Comparator with hysteresis


The hysteresis comparator without opamp do not have the need to eliminate the uncertainty of switching even with infinitesimal oscillations with respect to the threshold value or deleted because before the test or irrelevant in the next circuit.

A solution “cold” through a modest modification to the circuit it allows to include the hysteresis which in turn completely eliminates the exit stirred during the change threshold.

The input signal must exceed the upper threshold (Articles) to generate a switching output of low or lower than the lower limit of the preset threshold (VL) to switch to high output.

The figure indicates the hysteresis of a comparator. The resistor Rh determines the level of the hysteresis threshold.

Each time that the output is at a high logic level (5 V), Rh remains in parallel with Rx. This pushes the additional current in Ry, raising the threshold limit voltage (Articles) a 2.7V. The input signal will probably go above VH = 2,7 V to request the output response of the switch to a low logic value (0 V).

While the output is at a low logic level (0 V), Rh is parallel to Ry. This reduces the current in Ry, lowering the threshold voltage to 2.3V. The input signal will go below VL = 2.3V to adjust the output to a high logic value (5V).

Output Comparator with fluctuating input

The figure indicates the output of a comparator with hysteresis with a flottuante input voltage. It is assumed that the input signal level moves beyond the highest threshold limit (VH = 2.7V) so that the amplifier output op steps in low logic (0V).

Furthermore, the input signal level must move under the Lower threshold so that the op amp output may rise to logic high (5 V).

The disorder in this sample can be ignored, due to hysteresis.

However, said this, in cases where the levels of the input signal were greater than the interval calculated for the hysteresis (2,7 V – 2,3 V), You could generate additional fluctuating output transition responses.

To remedy this, it is necessary that the interval setting hysteresis is sufficiently extended to eliminate the disturbance induced in the specific model of the specified circuit.

Designing of the hysteresis comparator

equations (1) e (2) They can be of help to decide if the resistances wish to create the voltages of VH and VL hysteresis threshold. A single value (RX) It is required to be arbitrarily chosen.

Within this illustration, RX was determined to 47k to help reduce the current consumption. Rh was calculated to be 270,25k, consequently was implemented immediately standard value of 270k.

The hysteresis with a practical example

These days I am dedicated to the design of a charger, the one below is a simplified version used by me on breadboard to establish on the actual values ​​field to be used. The preset is initially set in such a way that the op amp output becomes high when the voltage pin3 goes just above the value of the zener pin2. When that happens, the pin6 becomes high and reaches a next potential to the supply voltage. This means that the RF feedback resistor (in viola) It is virtually in parallel with the preset resistance in the upper half of the potentiometer (shown in pink)

This means that pin voltage 3 increases even more. Now, when the battery voltage drops, pin 6 does not respond because the pin 3 It must fall to become much lower than the pin 2, which means that the battery level has to go down relatively long to allow the pin 3 to fall below the pin 2. This forced delay between the op-amp switch ON and OFF due to the feedback resistor is called hysteresis in operational amplifier. Once the pin6 is switched to low level this time RF is located in parallel with the lower half of the potentiometer (in verde) bringing the threshold pin3 even lower and thus avoiding that a few mV variation of the battery can of Back to switch the output.

In the drawing, the Green LED on means the battery is charged while the red one means it is being charged. This operational amplifier output can drive now the charging of the battery in an autonomous manner without the intervention circuits ne ne activation deactivation.

Greetings from A_X_C and Amilcare

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