A multimeter is one of the most useful tools for troubleshooting or installing a car audio system. Most meters have a rotary selector switch but a few have pushbuttons to select the desired mode (resistance, AC voltage, DC voltage...). If there are buttons and a rotary selector switch, the buttons are likely to select options for the various modes. The meter below has a rotary selector switch. You should also notice that it has four terminals (banana type jacks below the selector switch) in which you place the leads. The jacks are marked to tell you when they will be used.
Many meters have an auto-off function but you should switch the meter off manually if you won't need it for a while. When it's switched off via the auto-off feature, the meter will still draw a bit of current. If you don't manually switch it off and don't use it for weeks or longer, the battery is likely to be dead when you need it again.
Most hand held amp meters have 2 ranges. The high range is generally limited to 10 amps. The low range is usually limited to less than one amp. Both ranges usually require moving at least one of the meter's leads. When measuring current, the meter is inserted in the circuit path. When the meter leads are in place to measure current, you must be careful NOT to connect the meter leads ACROSS any source of power. Most meters have internal fuses in the current measuring circuits for protection but you should still be careful.
The demo below explains how you measure the impedance/resistance of a speaker with a DMM. Actually, you're measuring the DC resistance of the voice coil. Since the DC resistance is very close to the speaker's rated impedance, you can determine the speaker's rated impedance.
The meter used as an example in this section is more than most people will ever need. Many of the features are not needed for most work related to car audio installations or even amplifier repair. This is a Fluke model 87V. The V indicates that it's the 5th in the series of model 87 meters. This meter is expensive but for someone who needs a meter that they can really rely on in dangerous/critical situations, this is a popular meter. Below, you'll find a few of the features for this meter. You can click on the image to open a larger version.
This is obviously for measuring AC voltage like you'll find in your home or workplace. This meter can measure AC voltage for AC voltage sources operating at high frequencies. Many meters are optimized for measuring common AC mains frequencies (nothing greater than 500Hz). This one is accurate to well over 100KHz. Since this meter has such a great frequency response, sometimes high frequency noise can interfere with it. To allow it to display a stable, reliable reading, it has a feature that allows it to filter the high frequencies. The 'LO' symbol essentially inserts a low-pass filter (like on a crossover) in series with the input (although it's likely done digitally, not with an analog filter).
This is straightforward. In this mode, the meter measure DC voltage.
For added accuracy, this meter has a millivolt range. It can read millivolts in the standard DC mode but it can provide higher resolution for very low voltages in this mode. You can also see that it has a thermometer for this position on the dial. This is the range you set it to when you are using the meter to measure temperature. It requires a special probe to measure temperature.
In this mode, the meter applies a low voltage to the meter leads. When touched across the resistor (or whatever is being tested), the voltage is pulled down. The meter determines the resistance of the device by the amount that the voltage is pulled down. Most meters apply about 3.5v. Other meters, like the 87v, apply more voltage (I think it's about 7v for this meter). The voltage is applied in resistance and diode-check modes.
In diode-check mode, the meter again applies a voltage to the device being tested but instead of converting the reading to resistance, it displays the voltage across the device. For diodes, the voltage is pulled down to about 0.6v. If you read the diode in resistance mode, the meter will give a resistance reading but it will largely be meaningless unless the diode is leaking or shorted.
In this mode, the meter is inserted into the circuit and all of the current that flows through the circuit flows through the meter. This allows the meter to measure the current flowing in the circuit. In this mode, you have to be careful not to exceed the current capacity of the meter. There are two different sockets for the high and low amperage ranges. You must use the one that's rated higher than the current flowing in the circuit. The circuits are fused but the fuses aren't cheap. The fuse for the 10 amp range is generally about $11 and you must use the correct fuse to ensure that the meter is properly protected. This isn't a significant issue for car audio work but it's extremely important for those who use their meters with high voltage. On this meter, you use the yellow button to select AC or DC amps.
The Min/Max function allows the meter to record the highest and lowest values so that you don't have to constantly watch the display.
This is an auto-ranging meter but sometimes, it's better to keep the meter on one range. This button allows you to switch from auto to manual range selection. The °C/°F means that you can change between Celcius and Fahrenheit temperature readings when using the temperature probe.
This causes the meter to lock onto the last stable reading. A lot of meters have a 'hold' function but they require that you press a button to lock the display.
This button allows you to see the difference between two readings. If you have a point that generally has a constant voltage and you want to see how much it fluctuates from that voltage, you can connect the meter to the circuit and press the rel button. The meter's display will go to all zeros. If the voltage goes to 1v above the original voltage, the meter will display 1v. If it drops 3v from the original voltage, it will display -3v . This can be used in other modes also. It's useful to zero out the meter leads. When measuring low value resistors, the resistance of the leads will skew the results. If you touch the probes together and the meter reads 0.1 ohms and you push the rel button, the meter will go to 0.0 ohms. Then, when you measure the value of a low value resistor (common when measuring the value of emitter resistors in amplifiers), you will get a more accurate reading.
Other Multimeter Information
There are some things that you should be aware of when using a multimeter.
Reading DC Biased AC Voltage:
Many times, you'll need to read AC voltage that's biased (riding on top of) DC voltage. Many meters don't actually read AC voltage. They simply use filters to get an approximate reading. This will cause huge errors when reading DC biased AC voltages. Even though most meters (even relatively good ones) aren't 'true RMS' meters, they use a more sophisticated circuit (one that ignores the DC bias) to measure the AC voltage.
True RMS Meters:
As was stated above, many meters don't actually read AC voltage. A true RMS meter samples the instantaneous voltage (many times for each cycle of the AC waveform) and then calculate the true RMS value of the AC waveform. Normal meters can only read accurately for sinusoidal waveforms (like those used for household mains). For any other type of waveform, you need a true RMS meter. This is covered in more detail on the 'Quantifying AC Voltage' page of the site.
Voltage Applied by Meter:
Previously, it was stated that multimeters applied voltage across the meter leads in resistance and diode check modes. The 3.5v applied by most meters is enough to check most semiconductors. Standard diodes and most LEDs can be forward biased by the 3.5v. Some of the newer, higher power LEDs require more voltage to forward bias them. This is one instance where the extra voltage is required.
Above, it was stated that you must use the correct socket/range when measuring current with a multimeter to prevent blowing fuses (or the meter, for meters that are not internally fused). A better way to measure current is to use an external current shunt. When measuring current directly with a multimeter, the meter passes the current through a precision resistor. It measures the voltage across that shunt and calculates the current flowing through the shunt. Using an external shunt, you can do the same thing. The great benefit is that you cannot damage the meter or blow the fuse. The shunt must be selected to easily withstand the greatest current that's expected to flow through the circuit. If there is a miscalculation and significantly more current than expected flows through the circuit, only the shunt will be damaged. Generally, the shunt can handle short term overloads and will survive if you quickly disconnect power from the circuit when you see too much voltage across the shunt. To measure the current draw by amplifiers at idle, I use twenty 3 watt. 0.2 ohm, 1% tolerance resistors connected in parallel. This is a 0.1 ohm shunt and I can read the current draw by reading the voltage across the shunt. 0.05v across the shunt is 5 amps.
4-Wire Ohm Meters:
It was previously stated that the meter leads can induce errors into resistance readings. Some meters can cancel out the error by 'zeroing' the meter (like the Fluke 87 with the rel button). The error is caused by the current flowing through the leads that are measuring the voltage drop across the device being tested. To eliminate this problem, some meters use 4 wires to measure resistance. The meter sends a very precise current through two of the leads. This is generally 1mA. They then use the other two leads to measure the voltage drop across the device being tested. Since there is essentially no current flowing through the leads that are used to read the voltage across the device, there is no induced error.
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