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Chapter 7 BONUS! For most measurements, the black probe plugs into the meter’s “COM” (common) socket and the red lead into the “V-Ω-mA” socket. Push them as far into the sockets as they’ll go to make sure there’s a good connection (and to avoid accidental shocks if you’re measuring high voltage). Before measuring a DC voltage, set the meter to its highest DC voltage range (here 600V). That way, there shouldn’t be any damage done if the voltage is higher than you expect. You can always click down a range or two to make the measurement more accurately, if you need to. The same applies when you’re about to measure an AC voltage – set the meter to its highest AC voltage range first, to avoid mishaps. Here the switch is set to 600V again but this time on the AC voltage scale (ie, V~). Exercise extreme caution if making high voltage measurements. Using A Multimeter You can’t make do without a multimeter! Here’s how it’s used to measure voltage, current and resistance. M ULTIMETERS ARE great tools for tracing your car’s wiring. They are also invaluable when it comes to checking circuits and fault-finding. However, there are some basic “DOs” and “DON’Ts” to remember, to ensure you take accurate measurements and don’t damage either your meter or the circuit you’re testing. The main thing to remember when using a multimeter is that before connecting the probes to the circuit or component to be tested, make sure you have it set for: (1.) the correct kind of measurement – ie, VOLTS DC when measuring DC voltages; VOLTS AC for measuring AC voltages or you are not sure if the voltage is AC or DC; AMPS (or more likely MILLIAMPS) for measuring current; or OHMS for measuring resistance. (2.) the correct range – that is, a range higher than the highest voltage, current or resistance you’re likely to measure (if you don’t know, select the highest range). If you don’t do this, there could be an expensive BANG when the probes contact the circuit! Note that some meters are auto-ranging, so you don’t have to worry about range selection. 40 PERFORMANCE ELECTRONICS FOR CARS A digital multimeter (or DMM) is much preferred for probing your car’s electrics because it will invariably have a high input impedance (10MΩ). This means that it won’t load down the circuit it’s measuring. If you choose to use an analog meter (right), make sure it is a high-impedance type. Most analog meters are only low-impedance types (typically 20kΩ/V) and so they will load down the circuit you are testing and give false readings. siliconchip.com.au HOW TO MEASURE VOLTAGE Voltage is measured by connecting the meter across the component or circuit under test while power is applied. In other words, the meter is in PARALLEL with the circuit or part of the circuit under test. In practice, the two meter probes are simply connected between the two points concerned – such as the terminals of a battery or the terminals of a lamp. 6.00 DC VOLTS 6V BATTERY VOLTAGE IN PARALLEL WITH COMPONENT UNDER TEST With many multimeters, the red (positive) probe lead needs to be changed over to a special “high current” socket before you can measure currents of more than a few hundred milliamps – as well as switching to the appropriate range. Here the red lead has been plugged into the “10A DC” socket on the left, to measure currents up to 10A. HOW TO MEASURE CURRENT BREAK CIRCUIT 100.0 DC mA CURRENT IN SERIES WITH COMPONENT UNDER TEST When you want to measure the current in a circuit, you need to break the circuit at that point and connect the probes so that the current to be measured flows THROUGH the meter – switched to the correct current range, of course. In other words, the meter is in SERIES with the circuit or part of the circuit under test. Here the small lamp is drawing 100mA. When you switch to any of the resistance ranges on a DMM, it generally gives this kind of “over range” indication when the probes aren’t connected to anything. If it doesn’t, the battery inside the meter may need replacing. HOW TO MEASURE RESISTANCE Resistance is measured by passing a tiny current (provided by a battery inside the meter) through the com­ponent under test. In this case, to obtain an accurate measurement, the component being measured must be isolated from other components and any other source of current. For example, to make an accurate measurement of a resistor on a PC board, one end must be disconnected from the circuit so that the meter can measure the component by itself. Note also that many meters have a “continuity” range. This is a low resistance setting which is used to check for breaks in cables and PC board tracks. When the two probes are touched together (or connected via a low-resistance circuit), a buzzer or beeper sounds, so that you don’t have to look at the meter to know that the circuit is OK siliconchip.com.au PERFORMANCE ELECTRONICS FOR CARS 41