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
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
Comment