 Girdhar Jain, Assistant Professor in Electronics and Telecommunication Engineering, Valchin Institute of Technology, Sholapur. Now, today I am going to deliver a lecture on Autopolarity Detector and Indicator and Attenuator Circuit for DVM. Now, learning outcomes of the session are at the end of the session, students will be able to draw and design Autopolarity Detector and Indicator and Attenuator Circuit for digital voltmeter. Now, contents are Autopolarity Detector and Indicator Circuit, Attenuator Circuit and design considerations of the circuits. Now, this figure one shows the circuit for Autopolarity Detector and Indicator. Now, this circuit consists of the comparator A1 and A2 using op-amp and op-amp A3 is connected as a inverting amplifier. Now, output of comparator A1 is given to a Zener diode. So, 180 ohm and Zener diode are connected as shown in figure. So, output of comparator because this operational amplifier is driven with plus 12 volt and minus 12 volt therefore, when output of comparator is high. So, it will be plus 12 volt and to clamp it to logic 1, the 4.7 volt Zener diode is used and 180 ohm resistance will limit the current through Zener diode. Now, similar circuit is used at the output of comparator A2 and op-amp A3 is connected as a inverting amplifier. So, for the inverting amplifier, the gain is minus Rf by R1. So, R1 and Rf are 1k therefore, gain of this inverting amplifier is minus 1. Then at the output of comparator A, it is output is connected to control switch Sx, output of comparator form by A2. So, point B is connected to control switch Sx. Now, let us understand how the circuit operates. So, this circuit will detect the polarity of input and it will indicate if the input is negative by displaying this LED by glowing this LED. Now, let input is positive V in. Let input is positive. If input is positive, then output of comparator A1 is high and output of comparator A2 is low. Therefore, LED will be off and as output of upper comparator A1 is high. So, point A, it is connected to control switch Sx and therefore, switch Sx is closed. Now, here switch Sx and Sy are the solid state switches which are control electronically means when the output A is high. So, it will close the solid state switch Sx. Now, when input is positive, output of comparator A1 is high, switch Sx is closed, output of comparator A2 is low, switch Sy is open. And therefore, the input V in, therefore, input V in through switch Sx, it comes at the output and it is connected to input of a integrator input. So, this auto polarity detector and indicator circuit is used at the input side. So, input is applied to the circuit and output of the circuit is given to a integrator input for the DVM which we have seen in the earlier lecture. Means when input is positive, it is transfer to input of integrator as it is. Now, let us understand if input is negative, what will happen? When input is negative, output of comparator A1 is low and output of comparator A2 is high. Therefore, the LED is glowing, right? 180 ohm resistance will limit the current through LED and that is the intensity of LED. So, LED is glowing which indicates that input is negative. When LED is not glowing or LED is off, input is positive and when LED is on, input is negative, okay? Now, when output of comparator A2 is high which is point B and it will close the switch SY and output of comparator A1 is low means switch Sx is open. Now, when Sx is open and SY is closed, now negative input is passed through the inverting amplifier which is having a gain of minus 1 and therefore, output of this inverting amplifier is a positive voltage. For example, if input is minus 2 volt and gain of this inverting amplifier is minus 1 means output of inverting amplifier is plus 2 volt and as SY is closed, so output of the inverting amplifier, so it is connected to input of integrator. So, in this way, this circuit will pass the input as it is by closing switch Sx and it will invert the input by closing switch SY and passing the input through a inverting amplifier, okay? So, this circuit will detect the polarity of input by glowing the LED, right? If input is negative, LED is on. Now, pause this video and think on the following question. So, shall we measure the 2000-hole DC using DVM? Yes, 2000-hole DC can be measured, but that 2000-hole DC cannot be connected directly to the integrator input because the op-amp can accept the input voltage up to the rated input, right? It cannot accept the 2000-hole and therefore, if you want to measure up to 2000-hole, say 0 to 2000-hole, then that voltage is to be attenuated by using a attenuator circuit. Now, this figure shows the attenuator circuit for DVM and this attenuator circuit is also we call as a range selector circuit, means we select the range of input voltage by operating the switches. Now, this attenuator circuit consists of the 9 meg, 900 k, 90 k, 9 k and 1 k connected in series, then switches S a, S b, S c, S d and S c as shown and then 100 k, then two Zener diodes connected in anti-parallel, okay? And there is a buffer, voltage follower. Now, input resistance of this buffer is high, which will avoid the loading effect and the output of voltage follower which is 0 to 0.2 volt which is connected to the input of integrator. Now, you can see depending on the magnitude of input hold or the range of input voltage, a appropriate switch is to be selected and this is given in the table 1 range selection. Now, if input voltage is in between 0 to 0.2 volt, switch S a is closed and remaining switches are open, means input is passed as it is and through buffer, voltage gain of the buffer is unity, so output is 0 to 0.2 volt. Similarly, if input is 0 to 2 volt, then switch S b is closed with other switches kept open. So, in this case, what will happen if switch S b is closed? So, 9 mega ohm upper resistance and from that point to the ground, the sum of 900 k, 90 k, 9 k and 1 k, okay? So, if we proceed further for the equivalent circuit, now if switch S b is closed, so close the switch S 2 and here let input is say 2 volt, so this is 9 meg and the lower resistance. Now, make sum of the lower resistances, so it will come as 1 meg and the simplified equivalent circuit becomes 2 volt, 9 mega ohm in series with 1 meg and voltage across 1 meg is the output V o dash. Now, if we apply a potential divider, V o dash is 1 meg upon 1 meg plus 9 meg means 1 upon 10 into 2, so it is 0.2 volt, means 2 volt is converted to 0.2 volt by the circuit. Now, similarly, for the range of 20 volt, operate switch S c, you can draw the similar equivalent circuit and verify yourself and for the 200 volt, the switch S d is to be used and for 2000 volts switch S c is to be used. Now, this kind of circuit is used where there is a manual range selector switch provided for a multimeter means by observing the input voltage, you have to select the appropriate switch manually, right? Now, design consideration. Now, in the circuit, the comparators which are used in the circuit, so use high sensitivity comparator such as mu a 710 or lm 324 and for the operational amplifier which is used as a inverting amplifier, you can use lm 324 or lm 741, okay? So, these are references. Thank you for attending the lecture.