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Wednesday, March 15, 2017

Millivolt Milliamp Source - Field Calibration Current Loop

This is easy to rig millivolt source for field calibration or troubleshooting of 4-20 mA current loops. Here a Darlington pair is used for current amplification which reduces the Ib error as gain is very high.

EE Design Library - Electronic Product Design Resource.

Millivolt Source - Field Callibration Current Loop

A rotary switch selects, 4-12-20 mA Preset points. A Bourns multi-turn wirewound Pot can also be used with a digital dial. Enclose in a dust proof handheld box. Read more on process calibration.

A mA source terminated with a 10 Ohm or 100 Ohm can turn it into a mV Source. The Resistor should be above 1W and has good temperature stability. Use MFR 1 % many in parallel to get a Instrument shunt if you do not get any.

Thursday, March 09, 2017

Analog and Digital Voltmeter using ICL7107

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.

Friday, March 03, 2017

3-1/2 Digit ICL7107 DPM Digital Panel Meter

This is a Digital Panel Meter based on ICL7107. This was the workhorse Digital Readout chip before Low power uC designs were developed. Even now it is the easiest way an analog parameter can be displayed in an instrument.

Instrumentation and Measurement Circuits

3-1/2 Display is just enough resolution for recording process data or troubleshooting equipment. With 4-1/2 you notice the problems of thermoelectric EMF of probes and connectors. That is one reason gold plating is used another being low contact resistance. In 4 1/2 the reading can change due to contact resistance too, if the input impedance of the analog signal conditioning is low or the protection diodes, RC filter caps are leaky.

When you measure an analog parameter from a sensor; you attenuate or amplify it, to scale to the A/D converter range. You also need to protect or isolate the analog front end from high energy mis-connections, which will happen during field use. The operator should not get injured from any high energy leakage while measuring.

Resolution is the finest detail you can enumerate in a parameter or object. You can say the crowd was 3000 or 4000 people. Here 1000 is the resolution, finer data does not matter here. You would also hear things like, there were 80 to 90 boys in that classroom. 10 is the resolution there.

Accuracy is how precise a statement or number is. If there was 85 boys in that class, the above statement is accurate. If there was 65 boys, the measuring method or process needs calibration or examination.

In this circuit, the plastic caps should be of very low leakage. Multilayer plastic caps are good for instrumentation.