Multimeters are very useful test instruments. They are needed in every kind of robotic activity. Don’t ever forget to carry this whenever you are going for a competition.
- Analogue meters take a little power from the circuit under test to operate their pointer (a hand like in a clock to indicate the reading).
- They must have a high sensitivity of at least 20k /V or they may upset the circuit under test and give an incorrect reading.
- All digital meters contain a battery to power the display so they use virtually no power from the circuit under test.
- They have a digital display as shown.
Measuring resistance with a multimeter
Measuring resistance with a DIGITAL multimeter
- Set the meter to a resistance range greater than you expect the resistance to be.
Notice that the meter display shows "off the scale" (usually blank except for a 1 on the left). Don't worry, this is not a fault, it is correct - the resistance of air is very high!
- Touch the meter probes together and check that the meter reads zero.
If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again.
- Put the probes across the component.
Avoid touching more than one contact at a time or your resistance will upset the reading!
- Select a voltage range with a maximum greater than you expect the reading to be. If the reading goes off the scale immediately disconnect and select a higher range.
- Connect the red (positive +) lead to the point you where you need to measure the voltage
- The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points. (The black lead can be fitted by using a crocodile clip.)
- Digital multimeters have a special setting for testing a diode, usually labeled with the diode symbol.
- Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).
- Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).
Testing a transistor with a multimeter
- The base-emitter (BE) junction should behave like a diode and conduct one way only.
- The base-collector (BC) junction should behave like a diode and conduct one way only.
- The collector-emitter (CE) should not conduct either way.