A thermostat for a PSU has failed, the replacement needs to be closed
at room temperature and to open at 40C. Will that be an NC type or an
NO type please? I think an NC, but I always get confused over these
Rather unusually - The cooling fan runs, when the sat opens.
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.
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.
*I don't have a license to kill, but I do have a learner's permit.
Dave Plowman firstname.lastname@example.org London SW
On a related note, in the context of a traditional heating system
(mechanical timers and thermostat), what does a changeover stat get used
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.
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