Need help on a heater circuit using an Op-Amp comparator circuit

Hey guys, i need some help on a little project i am doing for fun. I have a warming strap which I took from a mothercare bottle warmer. It needs 2A to 12V to warm. I am using a comparator circuit with a NTC thermistor to detect the temperature of the strap and compare voltage at this temperature with reference voltage. Then the comparator circuit has a relay that goes ON and OFF depending on the temperature of the warmer strap. The comparator circuit needs 12V but around 30mA, so how can I integrate both the strap and circuit with the relay. Its alright if the solution for the problem needs two power source. Can anyone put me in a direction.

What you need is called an Schmitt trigger. It's easilly built from one opamp. You just need to decide the upper and lower voltage limits.

Hello there:

for such simple thing like a bottle warmer I would not waste time and money to design such complex circuit. Try one of the following:'

1. There are coffee-cup warmers available on the market; they run ~10W, 110 VAC, and the coffee is kept at ~ 60 deg.C.
2. If you remove a bimetal thermostat from a discarded iron, this can be adjusted to temperatures from 40 to 120 deg.C; the contact is rated to 110 VAC and ~1 kW, so you can use a suitable heating resistor and any voltage from 3 VDC to 11V AC.
3. Some dealers offer PTC devices, solid-state temperature-sensitive resistors. you can select one with a suitable temperature and in series with a heater resistor in a 12 to 24 VDC circuit, it will work for years. No contacts, the current stops at the threshold temperature and starts below.

---------- Post added at 22:58 ---------- Previous post was at 22:57 ----------

Under point two above, the voltage range should read 3VDC to 110V AC.

Heat propagation delay ( in time domain ) acts like such smith trigger circuit ( in temperature domain ).
The thermal capacity of matherials is modeled as reactive cumulative component.

Just a simple comparator is enought to perform what you need.

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