Electrical Units of Measurement

Voltage, current and resistance cannot be seen, but can be measured.Some of the numerical values can be very small, it is easier to use electrical shorthand.It’s is quicker to jot down 8M instead of 8,000,000 ohms resistance.

Circuit symbols

Any electrical schematic, or drawing can be compared with a diagram in which symbols are instead of pictures for the various components.The symbols are used in an effort to make the diagrams easier to draw and understand.

Battery Symbol

Resistance Symbol

Ground Symbol

Electrical Units of Measurement

Measurement Devices

A Voltmeter is used to measure voltage potential… Always: placed across the device. Never: placed in series with device. Units: Volts (V) and Millivolts (mV).

An ammeter is used to measure current flow… Always: placed in series with the

device. (May require a shunt). Never: placed across the device. Units: Amps (A) and Milliamps (mA)

An ohmmeter is to measure resistance… Always: placed across the device. (May

require device to be disconnected to isolate from parallel loads.)

Never: test while power is applied to circuit. (The ohmmeter supplies its own power.)

Units: Ohms ( ) 1.0 Kil-ohms (K ) 1,000 Megohms (M ) 1,000,000

Multimeter

Sometimes called a V.O.M.Measures voltage, current, and resistance.Uses a range select-switch to choose function and sensitivity.Read out is an analog-type swing needle.

Digital Multimeter

Sometimes called a D.M.M., D.V.O.M.Measures voltage, current, and resistance with a high degree of accuracy.Uses a range select-switch to choose function and sensitivity.Read out is digital-type liquid crystal display (LED) or light-emitting-diode (LED) display.

What’s the Big Deal About Digital Multimeters

Meters are meters… or are they? When working with circuits which

have a very small flow the meter used can have a significant effect on the current in the circuit.

If delicate components are involved, circuits can be damaged and measurement effectiveness can be questionable.

The internal resistance of the meter should be considered as a parallel resistor added to the circuit.

The higher the internal resistance of the meter, the less current it will drew.

If the meter is an inexpensive analog meter with a low internal resistance, it can actually draw more current than the circuit being measured can handle.

Old analog meter (1950-1960)

Typical internal resistance 1000 per volt. In a 9-volt circuit the meter would

present a 9000 parallel resistance to the circuit.

9 volt x 1000 = 9000

Good Quality Analog Meter

Typical internal resistance 20,000 per volt In a 9-volt circuit the meter would

present a 180,000 parallel resistance to the circuit.

9 volts x 20,000 = 180,000 If the circuits being measured have

larger branch resistance, accurate measurements are not impossible.

High Quality Digital Multimeter

Typical internal resistance 10,000,000 per volt. In a 9-volt circuit the meter would

present a 90,000,000 parallel resistance to the circuit.

9 volts x 10,000,000 = 90,000,000 This resistance is so high, its like not

even having the meter there. Sampling current dose not affect the

circuit.