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Simple hot plate

Why

Often there is a need to keep something at moderately high temperature, for example for resin curing or for solvent drying. In particular, the spray-on photoresist for circuitboards takes 24 hours to dry at 25 °C, but only 15 minutes to dry at 75 °C.

As for many such uses the temperature just below boiling point of water is sufficient, a simple approach was chosen; instead of a complicated electronics for which the designer would need the proper inspiration, a bimetallic switching thermostat was employed for the first makeshift version.

How

An old, decommissioned electrical clothes iron was taken apart and the bottom with the heater was removed.

Standoffs were made from long bolts by filing off sides of their center parts, to inhibit conduction heat transfer from the plate to the underlying controller housing.


Clothes iron, heater side

Standoffs

Standoffs

The power leads, a pair of naked resistive wires, were insulated with jewellery-grade glass beads; the small beads can survive being heated to red heat without cracking, mainly due to their small size. Silicone tubing was used on the ends of the leads to hold the beads together and make the  manipulation easier.


Heater with standoffs and bead insulation

A pair of holes was drilled into the bottom of the heater plate, taking care to not penetrate the other side, and a M3 thread was cut in them.

A bimetallic switching thermostat was mounted to the holes, using thermal paste to assure good thermal contact.


Thermostat mounting holes

Thermostat with thermal paste

Thermostat in place

Thermostat in place

Thermostat in place and wired in

Thermostat in place and wired in

An empty metal case from an ATX power supply, oriented upside-down, was used as the housing. Holes were drilled to its top (originally bottom), mating to the positions of the standoffs and the wiring for both the heater and the thermostat and a temperature sensor.

The power socket was connected to a switch on the front panel, the power was routed from there to the thermostat and then to the heater, then back to the socket. The power switch was placed to the cutout in the housing (originally for leading out the wire bundle for the low-voltage power) which exactly matched the switch diameter. Two holes were drilled in the panel and commercially available LEDs with serial resistors and diodes to match the 230V AC power were used as indication of the operation (green one, indicating temperature reached, parallel to the thermostat; yellow one, indicating the heater running, parallel to the heater).


Power wiring

Wiring, detail

Wiring, detail

Wiring, detail

Panel switch and lights

Power socket

A carrying handle was added to the front panel, for safer handling of the device when the plate is hot; carrying it by the box itself would be an invitation to drop it and touch the plate.

Results

The device works fairly well, despite its overwhelming simplicity. A potential pitfall is the very fast rise of temperature at the initial power-up, with which the thermostat cannot cope as the thermal equilibrium with its bimetallic switch does not follow fast enough. The plate therefore significantly overheats and reaches well above 100 °C, then it takes a long time to cool down to the desired temperature. A wet rag can be used for forced-cooldown of the plate; beware of the hot steam, though. (Alternatively, the plate can be manually switched on for a couple seconds, then powered off to give time to establish the thermal equilibrium between the plate and the switch, then switched on again.)

After the initial overshoot, the temperature stabilization is running well. The heater switches on once per couple minutes for only few seconds.

TODO


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