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Inverting Schmitt Trigger Calculator

This calculator first finds the exact resistor values to give the required circuit parameters of an Inverting Schmitt Trigger and then allows the substitution of preferred resistor values or those readily available to find the resulting circuit parameters.

[Inverting Schmitt Trigger Circuit]

1) Enter the required high and low input voltage switching thresholds, the high and low op-amp output voltages and the required current drawn from the reference voltage, then press "Calculate" below to obtain the Ideal resistor values.
High Threshold Voltage (Vth):   V Maximum Reference Input Current (Irefmax):    mA
Low Threshold Voltage (Vtl):   V Output Voltage High (Voh):   V
Reference Voltage (Vref):   V Output Voltage Low (Vol):   V

2) Change the calculated resistor values in the amber text boxes below to the nearest Preferred Resistor Values or to those available and then press " Calculate" below to get the resulting Schmitt Trigger switching thresholds.
Preferred Reference Input Resistor (R1):    Ω High Threshold Voltage (Vth):   V
Preferred Ground Resistor (R2):    Ω Low Threshold Voltage (Vtl):   V
Preferred Feedback Resistor (R3):    Ω Maximum Reference Input Current (Irefmax):   mA

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There are an infinite number of possible resistor values that will give the required High Trigger Threshold Voltage and the Low Trigger Threshold Voltage in an Inverting Schmitt Trigger because they are simply a ratio. Therefore, the required Maximum Reference Input current has been made selectable in order to set the required finite resistor values. The following formulas have been derived by myself and may not be in their simplest form.

Using the fact that both inputs to the op-amp will have virtually the same voltages, we first calculate the value of the Reference Input Resistor (R1) from the Reference Voltage (Vref) less the Low Threshold Voltage (Vtl) divided by the the selected Maximum Reference Input Current (Irefmax).

[Formula to find R1]Formula 1.

[Formula to find R3]Formula 2.

[Formula to find R2]Formula 3.

[Formula to find Vth]Formula 4.

[Formula to find Vtl]Formula 5.

[Formula to find Irefmax]Formula 6.

First published on by W J Highton on 15/6/2018


This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX

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