APPENDIX 6 FILE METERSDOC MULTIMETERS A MULTIMETERS IS THE

A multimeters is the most useful electrical measuring instrument. One instrument combines the capabilities to measure voltage, current, and resistance, often in both AC and DC circuits.

There are many different kinds used in this laboratory. The switches used to change functions come in a bewildering variety, but they all work on the same general principle. Some use push button switches, some large rotary switches, and some use different jacks to change the function. However, all meters have two leads which are connected to the circuit to make the measurement. In general, the switches select the type of measurement (AC or DC), the function (voltage, current, or resistance), and the range (maximum value which can be measured). All three selections must be made correctly for the measurement to be useful.

The most important thing to do is to make sure that the switches are set correctly before you connect the meter. To do so is important to prevent damage to the circuit, the meter, or you. Always double check the switch settings before connecting the meter.

The range switches set the maximum value of the quantity to be measured. If you do not know the approximate value you are measuring, start at a high value and work down. Some meters do not have range switches but select the correct range automatically.

The voltage measurement function is the most useful and easiest to use. To measure the potential (volts) between two points in the circuit you connect the + (red) meter leads to the highest potential point and the -(black) lead to the lower point. (If you guess wrong, the analog meters try to read backwards.) The ideal voltmeter has a very high resistance so it will usually not disturb the circuit you are measuring. Handle the leads carefully (do not touch any metal) if high voltages are involved. (These labs rarely use high voltages). Handle the leads carefully in any case so that you do not accidentally short (connect together) some parts of the circuit.

The current measurement often causes trouble. The ideal current meter has zero resistance. That means if you put it directly across a power supply or battery you will burn something up. Always put the current meter in series with the element through which the current you wish to measure is going. This means you will have to disconnect that element from the circuit and insert the meter in the circuit at the point of disconnection so that the current can go through the meter as well as through the element. The current should enter the meter through the +(red) lead and leave through the - (black) lead.

The most important thing to remember when making resistance measurements is that you must not make this type of measurement on an active circuit. (An active circuit is one which has any voltage connected to it anywhere.) Because a resistance measurement requires that the meter itself supply the needed voltage, any other voltage will just invalidate the measurement or damage the meter. The meter will measure the resistance between the points in the circuit to which the meter is attached. Remember that, if any part of the circuit is in parallel with the element whose resistance you are measuring, you will measure the equivalent parallel resistance, not the resistance of the element itself.

There are two types of meters, digital and analog, but both measure the same quantity. The digital type presents the results as a number on a screen. The analog type uses a meter pointer to indicate the reading on a scale. The digital type is often easier to read and is more accurate. The analog is less expensive and easier to use on varying signals.

The meter most of you will use is a small analog meter. The small end of the test leads go into the two jacks on the front panel. The switch selects the type of measurement and the scale to be used. There are

four voltage positions, three current positions, and one resistance position, and four AC voltage positions (printed in red). Do not turn the switch when the meter is connected to the circuit.

You have been supplied with two adapters which are put on the ends of the test leads so that it is easier to connect the test leads to wires on the breadboard.

The scale to be used depends on the setting of the range switch. The AC scale is in red at the bottom. The resistance scale is the non-linear scale at the top. All the other positions of the range switch use one of the scales in the middle. Use the one which has the same number on the left hand side number to which the range switch points. (You may have to mentally supply a decimal point.)