4 is better. Do not buy an analog storage scope in
order to learn about oscilloscopes, because these
are much harder to use. Get as much scope as you
can afford; like good books, good tools are cheap if
you use them.
POWER SUPPLY
A power supply (aka bench supply) provides “
known-good” power to your circuit. After we figure out how
much power our circuit actually needs, we can then
decide how to power it for regular use, like with
batteries or a wall wart.
Bench supplies let you adjust the output voltage, which is good for initial testing: while slowly
increasing the voltage to your circuit’s operating
value, you can look for burning components, smoke,
arcing, explosions, etc. The fanciest supplies also
have current limiting, which sets the maximum
amount of current the circuit is allowed to draw.
A circuit that draws way too much current could
be cooking itself, or might have a power short to
ground — maybe through that screwdriver you left
under the PC board. Some power supplies also have
meters that show voltage and current.
Many circuits require both positive and negative
DC voltages. To test these, you need a dual-output
supply, which has separate V+, V-, and ground
terminals. Expect to spend a minimum of $50
for a good supply, more if you want dual output
and meters.
MULTIMETER
A multimeter measures the voltages and currents in
a circuit, as well as the characteristics of individual
components such as resistors and diodes. They all
measure resistor values (ohms), but try to find one
that can also measure capacitance (microfarads),
since it is sometimes difficult to read the values on
capacitors.
Use a multimeter to “ohm out” an unknown circuit.
This means tracing out the connections on a circuit
board to see what is connected to what, so that you
can create a schematic drawing that shows all the
circuit’s connections.
Multimeters are designed to withstand wide ranges
of inputs, so you can use one to check for high
voltages in unknown circuits. The terms volt-ohm
meter (VOM), multimeter, ohmmeter, and voltmeter
all refer to pretty much the same thing. Expect to
spend a minimum of $10 for a handheld voltmeter,
or $50 for one with capacitance ranges.
FUNCTION GENERATOR
A function generator injects a regular signal at
a selectable frequency into a circuit. In certain
circumstances you can do this with a CD player or
other gear, but a function generator is more flexible.
Look for one that can generate sine waves (handy
for testing audio circuits), square waves (handy for
testing filters, amplifiers, and digital circuits), and
triangle or sawtooth waves (handy for graphing
circuit behavior). More costly units can produce
any waveform shape you want; these are called
programmable waveform function generators or
arbitrary waveform generators. Expect to spend
$75–$500, or more for ones that can generate arbitrary waveforms, or go up to higher frequencies.
HOW TO USE AN OSCILLOSCOPE
The oscilloscope draws a graph of voltage versus
time on the display screen (known as a graticule to
old-timers). By connecting probes to your circuit
and generating these scope traces, you can see
what’s going on.
Don’t be intimidated by the large number of knobs
and buttons. These controls are grouped into 3
basic function areas: timebase, sensitivity, and
trigger. Timebase and sensitivity controls change
the graph’s appearance, setting the horizontal and
vertical axes, respectively. Trigger controls tell the
oscilloscope when to start drawing the trace. Here’s
how to operate each control area.
Timebase — Adjust the Horizontal
Find the biggest knob on the front of the scope, and
“ratch” it around clockwise and counterclockwise. This is the horizontal scale adjustment, and it
determines the time represented by each grid line
(or “division”) on the display. A typical range is from
100 nanoseconds to 1 second per division. At the
short end of this scale, the trace zips left to right so
fast on the display that all you see is the line left
behind. At the long end, the trace appears as a
traveling dot. At any setting, the trace starts at
trigger time, which you can think of as time=0.
Sensitivity — Adjust the Vertical
Most scopes have multiple channels, which plot
separately to the display. Each channel corresponds
to its own electrical input, usually a BNC connector
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