Measuring
Voltage - Laboratory Exercise 3
Measuring
Power Supply Voltages
Power supplies are a very useful tool. They are designed to produce
different levels of constant (DC) voltages - positive and negative.
They often come in packages with multiple supplies because electronic circuits
often have a need for several different DC voltages.
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Logic circuits use 5.0
volts, although logic circuits are moving to lower voltage values to reduce
power consumption.
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Analog electronic circuits
often need two supplies, usually +12v and -12, or +15v and -15v.
As a result, you can buy inexpensive power supplies that provide three
voltages, +5v, +12v and -12v, all referenced to a common ground (0v).
In this section, you are going to measure the voltage produced by a power
supply, and you are going to examine some of the finer details of the voltage
that a power supply produces. Here's what you need to do.
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Get a power supply.
If you have one that produces +5v, +12v or -12v, you can use that.
If you have a variable power supply, use that and set it to +5v as closely
as you can.
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Next, you need to measure
the output voltage of the power supply. What kind of instrument do
you need?
Which of these instruments do you think you need?
You can't go wrong
whatever you chose with the buttons.. However, we are going to ask
you to do the following.
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Measure the output voltage
of your power supply with a voltmeter or a data acquisition unit.
If your power supply is adjustable, set the output to as close to 5v as
you can, then check it with a voltmeter
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Display the power supply
voltage on an oscilloscope and measure the DC value of the voltage.
Now, consider the following. When you measured the power supply voltage
with the oscilloscope, did you check whether it looks like the one shown
in the picture below.
Then, if you think about the situation pictured above, you do have a problem.
Here it is in a nutshell.
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Q:
If you measure a voltage with a voltmeter, how do you know you don't have
a voltage that looks like the one pictured above?
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A:
You won't be able to determine that from a voltmeter measurement.
You need to use an oscilloscope to see things like that!
So, now we're asking you to check your power supply. Measure the
output of the power supply with an oscilloscope, and check whether or not
you have any ripple (That's the technical term for a wiggly voltage on
top of a voltage that's supposed to be constant!) in your power supply
output. Measure it, and comment on it in your lab notebook and writeup.
Here are some hints on how to proceed.
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Oscilloscopes have settings
that let you choose between AC
and DC.
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In the DC setting, the
entire signal is presented.
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In the AC setting the
"DC component", i.e. the constant part of the signal, is subtracted out
and only the fluctuating "AC component" is presented.
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In the ripple signal,
the DC component is the DC voltage you set - or something near it, and
the AC component is the ripple itself.
Actually, for this lab, you should note the following.
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Every power supply has
ripple. It may be too small to see, but there has to be some there.
You may need to put the oscilloscope on AC and change the sensitivity so
you can see it.
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If you can't see that
ripple in your power supply, then somebody paid a premium for it.
Power supplies with very low ripple cost more than the other kind.
Here's an additional exercise
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Measure the ripple of
the power supply using an A/D board. (You'll have to read the section
on measuring voltages with the A/D board.)