NC - someone remind me please.

In which case you probably need a CO.

Normally Closed ^ ^ ^ ^ NC the clue's in the name.

Ta! I was aware that NC in regards to a relay, means closed when not powered, but I couldn't remember the designation for stats.

"Normal" is the state of unpowered, room temperature and atmospheric pressure.

So for the stat to open at 40 C it needs to be closed below 40C. ie a Normally Closed (NC) device.

How does that work then? Is it CO stat or isn't it directly controlling the fan?

Dave Liquorice presented the following explanation :

Its a 13.8v 20amp PSU for the caravan. The fan is a standard 5v computer fan, so the stat must have some intermediate control between stat and fan supply. It caught me out last time I looked at it, when I investigated it to make sure the fan was working. I shorted the stat pins, expecting the fan to start, but it didn't, then I realised it was triggered by the stat opening.

I recently noticed the fan running, when ever the 240v was powered, so I looked into it this morning. The stat was failing to close, so the fan ran continuously.

Get a changeover type. All bases covered. ;-)

Only if connected the right way :-)

NC

I've not knowingly come across a 5 V computer fan they've all been 12 V.

Simple single transistor emitter follower will invert a control signal. Seems a bit daft to use a 5 V fan and extra switching when you have 13.8 V avialable.

Dave Liquorice submitted this idea :

That was from memory of several years ago, so I could be wrong.

Maybe the extra switching is fail safe? It did in fact fail safe, with the stat open circuit and the fan running constantly.

But which is more likely to fail, the stat or the fan? My money would be on the fan ...

Andy Burns explained on 11/09/2018 :

In this case, the stat failed, but yes I agree - the fan would be the more likely to fail.

Its easy to put a resistor and fan in series across the normally closed terminals and then it will run when it opens if there is a suitable voltage.

It will, but at zero gain. I'd have the load in the collector and a potential divider biasing the transistor on. Use the thermostat and a resistor to bias it off. That way, one power transistor will do.

Dave Liquorice presented the following explanation :

Another reason (perhaps) for using a NC stat, is that perhaps they are more common/cheaper to source - just guessing? NC stats will be normally be used for heaters.

On a related note, in the context of a traditional heating system (mechanical timers and thermostat), what does a changeover stat get used for?

I get that typically mains is looped through the timer, then the stat, and then some combination of boiler, pumps and valves depending on the setup. But usually the thermostat contacts want to be closed for 'make heat' and open for 'hot enough'.

In what circumstance would you use the contacts that open when heat is desired? Bearing in mind that typically the circuit is dead if the timer is off so neither NO nor NC contacts are energised in that case.

Theo

The cylinder stat in a Y-Plan is a change over stat. Linked to the midposition valve.

And when the stat is closed?

You short out sizewell B and have a mushroom cloud for dinner.