DMM-Project

This Circuit  is a DPM or digital panel meter. It has a analog bar graph display and a 3-1/2 digit digital display. ICL7107 is used in the 200mV configuration.

U4A opamp LF353 amplifies the 200mV Full scale input to the level required for the LM3914 display circuit. D13-D14 are clamping protection diodes. Adjust P1 trimpot for a reading of 1000 counts when a 100.0 mV signal is fed at Vin. Adjust R8 trimpot to get the 5th led  to just turn on at 100mV input.

Analog and Digital Voltmeter using ICL7107

A combination of digital and analog display is helpful for quick decision making. Analog indicates even from a distance the process dimension.

Analog and Digital Voltmeter using ICL7107

Human Brain understands analog better. The digital is required to note down and record values for determining a setpoint or performance of a system.

This is a part of my Build a DMM or Digital Multi Meter

Sometimes a analog recorder with a ink-pen plot against time is a very good way of process analysis. Many systems are better studied using graphs not tables and lists of numbers. A Computer based data-logger gives greater power to this methods. You can measure and plot graphs of various types and at different points and for much longer periods.

U3 LF 356 is used as a constant current source (sink as the current is negative). R4, R5, R7 and R9 set the four resistance ranges by changing the constant current in decade steps. R2 is for calibration of resistance range. The A-B digital control of 4052 selects the range.

Resistance measurement – DMM Project

Let voltage current and resistance sockets be separate and of different color or use a high voltage electrical rotary switch or relays if you want the same sockets switched. D1, D2 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/- 3.5V only, with some FET it needs to be tweaked.

Resistance measurement - DMM Project

When you keep the current constant, the voltage across a resistor is directly proportional to the Resistor Value. This can be scaled to gat a usable reading on a Digital Voltmeter.

Here the 4053 selects or routes the voltage, current or resistance measurements to the A-D converter or display. It is selected with the mode selection when you want to measure Volts, Amps, Ohms and AC-DC. Some have to be polarity inverted and some signals just buffered this is selected and done by this circuit according to the digital control.

Now U1 OP07 circuit is a Buffer unity gain and low offset, U2 circuit is unity gain but polarity of output is opposite of input. D1-D2-R3 form a AND gate to select diode-buzzer test mode. The digital selection of 4053 Analog-Switch does not produce any errors in the analog-switching of even mV signals. But it works best at +/- 7.5V dual supply pin-16 is +7.5V, pin- 8 is digital ground and pin 7 alone should go to -7.5.

Analog Buffer and Inverter Switching

Analog ground can be same as digital ground, or the switched signals must be within +/-5V of digital ground. The switches should not carry any current and should be buffered at the output by FET opamps 1-Tera-Ohm. Then alone measurements are ok, as the switches have ohmic resistance.

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