h2. Voltage Meter Tester
A *voltmeter* is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter.
Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.
General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.
Part of the problem of making an accurate voltmeter is that of calibration to check its accuracy. In laboratories, the Weston Cell is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.
h3. Example 1: Testing batteries
Testing batteries is a super useful skill and is one of the best ways to practice with your multimeter\+
The first battery we’ll test is a new 1.5V alkaline. This one is a AAA but a AA, C or D cell will be the same voltage. Set the range to *2V DC* .
We read 1.588V, which you may think is a mistake, after all its a 1.5V battery so shouldn’t it be 1.5V? Not quite, the 1.5V written on the side is just a *nominal voltage*, or the “average” you may expect from the battery.In reality, an alkaline battery starts out higher, and then slowly drifts down to 1.3V and then finally to 1.0V and even lower. [Check out this graph from Duracell's page about alkaline battery voltage|http://www.duracell.com/oem/primary/alkaline/alkvoltage.asp]
Using this graph you can easy tell how fresh your battery is and how long you can expect it to last.
Next, we measure a 9V alkaline battery. If we still have the range set to 2VDC we will get a mysterious “*1.* ” display, indicating is it over-range.
display, indicating is it over-range.
Fix the range so that it’s 20V, and try again.
For this new battery we get 9.6V. Remember that battery voltage is _nominal_, which means that the “9V” is just the *average voltage* of the battery. In reality, it starts out as high as 9.5V and then drops down to 9 and then slowly drifts to 7V.
If we want to check a rechargeable AA battery, and it’s set to a 20VDC range, we will read 1.3V, which is about what a fully charged NiMH battery will measure.
If we fix the range so it’s *2VDC*, we can get an extra digit of precision. This meter probably isnt more than 0.5% accurate so the precision may not mean much.
Finally, I test a lithium 3V coin cell, its at 2.7V which means it’s getting near the end of it’s life.
h3. Example 2: Testing wall wart (adapter) plugs
Testing wall adapters is also very handy, especially when you build your own circuits.
The first kind we will test is a *transformer-based* adapter.
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Note that the label says *Transformer*, its also blocky and heavy which indicates a transformer as well. It requires 120VAC input, US power only. The nominal output is 9VDC at 300mA. The polarity symbol shows that the middle is positive, the outside is negative, thus we place the ground (black) probe on the outside and the positive (red) probe on the inside.
Yow\! 14V? That’s not anything like the 9V on the package, is this a broken wall wart? Turns out, its totally normal. Transformer-based wall adaptors are (almost always) *unregulated*, which means that the output is not guaranteed to be a particular value, only that it will be *at least* what is printed on the box. For example, with this adapter it means that when drawing 300mA, the voltage is guaranteed to be higher than 9V.
Since the output is unregulated, the voltage supplied will droop as more current is pulled from it, which means that open-circuit (connected to nothing) the measured output can be as high as 14V.
Next, lets check out a *Switch-mode* adapter
Notice that it’s not square, its much thinner and although you cant feel it, its quite light for its size: There is no big honking transformer inside\!
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Note that it says Switching (not Transformer) on the label, and you can input US or European power. Like the transformer adapter, it is center-positive polarity.
Switch-mode wall adapters are *regulated* which means that the output doesn’t droop from open-circuit to full load. Its not an ultra-high quality supply, the voltage is 12.2V which is less than 5% error. Still, its much better than the transformer’s 50% error\!
Lastly, we’ll test a 9VAC adaptor, which outputs AC voltage instead of DC. Basically this means that there’s still a transformer inside, but no rectifier. This is also an unregulated supply
Note that is is similar to the transformer-based DC supply we checked out first
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Note again that the label says transformer. It requires 120VAC input, US power only. The nominal output is 9VAC at 300mA. The output is indicated twice, once at the top “AC/AC” and then again in the output designator “9V AC”
There is no polarity because AC adaptors are not polarized: AC power oscillates between positive and negative voltages.
We test the output, but get 0V\! That’s when we remember that the multimeter has to be in AC voltage mode.
Switching over to AC, we get a good reading, 10.5VAC. This is an unregulated supply so again we are going to get a voltage higher than 9V.
h3. Example 3: Testing Wall output
This is the ‘easiest’ test, just shove the two probes into a wall socket. If you’re clumsy and think you’ll somehow electrocute yourself, don’t do this. Many people freak out about this test, but ironically it’s what the multimeter was designed to do.
About 120V, as expected
h3.
