Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2000-04-12
2001-10-30
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S483000, C219S486000, C219S510000, C219S493000, C307S041000, C337S042000
Reexamination Certificate
active
06310330
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heater power and control circuit for use in a heating, ventilation and air-conditioning (“HVAC”) system. In particular, the present invention relates to an improved system and method for controlling the temperature of a resistance heater in an HVAC system.
Although thermal overload protection has been used for many years with HVAC resistance heaters to sense and control when the heater temperature exceeds a predetermined level, there is still room for improvement in the design and operation of such circuits. In particular, as will be explained further below, prior art designs for sensing and controlling the temperature of resistance heaters in HVAC systems may not address certain issues encountered in situations of thermal overload. Accordingly, it would be desirable to be able to provide an improved heater power and control circuit for use in an HVAC system. It would also be desirable to be able to provide an improved method for controlling the temperature of a resistance heater in an HVAC system.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved circuit and method for controlling the temperature of a resistance heater in an HVAC system, which may be composed of one or more heater elements connected in parallel. The circuit comprises: (A) a power circuit including [i] a dual thermostat having a first and second thermostat in series with each other, [ii] a first switch coupled to a first contactor for controlling the operation of the switch, and [iii] a resistance heater element coupled in series between the dual thermostat and the switch, wherein the first thermostat cycles open and closed automatically and is set to trip at a first temperature T
1
, and the second thermostat is a manual thermostat and is set to trip at a second temperature T
2
, where T
2
is higher than T
1
; and (B) a control circuit including [i] a third thermostat coupled in series with the first contactor wherein energizing and de-energizing the first contactor causes the first switch to close and open, respectively, and wherein the third thermostat is a manual thermostat and is set to trip at a third temperature T
3
, where T
3
is higher than T
2
, and wherein when the temperature exceeds T
3
, the third thermostat trips, thereby de-energizing the first contactor, causing the first switch to open in the power circuit and preventing current from being supplied to one end of the resistance heater element. When the third thermostat trips, it also opens the circuits for any other heater elements connected in parallel and the blower motor that forces air over the heater elements. As a result of this design, continual oscillation in the opening and closing of the first thermostat as a result of oscillation in the temperature above and below T
1
, but less than T
3
, is prevented by said second thermostat having a trip temperature T
2
above T
1
but less than T
3
. Further, the circuit operating the blower motor is controlled by a thermostat that controls all of the heater elements, instead of less than all of those elements.
REFERENCES:
patent: 3046380 (1962-07-01), Carlson
patent: 3606970 (1971-09-01), Kompelien
Breen Thomas Bernard
Elasser Ahmed
Herzog Rollie Richard
Mihailov Wayne William
Stevanovic Ljubisa Dragoljub
Breedlove Jill M.
Paschall Mark
Stoner Douglas E.
Transport International Pool, Inc.
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