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Yes, Use less R on R5. You need to increase the set point range so that the NTC will be allowed to decrease in R to deliver more input to the - input which when it over shoots the + input will switch the output off. in that circuit, the RELAY remains on until the - input exceeds the
To add to the description of the circuit. the NTC decreases in resistance as it gets hotter. NTC = negative thermo coefficient (reduces in R as temp goes up) PTC = Positive xxxxxxxxxxxxxxxxx (increases in R as temp goes up).
This circuit compares the voltages from two resistive voltage dividers, one made with 3 fixed resistors, one of which has a variable tap, and one made with a fixed resistor and a temperature dependent resistor.
As temperature falls, the resistance of the thermistor rises, lowering the voltage from its divider. When that voltage falls below that from the adjustable tap, the comparator output swings positive, turning on the relay driver transistor.
A limitation of the circuit is that the comparator does not work when its two inputs are closer than 1.5 volts to its positive supply, but since the dividers are driven by only a fraction of that supply (regulated by a zener diode) I doubt this limitation will be a factor. But rather than change R5, I would lower R4 to raise the temperature range where the thermistor divider passes through the tap choices. This will also raise the lower end of the adjustment range, though, keeping the total temperature span about the same.
Thank You Jamie, John and John, All of your answers helped me to understand what's going on. And John Field's lengthy explanation was really helpful. Thanks especially for taking the time to write it all. Cheers, Eric R Snow
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