Test Equipment
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For explanation of many of the quantities described here, see Basic Electrical Theory.
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Voltmeter
Measures voltage.
The probes of a voltmeter should be put in parallel with whatever voltage is to be measured.
In order to minimise the amount of current drawn from the test circuit (which can cause errors, as well as load the test circuit), voltmeters have a high, but finite, input impedance. For a cheaper voltmeter, this is typically 1MΩ while more expensive voltmeters, using JFET inputs, have an input impedance of about 10MΩ. This should be kept in mind if measuring voltages across resistances anything comparable to, or greater then, the input impedance.
Ammeter
Measures current.
An ammeter should be put in series with whatever current it being measured. If put in parallel with a voltage source it is likely to be damaged (or at least suffer a blown fuse).
Ohmeter
Measures resistance. It is used in a similar way to a voltmeter - ie. with two wires in parallel.
Kelvin Four-wire Milliohmeter
This is a very accurate device for measuring small resistances. As the name suggests it uses four wires. It is simple enough for the amateur to make.
Small resistances are hard to measure because the contact with the probes has its own resistance. This meter puts a known current through the test item with two of the wires and then measures the voltage with the other two. Ohms law can then calculate the resistance. See Basic Electrical Theory. The contact resistance will develop a voltage at the contact points of the current carrying wires but this won't be measured by the voltmeter wires.
Continuity Tester
A simple piece of test equipment similar to an ohmeter. A continuity tester gives an indication (usually an audible beep) if there is a conducting path put between its probes, often defined in equipment specs as being any resistance below a given level (typically around 50Ω).
Although very simple and perhaps gimmicky-seeming, a continuity tester is invaluable in checking new PCBs.
Multimeter
A multimeter combines the functions of several pieces of test equipment in one unit. A basic multimeter should have a voltmeter, ammeter and ohmeter.
Multimeters come in two kinds: digital and analogue. This refers to the kind of display. Digital multimeters (DMMs), with numerical LCDs, are now very cheap and popular. They give precise and definite readouts for slow-changing inputs. For inputs that change more quickly, however, digital multimeters are useless (car indicator lights, for example, are hard to test with a DMM), which is why analogue meters are still available.
Wattmeter
A wattmeter measures current and voltage simultaneously to calculate power, where P = VIcosΘ
Note that while for DC power is simply voltage times current, for AC the two might not be in phase. That is the cosΘ in the above.
Oscilloscope
Displays an eletrical signal over time. The oscilloscope helps in finding frequency.
They are the best for figuring out signals that change very quickly.
Frequency Counter
Measures frequency.
Semiconductor Testers
Semiconductor testers can be a very handy tool when trying to determine if a component has died. This tester allows the user to clip onto the leads of a component rather then trying to push the component's already short leads into a socket on a Multimeter.
Custom-made test equipment
People that manufacture thousands of pieces of test equipment can (a) spread out development costs over thousands of units, and (b) take advantage of bulk quantity discounts on parts. Commercial off-the-shelf test equipment will be cheaper (and, of course, take less time) than developing your own equivalent equipment.
However, off-the-shelf equipment often has stacks of extra "features" that you may not need or want in your particular application. Building a piece of equipment that is just barely adequate for a single purpose may cost 1/100 the price of the general-purpose, all-singing, all-dancing off-the-shelf equipment. Building your own test equipment can also be an educational experience.
Someone once claimed that a $10 PIC-based system can emulate 90% of the functionality of almost any piece of test equipment (where did I see that quote? Was it at Electronics Bench in a Box or somewhere else?).
