Schematics of delabs

This circuit uses a LM339, a quad comparator. LM339 can work on single or dual supplies, it has a open collector output that can drive 15mA, low power consumption. The circuit is an untested design but it should work.

Mains Voltage and Power Circuits - Similar circuits for Mains Voltage Monitoring.

There are many better circuits in the various circuit archives i have linked on the front page, you just have to look around. When you measure the open circuit voltage of a battery with a high impedance DMM (10M), the value may be a bit misleading. Apply a dummy load to bleed the battery a bit so that proper readings can be taken on Load. The load below is a 100 ohms wire-wound fusible ceramic resistor which will heat a bit when you test 12V batteries.

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Auto Ranging for ICL7107 had to be done with window comparators. In ICL7135 Digital Outputs are available for autoranging. Here in this circuit is a close approximation of what i may have designed once.

Auto ranging 4-1/2 Digit Digital Voltmeter

The below Details from Datasheet - These pins are used in this circuit to Autorange the DPM with some peripheral CMOS Logic.

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I don't remember if this circuit worked properly. But a few were made and i might not have shown the modifications that were done to make it work. This was meant to be a portable, low cost, insulation tester for an electrician. If you try it out and debug it it may work well.

High Resistance Insulation Materials

A negative voltage is derived by shifting gnd with two diodes, i feel this did not work very well. Two pins of CD4028 pins are also used to boost the reference to get two extra ranges as 4051 has a 100E on resistance.

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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.

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Here is a easy to read 'Analog' Millivoltmeter. Just like the Moving Coil Voltmeter, but does not have that resolution. This gives a easy indication of process progress or parameter magnitude from a large distance. A bargraph is easy on decision making too, compared to a digital readout.

How this Works ? - The analog input in mV - millivolts is fed to R18, RC reduces Noise and the Zener Clamps protect. The LF353 FET Opamp offers High Impedance as a Non-Inverting Amplifier, which nullifies measurement burden, Remember the Moving coil voltmeter loads the measured circuit, causing sizable errors. Then they invented the Vacuum Tube Voltmeter to solve this issue. An FET voltmeter is near ideal, they ought to have inventing this first.

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