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Current Amp digital control – DMM Project

R6 is the Shunt thru which the current to be measured passes. F1 fuse is to protect shunt. D1-D4 ensures that the current flow is not broken if the shunt blows. The Ammeter is always used in series in a circuit.

OP07 is used here as a digitally controlled amplifier as the voltage offset error is around 75uV. 4052’s digital controls A-B set the range by selecting R1, R2, R3, R5 for the digital code at A-B. The inverting amplifier changes the gain and four current ranges are got. The output of this circuit has to go to a buffer and cannot be loaded directly. R8 is offset trim at very high gains, it has to be adjusted for a zero output of opamp for zero current measured.

Current Amp digital control - DMM Project

Rf and Ri are 0.1% MFR, if costly, use bourns 10T trimpot or a difficult way – use series parallel combination and scratch 10% part of resistor network to increase value.

Read more at my – Current Shunt Amplifier with digital control – del20014

Voltage Attenuator Amp – DMM Project

LF356 opamp FET input is in a inverting amplifier configuration here. R4 + R5 make up Ri each 1/4 W MFR withstands 250V so both will take upto 500V. Use more in series for higher voltage withstand with care in PCB layout and cabinet insulation.

Voltage Attenuator Amp – DMM Project

Rf is selected by a digital value at A-B inputs of 4052, that way R1, R2, R3, R6 are selected for four ranges giving various attenuation levels. The important thing in this circuit is the on resistance of 4052 of 100 ohms comes in series with the output resistance of opamp, so the output is taken before the cmos switch . The output of this digital attenuator should not be loaded and should be buffered before use.

Gain = Av = Rf/Ri and Vout = -(Rf/Ri) * Vin

Voltage Attenuator Amp - DMM Project

Rf and Ri are 0.1% MFR, if costly, use bourns 10T trimpot or a difficult way use series parallel combination and scratch 10% part of a network R to increase value.

DMM range and AC-DC mode Logic

This is the third circuit that you will need to build a Simple Benchtop DMM with no Microcontroller.

Here the U1D and U1B 4093 acts like a de-bouncing circuit for the push keys. The 4029 counts up scrolling to binary values 00, 01, 10, 11 for the four ranges.

The binary value of 4029 is decoded to decimal by 4028 in order to light four LEDs which indicates the range or mode on the front panel. When any of these pushbuttons are pressed and held, the nand schmitt 4093 clocks to scroll the range continuously. The binary output of these counters control CMOS switches 4052 which are analog multiplexers.

DMM range and AC-DC mode Logic

The DPM or DVM gives a readout of an Analog Value or process. The Analog reading we obtain from a Circuit Measurement Jig represents some real world parameter.

In a multi-parameter instrument like DMM, the measurement jig functions and the analog signal routing is done by ganged range selection switches.

Read More here DMM range and AC-DC mode Logic

Power supply with battery backup – DMM

This is a simple charger circuit which will work for a light load like a DMM, the Battery can be a sealed maintenance free battery of 9V-2AH or better. The circuit will work best if the Unit is powered on many times daily on regular use, else battery will drain down.

The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply.

Power supply with battery backup - DMM

Many dual supplies are derived from one DC Source. A SMPS solution is the best. A Series Regulator is simple to troubleshoot in comparison to Switching Types, there is no EMI-RFI too.

Power supply with battery backup – DMM

This simple circuit is ok only for Low Current gadgets, Whereas SMPS is green and efficient. In SMPS a greater care for Product Safety is required. In a Linear Supply with Step-Down Mains-Frequency Transformer. The Transformer is the only place, where you look into safety the most. In SMPS it is the PCB, the feedback components and also The High-Frequency (200 kHz) Mains Ferrite Transformer.