Multimeters and all that magic stuff

Feb. 1, 1998

Multimeters and All That Magic Stuff

By Michael D. Faircloth

February 1998

Working with electrical circuits on aircraft can be unbelievably simple or (hair pulling) complex. How many times have we become frustrated tracing out lines on a wiring schematic only to find that for some reason the aircraft doesn't seem to reflect the print?

To compound the stress we are warned, and rightfully so, that electricity is lethal if handled incorrectly. Then there is the multimeter. The biggest hang-up for many aircraft technicians is not so much using a meter but understanding what the meter is telling them.

The problem goes back to our initial training in A&P school. We spent so much time learning the laws and getting the math down pat that we didn't make the connection when it came time to learn how to use the multimeter. Whether we are working on simple or difficult circuits, a multimeter is an invaluable tool.

Although meters can be purchased that solely read voltage, resistance, watts, or amperes, most A&Ps cannot afford to own so many different types of meters. Because the multimeter is so versatile, portable, and relatively inexpensive, it becomes the tool of choice by most technicians.

There are two types of meters in wide use today. They are the D'Arsonval pointer type (swing meter) and the digital multimeter. Although the pointer type is still more prevalent, the growing popularity of the digital multimeter is not far behind. It is more compact, more accurate, comes with more features, and oftentimes challenges the price of the traditional swing meter.

Technically speaking
The D'Arsonval-type meter movement has a pointer which deflects an amount proportional to the current flowing through its moving coil. A reference magnetic field is created by a horseshoe-shaped magnet and its field is concentrated by a cylinder-shaped keeper in the center of the open end of the magnet. The current that is being measured flows through a coil which is surrounding the cylindrical keeper. It is supported by hardened steel pivots riding in smooth jewel bearings. Current enters and leaves the coil through calibrated hairsprings, one surrounding each of the pivots. Current flowing through the coil creates a field which opposes the magnet field. This causes the coil to rotate on its pivots. The hairsprings will exactly balance the magnetic forces, and the pointer (which is attached to the pivot) will indicate the current being measured. Did you catch that? Who cares about how it works, right? As long as we know what to do with it.

An ammeter is used to measure current flow in units called amperes. Most multimeters have ammeters incorporated, but they rarely have much use and the values they measure are quite small. Many aircraft have ammeters installed to show current demands on a system. They are usually located in the cockpit.

Whether installed on the aircraft or used with a handheld meter, they will always be hooked up in series. An ammeter must use a shunt either internally or externally to provide a parallel path for a voltage drop. The meter is set up to read amperes.

OK, the cat is out of the bag. The voltmeter we use out on the line isn't really a voltmeter at all. It is actually a milliammeter in disguise. That is, it is a milliammeter that is set up to read voltage. A resistor is connected in series with a milliammeter. The amount of current flowing through the meter circuit will depend on the resistance of the resistor. The meter indicates the current flowing through the meter, but the scale is calibrated to read the voltage applied across the combination of the meter and the resistor. The meter is calibrated to read voltages of different values by changing the value of the resistor within the meter. This is accomplished by incorporating a rotary dial which selects the appropriate resistor in which to send the current through. A 1,000,000 ohm resistor in series with a milliammeter could be used to measure up to 100 volts.

The selection of the proper range with the rotary dial should be based on the voltage we expect to read. The most accurate reading will be on a range which deflects the needle as close to the middle of the scale as possible. AC voltages are read by sending the current through a bridge rectifier within the meter. Select the appropriate AC scale and read the scale in red.

Since current only flows when there is a difference of potential and a complete path, it stands to reason that a voltmeter will only measure a difference of potential. Of course, the resistance is so high within the meter that the voltmeter has very little effect on the circuit in question.

Since voltage is constant in parallel branches within a circuit, the meter should always be in parallel with the load in question. There are two main ways to take the measurement.

One way is directly across the load in order to see the voltage drop across only that particular load. In Figure 1, with two loads of equal resistance, the voltmeter reads 14 volts across the first load. We would have seen 14 volts across the second load as well.

Another way is from either side of a load to direct circuit ground in order to see the amount of voltage remaining (Figure 2).

Let's look at one of those laws that we learned back in A&P school and see how we can put it to practical use.

Kirchhoff's Law states: The algebraic sum of the applied emfs in a closed loop circuit is equal to zero.

In plain English, it means that all of the voltage will be used up in a given circuit. It also means that the voltage will be distributed throughout the circuit based on the loads.

Figure 2 shows a simple circuit with two loads of equal resistance. The voltmeter reads 28 volts before the first load. None of the source voltage has been dropped yet.

In Figure 3, the voltmeter reads only half of the source voltage with the positive lead now on the ground side of the first load. This is because the first and second load are of equal resistance value so the voltage drop is split equally between them.

In Figure 4, all of the voltage has been used up on the negative side of the second load. There is no difference of potential so the meter reads 0 volts. Even though the meter reads 0 volts, current is still flowing through the ground lead. Current will take the path of least resistance so no current will flow through the meter.

In Figure 5, since there is an open to ground, none of the voltage will be dropped. The meter will be reading 28 volts with the positive lead anywhere before they open. This can be a handy way to determine if a ground is good or bad, particularly if the circuit has a lot of relays.

In Figure 6, with a short to ground between the first and second loads, all the voltage is used up. In the example here the first lightbulb would be brighter than usual. Because the meter reads zero it tells us that the second bulb is not burned out. (Of course, had the load been something less obvious, this could also mean that voltage never got there to start with. Verify that there is not an open from the bus and through the first load). If the positive lead had been placed on the positive side of the first load, the meter would have read the full 28 volts.

In Figure 7, the meter shows a voltage drop lower than normal with corrosion in the circuit. The first load only used up 9 volts because the corroded ground used up 10 volts (9v + 9v + 10v = 28v). The total voltage is redistributed throughout the circuit based on total resistance and the current will go down (Ohm's Law). The lightbulbs in this case would be less brilliant.

Ohmmeters are exactly like voltmeters (which are secretly milliammeters, right?). The only difference is the ohmmeter provides its own current source so we don't need to help it out by hooking it to a powered up circuit. In fact, this would quite possibly send you off to the store to buy a new one. Perhaps if you're lucky, yours has a circuit breaker or a fuse that will protect it from boneheaded mistakes like these.

Ohmmeters measure the resistance of a circuit or load. Its units of measurement are in ohms. The leads are always hooked up across the load in question. Always isolate the component from the rest of the circuit because of possible sneak circuits.

When using a swing meter, short the leads together and adjust the zero adjustments to read full scale right and resting on zero. Once the test leads are separated, the pointer will be pulled to the left of the meter resting on infinite resistance. Digital ohmmeters are much easier because they are set up automatically. You should still, however, short the leads in order to check the meter's status.

Ohmmeters are great troubleshooting tools. They can be used to find open circuits or short circuits. They can be used to check diodes (remember the side on the diode with the mark is the cathode or negative side). They can be used to verify switches and relays. Ohmmeters can measure the resistance of resistors or any other component.

Remember to turn off the power before hooking an ohmmeter to a circuit. Isolate components to avoid erroneous readings. Be careful measuring circuits with capacitors. They store a charge and can still have voltage even after the power has been removed.

Also, remember that you should have some idea what the normal value should be before you measure it. Don't be surprised when you don't have a normal value. After all, we don't normally troubleshoot normal systems.