Analog circuit analysis and description

We have an existing circuit that is an H-bridge for controlling a thermoelectric cooler. It works and is controlled by software as far as the control algorithm, but this circuit has two resistors that set the heating and cooling current limits. We want to change these fixed resistors to trimpots, so that the heat and cool current limits can be adjusted. We also need to know the relationship between the voltage at the opamp side of these current limit resistors, and the current limit, so that when we dial up the current limits by adjusting the trimpots we can measure what the current limits are that we are setting.

Attached is a schematic of this circuit. We feed in the voltage from a DAC (0-3.3V), and this is multiplied by x7 by U1B. The output of U1B determines which MOSFET pair in the H-bridge fires to heat, or cool. If the voltage out of U1B is less than Vtec (input at J4 over a range of 5 - 20V), then the system cools. If this voltage is greater than Vtec the system heats. We monitor the load thermistor as well as Vtec with our microcontroller and run the control algorithm.

The cooling limit resistor is R6 (fixed at 20K now) and the heating current limit is R8 (fixed at 15K now so the the heating limit is somewhat less than the cooling limit). With these values, the maximum current is equal to Vtec/4, and we want to know why that is the case, and how R6 and R8 determine this.

All of this works, and we only want to change R6 and R8 to trimpots, and know the relationships between their values and the heat/cool current limits. Also, we need to know the relationship between the voltage at the opamp sides of R6 and R8, and the cool and heat current limits so that we can measure the current limits as we adjust R6 and R8 (after changing these to trimpots).

Skills: Circuit Design, Electrical Engineering, Electronics, Microcontroller, PCB Layout

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