I'm fixing a heated water dish, used so that the dogs don't have to eat snow. The previous dish was heated with a wire embedded in what appears to be ceramic, on a foil back on the bottom side of the dish. This failed. Previous experience with this company's dish (Farm Innovators) indicates that 1-2 years is a typical life span for this type of dish.

The rated power for this assembly was 60W. It had a disk thermostat with about a 15 degree F hysterisis in series with the heating element.

It occurs to me that I could replace this heating foil with a handfull of 10 ohm, 10 W power resistors, connected in series, and glued to the bottom of the plastic bowl using thermal glue.

However, this puts the heat in a much smaller space. It's not clear to me if this poses a fire hazard. I'd like to find a guide that gives typical operating temperatures for a given geometry of resistor and surrounds.

In normal operation, the thermostat should be sufficient to protect the bowl and floor, however, I figure I need to design so that failure of the thermostat to open doesn't result in too high a temperature.

I would like to include a temperature sensitive fuse in the circuit. Some that melts at, say 80 C -- well above the operating temperature, but low enough to not catch fire.

Alternate method: Use 32 guage nichrome wire (10 ohms/foot) I would need 20 feet of this with 130v (rural power) to get approximately the same heating effect. Invert bowl. Glue roofing nails to bottom of basin. Pour 1/8" plaster of paris or tile set mortar. Allow to set. Trim nails to 1/8 above mortar. Wind wire from nail to nail in zigzag. Place thermostat. Add 2nd layer of bedding substance.

Nichrome reaction with highly basic mortar?

32 guage wire would puts a lot of heat flux through small area. Would the wire embedded like this overheat?

  • \$\begingroup\$ Figure out how much power their thing dissipates. Dissipate the same amount with resistors. No need for temperature calculations \$\endgroup\$ – PlasmaHH Dec 4 '15 at 17:43
  • \$\begingroup\$ Is it the heater that fails or the thermostat? \$\endgroup\$ – Spehro Pefhany Dec 4 '15 at 21:51
  • \$\begingroup\$ Ditto @PlasmaHH - If it were a 60W heater, get 10 pieces of some identical resistor that will dissipate 60W in series, 10W rating each (ceramic.) Mix up some J-B Weld epoxy, and "glue" them to the bottom. I'd include a thermal fuse too, glued to a resistor. Be very careful to not expose any current-carrying circuitry in a manner that could shock the animals. (Think, inquisitive raccoon, chipmunks...) \$\endgroup\$ – rdtsc Dec 5 '15 at 0:09
  • \$\begingroup\$ @SpehroPefhany The zigzag wire embedded in foil has a crisp inch that has been reduced to powder. \$\endgroup\$ – Sherwood Botsford Dec 5 '15 at 2:10
  • \$\begingroup\$ the present setup uses several feet of element on about 1 cm spacing. If I used 8 10ohm 10 watt resistors each each 1/2" x 2" then the total 60 watts is on a space of 5 sq inches. While the average heat dissipated is similar, I could get local temps substantially higher. Hence the request. \$\endgroup\$ – Sherwood Botsford Dec 5 '15 at 2:14

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