Electrical 101

Input Impedance

When a meter is connected to a circuit, it acts like a load with current going through it.

Meters with a high input impedance draw almost no current through the meter while testing a circuit. Multimeters have high input impedance (you can measure low voltage on logic circuits). They give precise measurements but may give a false voltage reading with photocells, dimmer switches, a poor connection, or parallel wires (voltage may be present, but not be able to operate a load).

Meters with a low input impedance draw a small amount of current through the meter while putting a small load on the circuit. You will get false readings on low voltage logic circuits with a low input impedance meter. Solenoid testers have low input impedance. They give approximate voltage measurements and accurate (but not precise) voltage reading with photocells, dimmer switches, or parallel wires. When a voltage is present, but not able to operate a load, a solenoid tester will absorb the voltage and show there is no voltage.

The high input impedance multimeter at left shows 48.2 volts on a 120 volts outlet where power is turned off. This outlet is fed by one of the line wires of a 12-3 NM cable. The other line wire is turned on. A low input impedance solenoid meter will show 0 volts on a circuit that is turned off.

This 12-3 NM cable contains four wires, two line wires (black and red), one neutral wire (white), and one ground wire (bare). All four wires are bundled close together inside NM cables from this outlet to the electrical panel.

The meter at left is measuring the voltage of the black wire with power shut off. The red wire that travels next to the black wire has power turned on.

The magnetic field from the red wire induces voltage to the black wire. This voltage is unable to supply a load or an electrical shock.

When measuring the output of certain photocells or dimmer switches with a high impedance multimeter, it may show a reading of 120 volts when they are turned off or have failed.

120V

Magnetic field is present when a wire has voltage on it.

Voltage induced from adjacent 120V wire. 48.2 volts measured with high impedance multimeter.

0 volts measured with low impedance solenoid meter.

Meter connects to an outlet that has power turned off.