The Digital Multimeter





Overview


A meter is an instrument for measuring electrical quantities, such as voltage, current and continuity. A digital multimeter (DMM) can measure any of these quantities, depending on how it is set up, and display the result on a digital readout, like a pocket calculator. Using a DMM, you can troubleshoot a circuit quickly to find a bad connection or component. A meter also helps to answer questions about circuits, such as:
  • Why does a red LED make a blue LED turn off, when both are connected in parallel with a coin battery?
  • Why does it matter how two batteries are connected to each other, in order to double the voltage?
  • Why do batteries come in different sizes?
  • Why won't a coin battery run a motor?
  • What happens to a motor when it is working harder?
This Appendix develops troubleshooting techniques, and explorations of circuit questions, using a digital multimeter. The activities can be done as a complete sequence, or performed separately whenever they are needed.

Procedure


1. Setting up the Digital Multimeter. The first step in using the DMM is to unpack the box and plug in the probes. Click here to see a video and diagram showing how to set up the meter.

2. Learning about LEDs. In Lesson 1, students learned that some LEDs of different colors will both come on in parallel, but others won't. For example, the same battery will turn on a red and yellow LED, or a blue one and a green one, but either red or yellow will turn a blue or green one off. Click here for an experiment to find out why this happens.

3. Testing a battery. If you suspect that a battery is dead, you can test it using a DMM. Click here to find out how.

4. Testing a switch. A switch should close the circuit in one position, and open it in the other. Click here to see how to use a DMM to check out a switch.

5. Testing a battery connection. A battery might be OK, but the contacts to it might not be tight enough. Click here to see how to use a DMM to check the connections to a battery.

6. Why will a LED or buzzer run from a coin battery, but a motor won't? Click here for an experiment to find out the answer.

7. Putting two batteries in series. Two batteries will work in series only if they are connected properly. Click here for an experiment showing what happens when batteries are connected in various ways.

8. More experiments with batteries and motors. You can use a DMM to find out how a D cell is different from an AA battery, or what happens to motor when it is doing more work. Click here for these experiments.