Unidentified component in water boiler
Can anyone suggest what this strange component is beneath the 28 pin DIL chip? It's on the controller board in an autofill counter top water boiler. I can't easily see the other side of the board but it doesn't look like there are any pipes going to it.
Regards - Philip
In fact it's not as @jayeff described at all. Here's a picture on the inside of the tank. There are 4 sensors you can see on the right. Two at the bottom are described in a parts list as thermistors, and the two at the top simply as "probes". On the basis of what they tell the control board, it:
- Opens and closes the auto-fill inlet stopcock as necessary in order to maintain the water level (you can see the water inlet on the left)
- I presume, disables the heater element if the water falls below a certain level
- Switches on the heater until the water starts to boil or if the temperature falls below a certain value (e.g. on being left to stand, or on the water being topped up with cold).
The tip of the bent probe is at the maximum fill level, judging by the limescale marks on the tank. I assume it must sense the water electrically, either capacitively or conductively. The top probe must be what detects the steam once the water boils. (The heater is definitely turned off when the water boils, not when it reaches a set temperature.)
I think the pot which @oldturkey03 identified most likely sets the temperature at which the heater is turned on again after topping up or being left to stand.
The fault, in fact, was a failed resettable thermal cut-out mounted beneath the tank close to the element but the root cause isn't entirely clear. A rivet holding one of the spade terminals to the body of the cut-out was loose - possibly the electrical contact was poor due to oxidation or the rivet may never have been fitted tightly enough. Heat generated by the poor contact then degraded the plastic until the rivet was too loose to make contact at all.
I've now reinstalled it and reconnected it to the water supply. How it works is interesting. On switching it on empty it first auto-filled with maybe just a litre or so of water and brought that to the boil. It then repeatedly added a bit more water and brought it back to the boil. In normal use it evidently tops up and reheats a little at a time as hot water is drawn off, so that the temperature never falls below a certain value, maybe 90 or 95 degrees.