ELKO Thermostat D3200 Schematic

[Per Hansson]

As I was looking at capacitive dropper supplies I drew up a literal "back of the envelope" schematic of this thing.
To not let it go to waste I'm uploading it here.
In this thermostat the problem was twofold: the X2 capacitors capacitance had dropped from the rated 330nF to 165nF
I had a slightly larger 330nF yellow X2 cap that measured 215nF, it was able to switch the relay when I put this in its place.
However that showed another problem: The NPN transistor that drives the relay was shorted so it was always on.
I scavenged one from another board and then the device works fine.
The NPN transistor is marked 1GW and is probably a BC847C this is underrated to drive a relay that will present a dead short for a few ms before the coil energizes fully.
A sensible replacement would state that it is rated for inductive load usage in relays, like PBSS4160T for example.
As for the X2 capacitor that is a larger subject, but a nice replacement could be Epcos / TDK B32932A3334K

[budm]

Re: ELKO Thermostat D3200 Schematic

https://www.badcaps.net/forum/attach...9&d=1571846138
No snubber diode for the relay coil?

[Per Hansson]

Re: ELKO Thermostat D3200 Schematic

There is a component in parallel with the relay coil.
It is the one on the far left on the PCB in this picture:
https://www.badcaps.net/forum/attach...6&d=1571846138

I don't know how to decode the value so I left it out.

[redwire]

Re: ELKO Thermostat D3200 Schematic

Must have been hit by mains transient to kill the transistor. What is the relay voltage.
It's a cheapola Carli X-cap? MPXGMF40/100/21 is CBB62.
0.33uF has two thicknesses 7 or 8.5mm, P=22.5mm, 26.5mm width, 17mm H.
I would just put in a better part.

TVS 1.5SMCJ18CA bi-dir marking code BET, date code 4AA (2014) ~20V breakdown voltage.

BC847CW 45V 100mA. A better relay driver is BC817 45V 500mA.

[Per Hansson]

Re: ELKO Thermostat D3200 Schematic

The relay is 24VDC
The yellow X2 cap is not the original one, the original is pictured here in the OP.
I scavenged it from a PSU to test with it, as I mentioned in the OP.

[redwire]

Re: ELKO Thermostat D3200 Schematic

OK I 'm paying more attention now
Kemet R46 series PP film cap X2 "Not for use in 'series with mains' type applications."

We've got Kemet and Vishay advising against using some film caps in capacitive-dropper applications, and it's nothing to do with voltage as across the line X2 experience the same voltage transients. Still a mystery as to why.

The relay driver transistor failure, I have seen them heat up if their hFE is low or they aren't driven into saturation. I thought a transient hit it but the TVS should look after that, and the coil has a back EMF diode? That MELF part might be just a resistor for the LDO input.

[Per Hansson]

Re: ELKO Thermostat D3200 Schematic

Super thanks redwire, I never managed to find the manufacturer of the X2 cap!
I did a deep dive now, as I thought maybe Kemet added that warning to not use this X2 cap in series with the mains at some later date to their datasheets.
But I found a datasheet from 2014 (attached) that said the same exact thing back then!
So for sure ELKO has done a mistake here, I will send them an e-mail, be interesting to see what their reply is.
The capacitor is dated July 2014 and the end-user of this thermostat says it has been in use for only ca 4 years.

He had another thermostat that I mentioned in the other thread.
It uses a Vishay MKP339 X2 capacitor, and as you noted Vishay does not recommend it for use in capacitive droppers either, and it has the same failure mode.

[budm]

Re: ELKO Thermostat D3200 Schematic

Seems to me that it will lose it capacitance even if you use it as X cap.

[redwire]

Re: ELKO Thermostat D3200 Schematic

ELKO has no room really in that tiny box for a "real" X-cap, which is thicker. The choice might be driven by size.
It's strange Vishay says to contact their field application engineers on a case-by-case basis, to see if a vanilla X-cap could work.

[Per Hansson]

Re: ELKO Thermostat D3200 Schematic

Well, there is some space in there.
And a strange thing is the mechanical switch: it is huge!
Without it there would be allot of free space inside...
But anyway the yellow capacitor is larger but it does seems to fit.
I was bored this evening and started drawing the schematic in Kicad, still WIP.

[Per Hansson]

Re: ELKO Thermostat D3200 Schematic

I desoldered the little MELF that sits across the relay coil pads.
It is a diode that measures a 650mV drop on my transistor tester and 550mV on my Fluke 28-II
So probably it is a regular diode.
Attached the KiCad WIP schematic.
MCU_CONN is a ribbon cable that goes to a secondary board with the MCU, I will not make a schematic of it.
But on it of course is just the MCU that reads the temperature sensor and control the relay on or off state.
There is also the "setback" function via an optocoupler, that reduces the set temp by the MCU -4°C, controlled via an external switch if installed.
Don't be too harsh: it is only my second or third CAD schematic ever