So far we have used a voltmeter to measure voltage and an ammeter to measure current. On a real bench, most people use one tool that measures both, and resistance too. It is called a multimeter, and here is a typical one.
A handheld digital multimeter. Digital means it shows the reading in digits instead of with a needle.
Parts
- leads and probes
- The two wires are called leads, one red and one black. Each ends in a probe. Hold it by its plastic handle, and touch its metal tip to the point you want to measure.
- display
- Shows the reading and its unit. OL, short for overload, means the reading is too big to show, for example the resistance between two points that are not joined.
- dial
- Turn it to what you want to measure: V for voltage, mA for current and Ω for resistance. The sound symbol is for a beep test. Turn it to OFF when you are done, to save the meter’s battery.
- COM socket
- The black lead always goes here. COM is short for common, because every measurement shares it.
- VΩ socket
- The red lead goes here to measure voltage or resistance.
- small-current socket
- Marked mA. The red lead moves here to measure currents up to 400 mA. Inside, a fuse protects the meter. A fuse is a thin strip of metal that melts if too much current flows through it.
- 10A socket
- For currents bigger than 400 mA, up to 10 A, the red lead goes here and the dial to A. We do not need it in this lesson.
We measure voltage with a multimeter the same way we did with the voltmeter. The black lead goes in COM and the red lead in VΩ. We turn the dial to V and touch the probes to the two sides of a part while the circuit is running. On the bench below there is an LED circuit with a 9 V battery and a 470 Ω resistor. What do you think the meter will read across the LED?
The battery gives 8.97 V rather than 9 V, because a little voltage is lost inside it. A megohm, written MΩ with a capital M, is a million ohms. On V this meter is like a 10 MΩ resistor between its probes. With a tip touching nothing, this display shows two dashes, while a real one shows a small number that keeps changing.
The LED and the resistor together had all of the battery’s voltage across them, as Kirchhoff’s voltage law says. On V the meter lets almost no current through itself, so measuring a voltage hardly changes the circuit.
To measure a current, the multimeter has to work like the ammeter. All of the current must go through it, so we open the circuit and put the meter in the gap. On the next bench, the dial is on mA and the red lead is in the mA socket. The resistor and the LED are joined by a jumper wire. It is a short wire with bare ends, easy to pull out. How much current do you expect the meter to show?
If the meter is put across the LED without opening the circuit, the current goes through the meter instead of the LED. Across the resistor, about 2.7 A flows through the meter and the LED. That melts the meter’s fuse, and would burn out a real LED too.
When the tips touched the two ends of the gap, the LED lit again, because the meter was now part of the circuit. The current is the same all the way round a circuit like this, so the gap could have been anywhere in it. On mA the meter is almost a plain wire. So its probes go only in a gap, and never across a part as they do for voltage.
A multimeter can also measure resistance, which neither the voltmeter nor the ammeter could do. On Ω it sends a small current of its own through the part and measures the voltage across it. Then it works out the resistance with Ohm’s law. If the battery is still connected, its voltage gets mixed into that measurement, so we always switch the power off first. The circuit below has a knife switch, whose metal blade swings open on a hinge. Will the meter show a resistance for the LED, as it does for the resistor?
With the switch closed, this meter shows Err, short for error. A real meter would show a wrong number instead, or could even be damaged. A real 470 Ω resistor would read a little more or less, within its tolerance.
Across the LED the display showed OL, as if the LED were not joined at all. An LED lets current through only when the voltage across it reaches about 1.9 V. The meter tests with less than that, so no current got through, and to the meter that looks like a huge resistance.
Very often all we need to know is whether two points are joined at all, for example whether a wire is broken. For that, the multimeter has a setting called continuity, the one with the sound symbol on the dial. It beeps when the resistance between its probes is very small, as it is along an unbroken wire. It tests with the same small current as on Ω, so the power must be off here too. The plastic coating on a wire is called insulation. One of these three jumper wires has its copper broken inside the insulation. Can you find which one?
One wire’s copper is broken. An unbroken wire reads a fraction of an ohm. If your sound is on, you will hear a soft beep too.
A broken wire looks just like a good one. So when a circuit that should work stays dark, a quick beep along each wire is often the first thing to try.
Say you have just measured a current, so the red lead is still in the mA socket and the dial is on mA. Next you want to check a battery. You put the probes on its terminals, its + and − ends, as you would for a voltage. Many people melt a meter’s fuse this way. On mA this meter is like a 0.6 Ω resistor. A battery also has a little resistance inside it, about 2 Ω for this new 9 V one. So the whole circuit is about 2.6 Ω. Can you work out how much current will flow through the meter?
The fuse is made for 400 mA. The melting is slowed down so you can watch it, but a real fuse melts in a fraction of a second.
Ohm’s law gives about 3.5 A, nearly nine times what the fuse is made for, so the fuse melted. Do not try this with a real meter, because its fuse melts just the same. Until the fuse is replaced, the mA setting reads 0. Voltage and resistance still work, because their socket, VΩ, does not use that fuse.
Four questions, if you want to check yourself.
You want to measure the current through an LED in a working circuit. Where do the probes go?
Since yesterday, your meter reads 0 on mA every time, even in circuits where the LED is lit. The red lead is in the mA socket, and voltages still read fine. What happened?
A jumper wire in your circuit might be broken inside its insulation. What is the surest way to check it?
You want to measure a resistor that is part of a circuit. What must you do first?
In most circuits so far, we joined the parts by twisting their wires together, with a jumper wire here and there. That is slow when we want to try a circuit and then change it. In The Breadboard we will learn a much quicker way.
Lesson 6 done.
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