Mike Scott wrote:
...If it overheats, the safety cutoff, which is designed for AC, is apt to weld
closed on DC."
Good advice. The switches in most heaters are AC *only*, and can NOT
switch high voltage DC. They are apt to weld CLOSED.
You can get thermal switches that *are* rated for high voltage DC. They
just cost more, and are harder to find. They need to be snap-acting, and
have large open contact spacing. Perhaps the easiest ones to find are
the surface-mount overtemperature cutout switches used on water heaters.
I just finished wiring the thermal fuses in my ceramic heaters in series
with the contactor coil so that the DC will be shut down by the contactor's
main contacts.
This works as long as the contactor is rated to safely switch the
intended DC voltage. You can't skimp here, either. See
www.sunrise-ev.com for examples of relays rated for high voltage DC that
have been successfully used for EV heaters.
trying to sort out an air-flow switch that would inhibit the contactor
Mechanical switches with an air vane aren't very reliable in a moving
vehicle. Bumps and potholes can falsely trigger it.
A more reliable setup uses a pair of thermistors in series. Both are
placed in the airflow to be monitored, but one is "right out in front"
where it gets direct air, and the other is behind a baffle so the air
around it is barely moving. The thermistor resistance is chosen so they
self-heat due to the current flowing in them.
A voltage detector (like a TL431) monitors the voltage at the center tap
between the thermistors, to operate a relay or other circuit. If there
is airflow, the front thermistor gets cooled by the airflow, so its
resistance is higher, and the voltage between thermistors is low. With
no airflow, both thermistors heat the same, so the voltage between them
is high (half the supply voltage).
--
There are few industries with more BS than the battery industry.
Elon Musk
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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