Analog Vs Digital Meters

Ultimately, your diagnosis of vehicle electrical system problems will come down to using a voltmeter, ammeter, or ohmmeter to pinpoint the exact location of the problem. There are two types of each meter—analog and digital.

Analog meters use a needle and calibrated scale to indicate values.

Digital meters display those values on a digital display.

This chapter will help you understand how these meters work as well as the advantages and disadvantages of each.

Before using a meter, read the manufacturer's operating instructions. Reading analog meters usually requires simple mental calculations. For example, a meter might have three voltage ranges: 4.0 V, 20 V and 40 V, but only two scales: 4.0 V and 20 V. In order to use the 40 V range, you need to multiply the needle reading on the 4.0 V scale by 10 (or for that matter, the 20 V scale by 2).

Digital meters are usually simpler to read and many will adjust to the proper range required for the circuit or device they are connected to. These meters are known as auto-ranging meters. Other digital meters require the operator to select the proper range. In any case it is important to learn the symbols used in a digital readout so you can interpret the reading. The electrical units of measure symbols are:

M for mega or million K for kilo or thousand m for milli or one-thousandth u for micro or one-millionth

The three types of meters—voltmeters, ammeters and ohmmeters—connect to the circuits or devices in different ways. This is necessary to get accurate measurements and to prevent damage to the meters.

Voltmeters measure voltage or voltage drop in a circuit. Voltage drop can be used to locate excessive resistance in the circuit which could cause poor performance or improper operation. Lack of voltage at a given point may indicate an open circuit or ground. On the other hand, low voltage or high voltage drop, may indicate a high resistance problem like a poor connection.

Voltmeters must be connected in parallel with the device or circuit so that the meter can tap off a small amount of current. That is, the positive or red lead is connected to the circuit closest to the positive side of the battery. The negative or black lead is connected to ground or the negative side of the circuit. If a voltmeter is connected in series, its high resistance would reduce circuit current and cause a false reading.

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