Using a Multimeter, Voltmeter, Ammeter, and an Ohmmeter
What is a multimeter? A multi-meter is an tool that is capable of measuring two or more electrical values. Most all will be capable of measuring voltage, amperage, and ohms. Earlier meters were only capable of measuring one type of electrical value, so a person would have to carry a meter for each unit they needed to measure. Later meters were designed with a rotary switch so they could measure multiple units of value, hence the term multimeter.
What is the difference between a multmeter, a voltmeter, an ammeter, and an ohmmeter? In modern times, these terms are used interchangeably and all refer to a multimeter. Volt meters measure voltage, amp meters measure amps, ohm meters measure ohms, and multi meters measure a combination of two or more of these.
Analog meters, are the obsolete? An analog meter moves a needle along a scale. Switched range analogue multimeters are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it!
Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better because its resistance is much higher, 1 MΩ or 10 MΩ, compared to 200 Ω for a analogue multimeter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an anlaogue display.
Used as an ammeter, an analog multimeter has a very low resistance and is very sensitive, with scales down to 50 µA. More expensive digital multimeters can equal or better this performance.
Analog meters still have some applications, however for the beginner or even the experienced electrician, I would recommend a digital meter and that is why you will not find any analog meters for sale here.
Using a multimeter to measure amps, voltage, and ohms
Before going in to detail about multimeters, it is important for you to have a clear idea of how meters are connected into circuits.
Measuring Amps. Diagrams A and B below show a circuit before and after connecting an ammeter:
|To measure current, the circuit must be broken to allow the
ammeter to be connected in series
|Ammeters must have a LOW resistance|
Think about the changes you would have to make to a practical circuit in order to include the ammeter. To start with, you need to break the circuit so that the ammeter can be connected in series. All the current flowing in the circuit must pass through the ammeter. Meters are not supposed to alter the behaviour of the circuit, or at least not significantly, and it follows that an ammeter must have a very LOW resistance.
Measuring Voltage. Diagram C shows the same circuit after connecting a voltmeter:
|To measure potential difference (voltage), the circuit is not changed:
the voltmeter is connected in parallel
|Voltmeters must have a HIGH resistance|
This time, you do not need to break the circuit. The voltmeter is connected in parallel between the two points where the measurement is to be made. Since the voltmeter provides a parallel pathway, it should take as little current as possible. In other words, a voltmeter should have a very HIGH resistance.
Which measurement technique do you think will be the more useful? In fact, voltage measurements are used much more often than current measurements.
The processing of electronic signals is usually thought of in voltage terms. It is an added advantage that a voltage measurement is easier to make. The orginal circuit does not need to be changed. Often, the meter probes are connected simply by touching them to the points of interest.
Measuring Ohms. An ohmmeter does not function with a circuit connected to a power supply. If you want to measure the resistance of a particular component, you must take it out of the circuit altogether and test it separately, as shown in diagram D:
|To measure resistance, the component must be removed from the circuit altogether|
|Ohmmeters work by passing a current through the component being tested|
Ohmmeters work by passing a small current through the component and measuring the voltage produced. If you try this with the component connected into a circuit with a power supply, the most likely result is that the meter will be damaged. Most multimeters have a fuse to help protect against misuse.
Operating a multimeter.
The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20 V DC, for example, then 20 V is the maximum voltage which can be measured, This is sometimes called 20 V fsd, where fsd is short for full scale deflection.
For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build, the 20 V DC voltage range is the most useful. DC ranges are indicated by on the meter. Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used.
What does DC mean? DC means direct current. In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction. Every constructional project descirbed in Design Electronics works in this way.
AC means alternating current. In an electric lamp connected to the domestic mains electricity, current flows first one way, then the other. That is, the current reverses, or alternates, in direction.
An alternative style of multimeter is the autoranging multimeter:
The central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure. Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV. This type of meter is more expensive, but obviously much easier to use.
Where are the two meter probes connected? The black lead is always connected into the socket marked COM, short for COMMON. The red lead is connected into the socket labelled VΩ mA. The 10A socket is very rarely used.