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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.

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