Organize and Share your Electronics the way you want. Sign-Up for a free account now. It takes only 30 seconds!

High Impedance Voltmeter

High Impedance Voltmeter

This circuit is designed to provide an inexpensive way to to create a High Impedance Voltmeter while making use of an inexpensive analog or digital multimeter. The circuit is specifically designed for testing phototransistors when they are used in the circuits shown at this site. It has a very high input impedance that will not “Load Down” the sensor that is being checked.
When measuring voltages in high resistance circuits the resistance of the voltmeter itself has an effect on the circuit. For example if the voltage across a 1 megohm resistor is measured with a voltmeter that has an internal resistance of 1 megohm then the total resistance in that part of the circuit is effectively halved (two 1 M resistors in parallel = 500K ohms).

In another example; If a voltmeter with a 1 megohm resistance is placed in series with a 1 megohm resistance there will in effect be two – 1 megohm resistances in series, the resistor in the circuit and the resistance of the voltmeter. Under these conditions the maximum voltage that the voltmeter could show would be 1/2 of the supply voltage.

Many inexpensive digital multimeters will have an internal input resistance in the 1 megohm range. Analog voltmeters that are not battery powered will have much lower internal resistances. The more expensive meters will have higher input impedances and therefore will have less loading effect on the circuit under test.

The circuit on this page will compensate for the low input impedances fo the meters and thereby provide more accurate voltage readings in circuits that have high resistances.

High Impedance Voltmeter Notes

This circuit should only be used for testing phototransistor sensors in circuits where the maximum available voltage is already known.

Limited protection for input Reverse and Over voltage conditions has be provided.

An analog or digital multimeter would be suitable for use with this circuit. An analog 15 volt meter such as Radio Shack part number 276-1754 could also be used.

Due to the very high impedance of this circuit the meter will read about 17.5 Volts when the input open circuited. This is caused by the leakage current from the input terminal of the OPAMP and is a normal condition. The false reading will disappear when the input is connected to a circuit.

The circuit should not be connected to a voltage source unless the batteries are installed.

An ON/OFF switch has not been shown as this would likely be the most expensive part of the circuit.

An ideal power supply would be the used batteries from smoke detectors as the circuit draws very little current and would only be used occasionally.

This circuit uses the LM358 OPAMP in a “Voltage Follower ” configuration. This type of amplifier circuit has a voltage gain of 1 and an input impedance of several hundred million ohms.

Visit Here for more.







 

More Articles to Read

Skill Sunday: Arrays
Skill Sunday: Arrays
App note: Infrared remote control implementation with MSP430FR4xx
App note: Infrared remote control implementation with MSP430FR4xx
Gesture Controlled Smart Home
Gesture Controlled Smart Home
Design for a Wideband, Differential Transimpedance DAC Output
Design for a Wideband, Differential Transimpedance DAC Output
Wrist thrusters let you fly through the water effortlessly
Wrist thrusters let you fly through the water effortlessly
Bertan/Spellman 225-20R HV power supply teardown
Bertan/Spellman 225-20R HV power supply teardown
Blinking Christmas Lights
Blinking Christmas Lights
Adjusting clock with alarm, hygrometer & thermometer on 1.8″ ST7735 display
Adjusting clock with alarm, hygrometer & thermometer on 1.8″ ST7735 display
Adding an “extra sense” with rangefinders
Adding an “extra sense” with rangefinders
This wireless game controller looks like a rug
This wireless game controller looks like a rug
How to run your ESP8266 for years on a battery
How to run your ESP8266 for years on a battery
Calibration of a 3-Phase Energy Meter Board on the ADE7754
Calibration of a 3-Phase Energy Meter Board on the ADE7754
Making beats on a tiny Arduino DJ controller
Making beats on a tiny Arduino DJ controller
Qinsi-QS5100 Sn63Pb37 solder profile
Qinsi-QS5100 Sn63Pb37 solder profile
Water Detection System
Water Detection System
Professional Hi-Fi 15W Tube Amplifier circuit
Professional Hi-Fi 15W Tube Amplifier circuit
This Arduino machine will judge how sick your ollies are
This Arduino machine will judge how sick your ollies are
Turn an old payphone into a boombox for ’90s hits
Turn an old payphone into a boombox for ’90s hits
Big F’n 3D printer build
Big F’n 3D printer build
Temperature alarm for boiling milk
Temperature alarm for boiling milk

Top


Shares