How to Measure the Amperage Drawn by a Speaker

To accurately measure the current, in amperes, drawn by a speaker, you can follow a series of detailed steps which include the setup of a simple circuit. This guide also covers important safety notes and considerations for dynamic measurements.

Tools Needed

Multimeter: Ensure it can measure current amperes. Resistor: A known resistor value, for example, 4 ohms or 8 ohms, to create a circuit with the speaker. Power Source: An appropriate amplifier or power source for the speaker.

Steps to Measure Current

Set Up the Circuit

Connect the known resistor in series with the speaker. This means you will connect one terminal of the power source to one terminal of the resistor, the other terminal of the resistor to one terminal of the speaker, and then the other terminal of the speaker back to the power source.

Measure Voltage Across the Resistor

Use the multimeter set to measure voltage to measure the voltage drop across the resistor while the speaker is producing sound.

Calculate Current

Use Ohm's law to calculate the current:

I (V / R)

Where V is the voltage measured across the resistor, and R is the resistance of the resistor in ohms.

Example Calculation

If you measure a voltage of 0.8 volts across a 4-ohm resistor, the calculation would be:

I (0.8V / 4Ω) 0.2A

This means the speaker is drawing 0.2 amperes.

Important Notes

Safety

Ensure that the power source and the speaker are compatible and that you do not exceed the speaker's power rating. Be cautious with the resistor's power rating; it should be rated to handle the power without overheating. The current draw can vary with the audio signal, so you may want to take measurements at different volume levels or types of signals.

Additional Considerations

I assume you mean amplifier speakers by "AMP" speakers. Most speakers can be tested with a normal multimeter on the Ohms range. Choose a range between 1 and 100 ohms or greater, as my meter has a 200 Ohm setting, some go to 1k or higher. Generally, this will be the lowest resistance setting on your multimeter unless it is a very unusual meter.

Short the two probe leads and you should read zero or near zero resistance. Always check this when measuring resistance before the actual measurement.

Simply place the multimeter leads across the two terminals of the speaker. You might hear a click or pop. You should read a value perhaps somewhere near 8 ohms, although 4 ohms is not uncommon. I've seen 3 Ohm speakers and 16 and 32 Ohm speakers but the majority of speakers are nominally either 4 or 8 Ohms.

Avoid leaving the meter across the terminals for a prolonged time. The actual value you read for a nominally 8 Ohm speaker can vary, 7 Ohms or 9 Ohms might not be uncommon for an 8 Ohm speaker. 99% of speakers are made from a coil of wire around a permanent magnet, you will be measuring the DC resistance of the coil. As you may be aware, the audio signals from an amplifier are AC, the exact impedance function of any speaker cannot be measured by a simple multimeter but a DC measurement will give you an idea if the speaker is in a suitable condition and working to a first degree.