What thermal fuse temp would be safe to use here?

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Joined: 21/06/2018
Location: Australia
Posts: 1018
Posted: 08:52pm 28 Jul 2023      

Roger, I have had second thoughts about that bit of circuitry and what it may really do.

In power supplies a soft start to a capacitor is often used, but usually not on a remote PCB.

I think it probably has an inductive load and works essentially the opposite to a soft starter.

An AC contactor may require a large pull in current but only for maybe a hundred or so milliseconds, as the armature pulls in completely it also completes the magnetic field which previously had a large air gap and therefore a low inductance (and higher current).

Once pulled in the holding current required is much smaller and the triac for the pulling in current can be turned off allowing holding current through the "efficiency" resistor.

This resistor will get hot as it continually passes the holding current (if its got a micro they could have just reduced the firing angle of the triac to reduce the  holding current...).

So if someone replaces the contactor with a different one with a lower impedance the resistor may run hotter and open the fuse, or if it is designed for 220V our mains sometimes >245V could make it run hotter, or if the contactor is worn or corroded the magnetic circuit may be lossy causing more current to flow etc.

Is it an imported unit for 60Hz maybe the lower Xl at 50Hz creates more current, so many things to consider ?

Replacing the thermal fuse may be treating the symptoms but not fixing the actual problem.

Of course this is still flying blind theory but I reckon closer to being on the money this time.
Edited 2023-07-29 09:20 by wiseguy