Hi,

I'm trying to refine a simple circuit design, barely armed with Ohm's law and a thin understanding of the Rule of Parallel Circuits. Please could someone help with a couple of novice questions?

The attached design, using B57891M103J thermistors, is intended to switch on the relay if two conditions are met:

- Th1-tank indicates a temperature greater than 32 Celsuis (or, Th1's resistance is less than 7.5K Ohm); and

- panel is hotter than tank (or, Th2-panel shows less resistance than Th1-tank).

I'll assume the usual convention of IN1/OUT1 and IN2/OUT2 referring to the two halves of the dual opamp.

My scant knowledge of electronics breaks down on that line between the "tank" potential divider and IN1-.

Here's a snippet from the thermistor's data sheet, for an example.

Celsius Ohms

30 8080

32 7432

34 6843

At 30 Celsius, the "tank" potential divider will give 6.22V. By the Rule of Parallel Circuits, I believe that 6.22V will reach both IN1- and IN2+. Do I have that right? If not, could someone please sketch the voltages?

Then, I'd like some hysteresis, limited by the 200K-B, from OUT2 pin IN2+. But I'm worried that the way I've laid this out, I'm also providing unwanted hysteresis to IN1-. Will I be? If so, could someone please suggest a solution?

Thanks very much.

I'm trying to refine a simple circuit design, barely armed with Ohm's law and a thin understanding of the Rule of Parallel Circuits. Please could someone help with a couple of novice questions?

The attached design, using B57891M103J thermistors, is intended to switch on the relay if two conditions are met:

- Th1-tank indicates a temperature greater than 32 Celsuis (or, Th1's resistance is less than 7.5K Ohm); and

- panel is hotter than tank (or, Th2-panel shows less resistance than Th1-tank).

I'll assume the usual convention of IN1/OUT1 and IN2/OUT2 referring to the two halves of the dual opamp.

My scant knowledge of electronics breaks down on that line between the "tank" potential divider and IN1-.

Here's a snippet from the thermistor's data sheet, for an example.

Celsius Ohms

30 8080

32 7432

34 6843

At 30 Celsius, the "tank" potential divider will give 6.22V. By the Rule of Parallel Circuits, I believe that 6.22V will reach both IN1- and IN2+. Do I have that right? If not, could someone please sketch the voltages?

Then, I'd like some hysteresis, limited by the 200K-B, from OUT2 pin IN2+. But I'm worried that the way I've laid this out, I'm also providing unwanted hysteresis to IN1-. Will I be? If so, could someone please suggest a solution?

Thanks very much.