Analog thermostatic control circuit for a heating pad
A heat mat with thermostatic control having a reference voltage generating source that provide high voltage DC for a power controller and low voltage for a temperature sensor and hysteresis circuit. The sensor and hysteresis circuit establish a temperature threshold signal that is delivered to the resistance heating element. The resistance heating element is sandwiched between two layers of material with adhesive. Two layers of PVC protects the sandwich. In manufacturing the heat mat, the resistance heating element is placed with adhesive between two layers of material then cured and degassed under vacuum. The thermostatic control is sealed within an overmold housing or flat pack.
1. A heating pad comprising:
a resistance heating element; and
a thermostatic control circuit coupled between a mains electrical source and said resistance heating coil, wherein said thermostatic control circuit comprises a variable-resistance, low power temperature sensor, a reference voltage generating source, a hysteresis circuit having a first and second thin film resistor and a power controller for said resistance heating element;
wherein said hysteresis circuit compares an analog signal from said temperature sensor across said first thin film resistor to an analog signal derived from said reference voltage generating source across said first thin film resistor to provide a control signal to said power controller that selectively varies the power output to said resistance heating element,
wherein said thermostatic control circuit comprises a small form factor integrated circuit board that is embedded within a plastic housing that is connected between a mains electrical source and said resistance heating element.
2. The heating pad of claim 1 , wherein said reference voltage generating source includes a bridge rectifier to convert AC from the mains electrical source to unregulated DC that is supplied to said power controller.
3. The heating pad of claim 2 , wherein said reference voltage generating source includes a voltage reference diode to create a stable 5v reference voltage regardless of load, changes in power supply or temperature that is supplied to said hysteresis circuit.
4. The heating pad of claim 1 , wherein said power controller comprises a MOSFET.
5. The heating pad of claim 1 , wherein said integrated circuit board containing said thermostatic control circuit is disposed within an overmolded power plug.
6. The heating pad of claim 1 , wherein said resistance heating element made of a high resistance metal alloy wire is sealed between two layers of material made of polyethylene terephthalate (PET) film forming a sandwich.
7. The heating pad of claim 6 , wherein said sandwich is sealed between two layers of polyvinyl chloride (PVC).