Voltage window comparator Circuit follows reference voltage

Engineeering Projects

The use of window comparators are essential in the test circuits which perform the ‘GO-NO-GO’ tests for the various electronic components likes zener diodes, transistors, relays etc. The upper and lower threshold voltages are set for the window comparators depending upon the values and tolerances of the components to be tested. Here is a window comparator circuit in which the window width is automatically adjusted to be a fixed percentage of the input reference voltage Vref proportional to the nominal value of the component under test and produces an upper threshold voltage Vref-Hi and the lower threshold voltage Vref-Lo. The two threshold voltages are equally spaced around the reference voltage.

Comparator b and c forms the window comparator. Op-amp a forms a non inverting amplifier. The gain of op-amp is set be resistor R1, R2, Rf and VR. When the value of R1 and R2 are equal, upper and lower threshold voltages are given by

Vref-Hi = Vref (1+(T/100)) and

Vref-Lo = Vref (1 – (T/100))

Where T is the percentage tolerance selected by the switch.

circuit diagram voltage window comparator follow reference voltage

It can be seen from the circuit that (R2/(R2 + Rf + VR)) = T/100. Therefore, (Rf + VR) = (R2(100-T ))/T. The values of resistors, forming the voltage dividers for different positions of switch SW1, are shown in table 1 for different tolerances. The LED indication for the values of Vsignal relative to upper and lower threshold voltage are shown in table 2.

TABLE 1

Switch Position

Tolerance

Fixed Resistor Rf

Preset Pot VR

5 %

R3

VR1

10 %

R4

VR2

20 %

R5

VR3

TABLE 2

Glowing LED

Indication

LED1

Vsignal > Vref-Hi

LED2

Vsignal < Vref-Lo

LED3

Vref-Hi > Vsignal > Vref-Lo

PARTS LIST OF  VOLTAGE WINDOW COMPARATOR FOLLOWS REFERENCE VOLTAGE

 Resistor (all ¼-watt, ± 5% Carbon)

R1, R2, R6 = 1 KΩ

R3 = 18 KΩ

R4 = 8.2 KΩ

R5 = 3.3 KΩ

R7 = 820 Ω

VR1, VR2 = 2 KΩ

VR3 = 1 KΩ

Capacitors

C1, C2 = 0.1 µF (Ceramic Disc)

Semiconductors

IC1 – IC3 = LM311 (voltage comparators)

T1 = SK100 (Medium Power PNP Transistor)

LED1, LED2, LED3

Miscellaneous

SW1 = 1-pole 3-way switch

 

Leave a Comment