 Giridhar Jain, Assistant Professor in Electronics and Telecommunication Engineering, Valkan Institute of Technology, Sallapur. Now today I am going to deliver a lecture on Auto-Ranging Circuit for DVM. Now learning outcomes of the session are at the end of the session students will be able to draw and design auto-ranging circuit for digital hold meter. Examples are auto-ranging circuit for DVM, circuit explanation and design considerations. Now auto-ranging circuit for DVM. Now using manual selector switch you have to change the or you select the appropriate switch depending on the magnitude of input that is a manual range selector. Now if you want a auto-ranging facility for your DVM, now these selection of the switches is to be done automatically by monitoring the input voltage, ok. Now here in the circuit the attenuator is provided by connecting the resistances 9 mega ohm 900 K, 90 K, 9 K and 1 K as shown and the switches are S A, S B, S C, S D and S E. These are the solid state switches. Then output is clamp to 4.7 hole by using the two Zener diodes and buffer voltage follower is connected at the output which will avoid the loading effect and this output of this voltage follower goes to integrator. Now below is the auto-ranging control circuit. Now this will consist of the four comparators as shown and the negative that is inverting terminal of all this comparator is connected together and that is connected to 1 K of the attenuator as shown. And the input to the comparators are as shown in the figure means after 9 mega it will go to upper comparator then after 900 K then lower next comparator like that. And the output of these four comparators is let it is V o 1, V o 2, V o 3 and V o 4 it will go to 4 H 16 decoder. Now 4 H 16 decoder output of these four as 16 decoder is now here there will be 16 outputs out of which the five are used here and output of the 4 H 16 decoders are active low. Now outputs Y 0 bar, Y 8 bar, Y 12, Y 14 and Y 15 are connected and these output of these decoders they will be connected to control switches S E, S D, S C, S B and S A respectively means these outputs will control the solid state switches S A, S B, S C, S D and S E right. Now in this table you will understand it will give the auto range selection how it is done. Now whenever input voltage is 0 to 0.2 then output of four comparators is high that is V o 1, V o 2, V o 3 and V o 4 is high. Now this circuit connections and the value of resistances are selected such that whenever input is in between 0 to 0.2, V o 1, V o 2, V o 3 and V o 4 that is output of all comparators is high and when output of all comparators is high then that is Y 15 of the decoder will go low and after inverter it is connected to switch S A means switch S A is made on in this way. Now whenever input voltage in between 0 to 2 volt then what happens the output of comparator 1, 2, 3 that is V o 1, V o 2 and V o 3 are high and V o 4 is low. So you can look at the circuit again. Whenever input voltage is between 0 to 2 volt V o 1, V o 2, V o 3, logic 1 and V o 4 is low and therefore you can see the Y 14 is selected. So here it is 1, 1, 1, 0 so Y 14 will go low and it will control switch S B. Similarly whenever input voltage is 20 volt then Y 12 is selected which will control switch S C then whenever input voltage is 200 volt V o 1 is 1 and remaining 3 output are 0 therefore Y 8 is low. So switch S D is selected and when your input voltage is 2000 volt output of all the comparators are low and therefore Y 0 is selected and you can see switch E, S E is selected. So in this way the circuit works in auto ranging mode. So here important components or elements of the circuit are the range selector switches which are solid state switches then 4 comparators and the 4S 16 decoder. Now pause this video and think on this question. So shall we measure the small AC voltages using digital voltmeter? Now small AC voltages now if the voltage measured is greater than 0.7 volt that can be rectified by using the diode and you can measure means you convert AC into the DC by using ordinary rectifiers using diode. But if input voltage is small less than say 0.7 of the order of say few milliholts then ordinary diode cannot be used as a rectifier because its diodes will not conduct and therefore precision rectifiers are used to convert small AC into the DC and then DC is given as a input to the DVM. Now this figure shows the precision full wave rectifier using the op-amp. Now first circuit first op-amp will act as a active half wave rectifier that is a half wave precision rectifier and the second circuit will act as a adder. Now second circuit will act as a adder which will add the AC input voltage and the output of rectifier and then the output of adder is a full wave rectified output and it is filtered by a capacitor CF. Now this output voltage of the second circuit is scaled to RMS failure of the input by adjusting gain of the adder that is by adjusting a potentiometer of 5 kilo ohm as shown in the figure okay. So in this way the small AC voltages can be rectified using a full wave precision rectifier as shown in the circuit and the output of this full wave precision rectifier is scaled to show the RMS failure of input by adjusting the gain of the circuit by adjusting the preset or potentiometer of 5k. Now design considerations, now in this auto ranging circuit the switches which are used that is SA to SE 5 switches are used. So these switches are not mechanical switches you have to use a solid state switches and for solid state switches a SWIMOS switch IC CD4066 can be used okay. Then second is comparators, now for the comparators we require a high sensitivity. So high sensitivity comparator mu A710 can be used then for the decoder. So we are using 4S16 decoder in the auto ranging circuit. So for the 4S16 decoder use the decoder IC 74156 which will have the 4 input and 16 outputs and outputs are active low. So this IC can be used as 4S16 decoder and op-amp which are used are LM324 or LM741 can be used as an operational amplifier for designing this circuit okay. So in this way by using this ICs you can design the circuit of auto ranging. Now these are the references of AMN linear integrated circuits by Ramakanth Gaikod and second is electronic system design by Vaibhav Tarate. Now thank you for watching the video.