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About this kind of hot plate I was told, quote:

That's very bad because potentiometer needs to emit power which is sufficient part of all heater consumption. In this case we cannot use potentiometer.

But surely there's a way to control it as it's done in hot plates for cooking etc.

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The most efficient way is to pulse the power to the heating element, this way the switch is either on or off and dissipates very little power (W = I^2 * R, so if switch off I = 0 so W = 0, and when switch is fully on, R = ~0 so W = ~0)

For the switch you can use a MOSFET of suitable rating, and control it with a samll microcontroller (or some simple control circuit) which uses feedback from a temperature sensor and either just switches on/off above/below setpoint, or (better) performs a PI, or PID calculation (Proportional Integral Derivative) to keep the temperature more stable.

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  • \$\begingroup\$ nice idea. But I have found a power NPN transistor probably via load line in a non inductive heater. For inductive heaters I think pulse method will work fine. \$\endgroup\$ – Standard Sandun Oct 13 '12 at 3:36
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    \$\begingroup\$ An important thing to note here is that if OP has a way to measure the resistance of his heating element, he can control the dissipated power nearly linearly using a DC/DC converter in Current Mode. \$\endgroup\$ – Christoph B Oct 13 '12 at 9:59
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This Polyester heater was designed for applications such as my car's side view mirror de-icer or perhaps you could use it as a hot-plate to keep your beverage warm.

Since the response time when adhered to the rear of the matching size glass mirror is in minutes and it uses a self-adhesive thermal transfer tape, the glass does not overheat unevenly and there is only a manual on/off switch.

The specs indicate in still free-air, this part reaches 90~95'C using 15W from 12V. This implies a resistance near 1 Ohm and a thermal resistance to air of 6 'C/W and the response time would be <10 seconds. Attached to Mirror would increase the response time to a minute but to a pot of tea, maybe 20 minutes. So it would be useful as a warming plate heater. Just add as many as you need to cover the area and apply appropriate DC power such as a laptop charger and bond a PolyFuse (*1) to it.

Attaching this to a mirror or a hotplate will change the heatsink factors. Mass increases the time constant, insulation could raise the thermal resistance, but may not be suitable for the adhesive for cooking but it would be similar adhesive to cooling a CPU. ( a 3M patented thermal tape material)

It may make a perfect coffee warming pad. In this case, it may burn the coffee if left to go dry in the cup. If you wanted to limit the temperature of the pad, I would bond a thermal PolyFuse (*1) to the polyester.

(*1) PolyFuse or PolySwitch PPTC are tradenames for a Polymer Positive Temperature Coeeficient radial disk switch. They are designed to trip below 85'C but start changing resistance near 75'C by more than a couple orders of magnitude. enter image description here

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  • \$\begingroup\$ PolySwitches come in a wide range of currents, so I would choose the holding current value near Point 3 which may be below 80'C, but is not precise. Point 4 maybe around 85'C. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 13 '12 at 9:44

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