My Thermostat stopped working a couple of days ago and I tried to debug it. The first thing I check is if there is any component that gives a strange result with the multimeter and I found these three components which look a lot like transistors (maybe BJT?):
I'm sorry for the bad quality but I couldn't get any better picture with my phone. Anyway, the package is like this one (only the package is the same, the component is different from those on my board!:
Two are the same component (5Bp or 58p is what is written on the package), and I can measure 0.6V (with the diode option of a multimeter) across any pins. This is strange for a BJT, but it may be related to how they are interconnected.
And the third one, pointed by the green arrow, with a horizontal "P" and a vertical "D3" written on the package, is what I would assume is a PNP BJT. Here is a sketch of the component (really bad proportion!):
My assumption is also that this is broken: I can't measure any diode for this component. Two pins are not connected (multimeter shows an open) and the other two couple of pins are connected but the multimeter measures a short with the diode option, and the resistance measures approximately 27 Ohm and 19 Ohm, regardless the polarity of the multimeter.
So, my question is if anyone knows by heart what this component is exactly, or if he can confirm that this is at least a pnp.
Unfortunately, I don't have any way to understand what type of bjt (assuming it is a bjt) without knowing the pin out and measuring the orientation of the diodes...
I may infer this from how the components are interconnected and especially if any terminal is connected to Vcc/ground, but without knowing the pin out of the component it's still quite bothersome, I think!
PS: I said I'm debugging my old thermostat, so I'll briefly describe the issue I had, maybe someone can help with the debug in general and not only recognizing the bad component! There may be some inaccuracies/errors in my explanation though, as it's not my business to work with this kind of component, but this is what I could understand from my trials. Please feel free to ask/correct me.
The relay that should give the start signal to the heater works (or the relay that should switch when the thermostat tells the heater to start). I can tell this because when I set the temperature so that the heater should start, I heard the typical noise of a relay that switches. This also means that the temperature sensor works, because it correctly switches the relay when I set a higher/lower temperature on the thermostat. Regardless of the relay that switches, the signal to the heater doesn't arrive, so the heater is stuck off.
There is also a quite big IC (NEC), which I suppose is a microcontroller, and I suppose it works correctly because: it drives correctly the relay, the LCD on the front panel works correctly, etc. There is also a quarz oscillator, which should work because the microcontroller works (I assume it's the clock generator for the microcontroller).
All the other components (15 components approximately in addition to these I already mentioned) are just passive components (capacitors and resistors), mainly SMD.
JRE pointed out that it could be the realy, and it really was. Now I'm trying to find a good replacement and asked a new question there Suitable Replacement for a Dual Coil Latching Relay. Feel free to help/see the end of this story! :)