Part 2What is voltage?So what is voltage anyhow? Well, its a pretty abstract term but a lot of people like to use the
term "potential energy" which is that thing you heard about in high school physics and then forgot
immediately.
Some people like to draw an analogy to water to describe voltage. A water pump is like a voltage
supply (also known as a battery).
The pump pushes water through a hydraulic system, and the voltage supply pushes electrons through
an electronic system.
The higher the rated pressure of the pump, the more 'work' the water can do.
Likewise, the higher the voltage the more 'work' (Watts) the electrons can do.
Voltage is used to provide power (via a battery or wall plug) and its also used as a way of
transmitting data. For example, music is recorded from a microphone as an analog voltage signal,
if that voltage waveform is applied to a speaker the voltage performs the work of making air move
and produces sound.
Voltage is also used to in digital circuits to talk back and forth in binary, usually 5V or 3.3V
is a "1" and 0V is a "0", by alternating the 1's and 0's millions of times a second, data can be
moved around rather quickly.
AC/DCNot just an 80's hair metal band! Voltage comes in two flavors (yum): Alternating Current (AC)
and Direct Current (DC). Here is a quick tour of the differences.
Direct current voltage is what comes out of batteries. The battery is at 9V, and it pretty much
keeps that voltage constant, until it dies. The chemical reactions inside the battery creates DC
voltage.
Electronic circuits really like DC voltage.
Alternating current voltage is what comes out of the wall. The generator at the US power plant
creates a voltage that oscillates, going from -60V to 0 to +60V to 0 again, 60 times a second.
At the European power plant its -120V to +120V at 50 times a second.
AC voltage is great for power plants because its easy to transform AC voltages (using a transformer)
up to 50KV for long distance travel and then down to 240V or 120V to safely power your home. Those
big honking grey things that you see next to buildings that hum are the huge transformers.
Motors (like your washing machine and refrigerator compressor pump) like running off of AC voltage.
You can turn AC voltage into DC voltage very easily by using a very small transformer to bring the
120V down to a reasonable level like say 16VAC and rectifier. This is basically what's inside a wall
wart plug or your laptop power supply.
Its much harder to turn DC into AC, you will need an inverter which are more expensive than
transformers/rectifiers.
Batteries only supply DC voltage and wall plugs only supply AC voltage. However, it is totally
possible to have both AC and DC voltage at a certain point:
If an AC voltage is oscillating between -60V and +60V it has 120V AC and 0V DC because the
average voltage of -60V and +60V is 0V.
If an AC voltage is oscilating between 0V and 120V then it has 120V AC and 60V DC because the
average voltage of 0V and 120V is 60V.
In the above oscilloscope image, the dashed horizontal line in the center is ground (0V) and each
dashed division is 5V. The scope is displaying a signal that has both AC and DC components. There
is an alternating voltage (a square wave) that is about 4V high at about 100Hz and a DC
(mean average) voltage that is around 7V. Use the dashed divisions to verify for yourself that
this is so.
What is voltage testing good for?Voltage testing is very common, you'll use it a lot
* Test if your power supply is working, are you getting 5V out of that 7805 regulator?
* Verify that your circuit is getting enough power: when all of the blinky lights are on,
is the power supply drooping too low?
* Verify signals to and from chips to make sure they are what you expect once the circuit
is up and running
* Testing batteries, solar cells, wall plugs, and power outlets (carefully!)
* With a current sense resistor you can perform current testing on a project without
possibly damaging your meter.
Remember!You can only test voltage when the circuit is powered If there is no voltage coming in
(power supply) then there will be no voltage in the circuit to test! It must be plugged
in (even if it doesn't seem to be working)
Voltage is always measured between two points There is no way to measure voltage with only
one probe, it is like trying to check continuity with only one probe. You must have two
probes in the circuit. If you are told to test at a point or read the voltage at this or
that location what it really means is that you should put the negative (reference, ground,
black) probe at ground (which you must determine by a schematic or somewhere else in the
instructions) and the positive (red) probe at the point you would like to measure.
If you're getting odd readings, use a reference voltage (even a 9V battery is a reasonable
one) to check your voltage readings. Old meter batteries and wonky meters are the bane of
your existence but they will eventually strike! Good places to take reference voltages are
regulated wall plugs such as those for cell phones. Two meters might also be good
Voltage is directional If you measure a battery with the red/positive probe on the black/
negative contact and the black probe on the positive contact you will read a negative voltage.
If you are reading a negative voltage in your ciruit and you're nearly positive (ha!) that this
cannot be, then make sure you are putting the black probe on the reference voltage
(usually ground)
DC voltage and AC voltage are very different Make sure you are testing the right kind of
voltage. This may require pressing a mode button or changing the dial.
Unless otherwise indicated, assume DC voltages
Get into the right mode
There are often two seperate modes for AC and DC voltage. Both will have a V but one will
have two lines, one dashed and one solid (DC) and one with have a wave next to it (AC).
This meter has the double line for DC voltage, and 5 ranges, from 200mV to 600V. The lightning
bolt symbol is a gentle reminder that this voltage is extremely dangerous.
There is also the V-wave symbol for AC, and two ranges since most AC voltages that are measured
are power voltages and are pretty big. (For small AC waveforms, a scope is best since you will
be able to see the waveform itself)
This autoranging meter makes it pretty clear which mode you want to be in
This ranged meter has 5 ranges, the top range is 750 VAC or 1000 VDC, to switch between DC and
AC you need to press the DC/AC button on the upper right.
When the probes are not connected to anything, they should display 0V. They might flicker a
bit if they pick up ambient voltage (your home is a big radiator of 60Hz voltage which can
couple into your meter probes).
Example 1: Testing batteriesTesting 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
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.
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. You can check out the discharge curve in
the Duracell 9V datasheet
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.
Example 2: Testing wall wart (adapter) plugsTesting wall adapters is also very handy, especially when you build your own circuits.
The first kind we will test is a transformer-based adapter.
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.
Glitchbuster has a long page that describes this.
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!
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
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.
Example 3: Testing Wall outputThis 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
ConclusionSo now you have seen the basics on using a meter. All you have to remember is most of the metal parts
on your PSP are grounded and you can take the black lead of your meter to use that for ground and
probe around your PSP for the voltages you are looking for.
PLEASE be careful as you will have the power on your PSP and of you short things out with the meter
you can blow out parts or fuses.