Multimeters and All That Magic Stuff
By Michael D. Faircloth
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.
ELECTRICAL MEASURING INSTRUMENTS
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.
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.
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