Filter condition sensor and indicator

摘要

A filter condition sensor and indicator is disclosed. A filter condition sensor and indicator that responds to the presence of a predetermined collection of material in a filter operatively positioned in an output air flow path. A small by-pass air flow path is connected to the output air flow path and is separate from the filter. A heated thermistor is positioned in the by-pass air flow path. Electrically connected in series with the heated thermistor is an indicator light that illuminates when the resistance in the circuit is decreased due to predetermined cooling of the heated thermistor. In this way, a predetermined collection of material in the filter increases the flow of air in the small by-pass air flow path for cooling the heated thermistor and for lowering the resistance in the circuit to illuminated the indicator light. Even in those instances where a multi-speed fan is located in the output air flow path, the present invention overcomes changing fan speeds and changing air velocity by coupling the thermistor in a circuit which changes the voltage and current through the thermistor depending on the fan setting. <IMAGE>