# Hysteresis

Discussion in 'General Electronics Discussion' started by jkele, Mar 18, 2011.

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Nov 21, 2010
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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The switching point is determined by the voltage on the non-inverting input.

The output of the op-amp will be at one supply rail or the other, so the voltage divider will consist of R1 and R2/R3 in parallel, or R1/R3 in parallel and R2.

Knowing this, you should be able to derive any formula you require.

3. ### jkele

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Nov 21, 2010
how can R2 and R3 be in parallel? Parallel resistors must have both branch out from some point in the circuit and meet up later, or is it because the output is at the same voltage as R1?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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The output of the op-amp will be at the potential of one of the supply rails. Therefore R3 will effectively be in parallel with either R1 o R2 depending on what supply rail the output is at.

5. ### Laplace

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Apr 4, 2010
It might be easier to visualize if you replace R1 & R2 with the Thevenin equivalent circuit. Then the Thevenin voltage is the nominal switching point. When you add in R3, the switching point will shift depending on whether the output driving R3 is high or low. The difference between the switching points is the hysteresis.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Easier for you maybe

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