Your multimeter

Four positions

Every multimeter is a little different, and the manual that came with yours is the authority on it. What follows is true of nearly all of them. Plug the black lead into COM, always. Plug the red lead into the jack marked with a V and the omega symbol for ohms. Leave the jack marked 10 A alone: it is for measuring large currents, and a lead in it while measuring volts is how meters get damaged.

On the dial these four positions are:

• DC volts: a V with a straight line over dashes beside it. The power module, an LED's drop and a divider's output are all DC.
• Resistance: the Greek letter omega. It makes its own small current and measures what comes back.
• Diode: the diode symbol. It pushes a small current through a part one way and shows the voltage it takes.
• Continuity: the sound-wave symbol, often sharing a position with diode. Press a button, if there is one, to cycle between them.

The reading in the figure is a sample. Your meter, your part and your supply will each differ by a few tenths.

Across, never through

A voltage is a difference between two points, so the meter's two probes go on two points, and the circuit stays as it was. Put the probes on the two ends of a part and the meter reads the voltage that part takes. Do that for each part around a loop and the voltages add up to the supply.

The last choice is the mistake worth seeing once. A meter on a voltage range has a very high resistance, typically about ten megohms. Put it in the loop in place of a wire and it stops the current: the LED goes out, and the number it shows means nothing. Measuring a current is the opposite job, putting the meter in the loop on purpose, and it comes later in this book. Voltage is the only thing the first circuits ask you to measure.

Resistance wants the part alone

The resistance range sends its own small current through the part and measures what comes back. That is why it works on a part that has no power: the meter supplies the power. It is also why it fails on a part that is wired into a circuit. Everything else joined across the part's two legs is a second path for the current, and the meter reads the two paths together.

The rule that follows is simple. To read a part's own value, lift one leg out of the breadboard so the part is on its own, or take it out entirely. And switch the circuit off first. A circuit that is on will push current into the meter, and the number is nonsense.

What an odd reading means

Most of the time the display is a number and you read it. Sometimes it is not.

OL means over limit, or open: more than the meter can read in that range, or no path at all. On resistance and diode it is often the right answer, since an open circuit really is more resistance than any meter can read. A minus sign on volts only says the red probe is on the lower of the two points. A minus sign on a resistance or diode range means the circuit has voltage on it and should be switched off.

A habit worth having

Before measuring anything in a new circuit, prove the meter on something you know: the module's rails on volts, the two probes touched together on continuity. A meter that is on the wrong range or has a loose lead gives a plausible zero, and a plausible zero is how you spend half an hour looking in the wrong place.