Lesson 5 Window Comparators ET 438 b Sequential

Lesson 5: Window Comparators ET 438 b Sequential Control and Data Acquisition Department of Technology Lesson 5_et 438 b. pptx 1

Learning Objectives After this presentation you will be able to: Ø Explain how a window comparator operates Ø Construct a window comparator using OP AMPs Ø Use a window comparator to detect voltages in three ranges Ø Design a window comparator given values of upper and lower trip voltages. Lesson 5_et 438 b. pptx 2

Window Comparators Circuit Logic U 1 Inverting Comparator Vcc = 5 V Vcc Vin U 1 Vo 1 Vref 1= 1 V Circuit Logic U 2 Non-Inverting Comparator Vcc = 5 V Vref 2 = 2 V Vo 2 U 2 Vin > Vref 1 (1 V) then Vo 1 = 0 V Vin < Vref 1 (1 V) then Vo 1 = Vcc (5 V) Vin > Vref 2 (2 V) then Vo 2 = Vcc (5 V) Vin < Vref 2 (2 V) then Vo 2 = 0 V V 0 Vo 2 1= Vref 1 Lesson 5_et 438 b. pptx 2=Vref 2 Vo 1 Vin 3

Window Comparators Add outputs through logic gates to get final Vo Assume Vcc = 5 Vo 1 Vo Vo Vo 2 Vref 1 LTP Vo 1 0 0 5 5 Vo 2 0 5 Vo 5 0 0 0 Vref 2 UTP Vo 1 when LTP< Vin < UTP when Vin > UTP when Vin < LTP this case never happens Lesson 5_et 438 b. pptx Vin Same logic as NOR gate. 4

Window Comparator Final Circuit Vin inverting comparator Vo 1 NOR LTP UTP V 0 Vo 2 non-inverting comparator V 0 Vref 1 LTP Vref 2 UTP Final Input/Output Vin Lesson 5_et 438 b. pptx 5

Window Comparator-Alternate Design Add outputs using resistive voltage dividers and use OP AMP for final output. Note bipolar power supplies. +15 V Analyze this part +15 V U 1 A LM 324 Vin + R 3 10 k Vltp R 2 10 k +15 V + -15 V U 1 C LM 324 Vo +15 V Vutp + U 1 B LM 324 -15 V R 4 10 k -15 V Lesson 5_et 438 b. pptx 6

Window Comparator-Alternate Design Analysis Circuit enclosed Must use superposition Lesson 5_et 438 b. pptx 7

Window Comparator-Alternate Design Analysis Lesson 5_et 438 b. pptx 8

Window Comparator-Alternate Design Analysis Lesson 5_et 438 b. pptx 9

Window Comparator-Alternate Design Analysis For VLTP < Vin < VUTP the following circuit applies This can be further reduced Lesson 5_et 438 b. pptx 10

Window Comparator-Alternate Design Analysis vin Lesson 5_et 438 b. pptx 11

Window Comparator Simulation of previous circuit using piece-wise linear input and transient analysis. Uses LM 324 Quad OP AMP. (4 in 1 package) Vutp=7 V Vltp=4 V Lesson 5_et 438 b. pptx 12

Design Example Design a circuit that will light one of three LEDs when the output from a pressure sensor that has a gain of. 25 V/psi is connected to its input. The output should follow the specification listed below Range 0 - 5 psi 5 - 10 psi 10 - 15 psi LED 1 on off LED 2 off on off LED 3 off on Assume there is a 5 Vdc power supply available and that all inputs and output are TTL compatible Lesson 5_et 438 b. pptx 13

Design Example Solution: Use window comparator with LEDs connected to the outputs of each device in the circuit. inverting comparator When Vin < LTP LED 1 energized +5 V R Vin LED 2 On V 0 LED 1 U 1 Vo 1 LTP R +5 V UTP non-inverting comparator Vin V 0 LED 2 Vref 1 LTP Vref 2 UTP V 0 Vo 2 Vin Vo 1 Vo 2 U 2 R LED 3 When Vin > UTP LED 3 Energized Vref 1 LTP Vref 2 UTP LED 1 On Lesson 5_et 438 b. pptx LED 3 On 14

Design Example (Continued) Determine the values of UTP and LTP from the pressure sensor gain. Ranges 0 - 5 psi 5 - 10 psi 10 - 15 psi VLTP = 5∙(0. 25 V/psi) = 1. 25 V VUTP = 10∙(0. 25 V/psi) = 2. 5 V Size resistors for LED current limiting. Determine the maximum current sourcing ability of the comparators. (assume 25 m. A) Forward voltage drop of LEDs Vf = 1. 2 V Vsat of U 1 , U 2 -0. 8 Vcc=4 V at high output Lesson 5_et 438 b. pptx 15

End Lesson 5: Window Comparators ET 438 b Sequential Control and Data Acquisition Department of Technology Lesson 5_et 438 b. pptx 16