Some of the device or system requires their input voltage should remain within fix upper (maximum) and lower (minimum) limits. The system or device may not be able to tolerate the input voltage outside these limits. For a system it may be specified that to get correct output the input voltage should be within these min and max limits otherwise it generates undesirable output. Similarly for a device it may be specified that the input voltage should not exceed these limits otherwise it may damage the device or affects the performance of device.
So to limit the voltage within min and max limit the circuit is used that is input voltage limiter or popularly known as window detector. The circuit is build using op-amps as comparator. It uses two comparators
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Upper comparator – for max input voltage limit – upper limit
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Lower comparator – for min input voltage limit – lower limit
Circuit description:
As shown in figure
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The input is given to non inverting terminal of upper comparator and inverting terminal of lower comparator after shorting them to gather
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The non inverting terminal of lower comparator is connected with sliding terminal of pot R3. Two fixed terminals of pot are given 5 V. Thus the pot R3 sets voltage at this terminal from 0 to 5 V
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Similarly pot R2 is connected to inverting terminal of upper comparator. It will also set the voltage for the terminal between 0 to 5 V.
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The output from both comparators are coupled through diodes D1 and D2 and it is taken out across load R1
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Both op-amps are given +Vcc and –Vee supply at terminals 7 and 4 for biasing
Here is the snap of circuit setup
Circuit operation:
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Using pot R3 voltage is set at non inverting terminal of lower comparator as 2 V. This is min input voltage limit
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Similarly using pot R2 voltage is set at inverting terminal of upper comparator as 4 V. This is max input voltage limit
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The sine wave input is given from function generator as 10 Vpp @ 1 KHz
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So the input voltage varies from -5 V to +5 V.
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Till the input is less than 2 V the lower comparator output is +Vcc but upper comparator output is –Vee
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Because final output is resultant of both comparator outputs coupled through diodes – the final output is +Vcc
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Now as input increases from 2 V but less than 4 V, the lower comparator output also becomes –Vee along with upper comparator output
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Because both comparator outputs are low, the final output is also low. So between 2 to 4 volt of input the output remains low
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As the input exceeds 4 V, the upper comparator output changes to +Vcc and lower comparator output remains –Vee. But the final output again becomes +Vcc
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So the output voltage remains low for the input voltage range from 2 to 4 V that is min and max limit for input voltage.
Here is the snap of final output of circuit on DSO
Circuit Diagrams
Filed Under: Electronic Projects
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