingroup\$ Looks like one of these by C.T.C Coils with glue on it to make it stop whining. It looks OK, as far as you can tell from a photo. \$\endgroup\$
    – ocrdu
    Nov 15, 2021 at 20:50
  • \$\begingroup\$ I don't think that part is bad, however those soldered-in batteries or supercap look suspicious to me. \$\endgroup\$ Nov 15, 2021 at 21:04

2 Answers 2

4
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you for the info. That is indeed the reading I got on that component :) \$\endgroup\$
    – Chris
    Nov 16, 2021 at 15:51
1
\$\begingroup\$

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.

\$\endgroup\$
3
  • \$\begingroup\$ 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. \$\endgroup\$
    – Chris
    Nov 15, 2021 at 21:44
  • 1
    \$\begingroup\$ If it is a (47,000µF supercap), a voltage measurement should show some DC voltage across it, perhaps 2.5 Volts. \$\endgroup\$
    – rdtsc
    Nov 15, 2021 at 22:07
  • \$\begingroup\$ @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… \$\endgroup\$
    – Chris
    Nov 16, 2021 at 15:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.