Interesting Chromalox electric heaters

JeffH · **Joined:** Fri Mar 14, 2014 11:44 pm **Posts:** 239

robertmacdowell wrote:

Yeah, that's how I'd do it. Nowhere is it written the thermal cutout must switch the load directly. It'll suffice to switch the control line to the contactor, wire them all in series. Good luck finding the bad one when it trips. Might help to wire an LED+resistor in series with each cutout.

Also these cutouts should probably be insulated for 600v/1500v. This is hard.

Trolley heater thermostat circuits were generally 600V line powered, big dropping resistor on the
high side of the contactor coil, and the thermostat element shunted across the coil of the contactor
to ground. These thermostats are "backwards" in that when it gets too HOT, the bi-metal contact
(or mercury) closes and shunts out the coil. If you try to do it the other way, where the thermostat
closes to call for heat and is in series with the contactor coil, then no matter what, even if the
thermostat is on the ground side of the coil circuit, it will need to break an inductive circuit with 600VDC
open circuit potential on it. Those little contacts with their tiny gap will not be able to do that, and
the thermostat would weld itself shut.

However, with the inverted operation, as the thermostat contacts break open, calling for heat, there is
the contactor coil in parallel, so the maximum voltage developable across the contact is limited to
the normal coil operating voltage