555 ramp generator
PARTS AND MATERIALS
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Two 6 volt batteries
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One capacitor, 470 �F electrolytic, 35
WVDC (Radio Shack catalog # 272-1030 or equivalent)
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One capacitor, 0.1 �F, non-polarized
(Radio Shack catalog # 272-135)
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One 555 timer IC (Radio Shack catalog #
276-1723)
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Two PNP transistors -- models 2N2907 or
2N3906 recommended (Radio Shack catalog # 276-1604 is a
package of fifteen PNP transistors ideal for this and
other experiments)
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Two light-emitting diodes (Radio Shack
catalog # 276-026 or equivalent)
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One 100 kΩ resistor
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One 47 kΩ resistor
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Two 510 Ω resistors
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Audio detector with headphones
The voltage rating on the 470 �F capacitor
is not critical, so long as it generously exceeds the
maximum power supply voltage. In this particular circuit,
that maximum voltage is 12 volts. Be sure you connect this
capacitor in the circuit properly, respecting polarity!
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
1, chapter 13: "Capacitors"
Lessons In Electric Circuits, Volume
4, chapter 10: "Multivibrators"
LEARNING OBJECTIVES
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How to use the 555 timer as an astable
multivibrator
-
A practical use for a current mirror
circuit
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Understanding the relationship between
capacitor current and capacitor voltage rate-of-change
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Again, we are using a 555 timer IC as an
astable multivibrator, or oscillator. This time, however, we
will compare its operation in two different
capacitor-charging modes: traditional RC and
constant-current.
Connecting test point #1 (TP1) to test point
#3 (TP3) using a jumper wire. This allows the capacitor to
charge through a 47 kΩ resistor. When the capacitor has
reached 2/3 supply voltage, the 555 timer switches to
"discharge" mode and discharges the capacitor to a level of
1/3 supply voltage almost immediately. The charging cycle
begins again at this point. Measure voltage directly across
the capacitor with a voltmeter (a digital voltmeter is
preferred), and note the rate of capacitor charging over
time. It should rise quickly at first, then taper off as it
builds up to 2/3 supply voltage, just as you would expect
from an RC charging circuit.
Remove the jumper wire from TP3, and
re-connect it to TP2. This allows the capacitor to be
charged through the controlled-current leg of a current
mirror circuit formed by the two PNP transistors. Measure
voltage directly across the capacitor again, noting the
difference in charging rate over time as compared to the
last circuit configuration.
By connecting TP1 to TP2, the capacitor
receives a nearly constant charging current. Constant
capacitor charging current yields a voltage curve that is
linear, as described by the equation i = C(de/dt). If the
capacitor's current is constant, so will be its
rate-of-change of voltage over time. The result is a "ramp"
waveform rather than a "sawtooth" waveform:
The capacitor's charging current may be
directly measured by substituting an ammeter in place of the
jumper wire. The ammeter will need to be set to measure a
current in the range of hundreds of microamps (tenths of a
milliamp). Connected between TP1 and TP3, you should see a
current that starts at a relatively high value at the
beginning of the charging cycle, and tapers off toward the
end. Connected between TP1 and TP2, however, the current
will be much more stable.
It is an interesting experiment at this
point to change the temperature of either current mirror
transistor by touching it with your finger. As the
transistor warms, it will conduct more collector current for
the same base-emitter voltage. If the controlling
transistor (the one connected to the 100 kΩ resistor) is
touched, the current decreases. If the controlled
transistor is touched, the current increases. For the most
stable current mirror operation, the two transistors should
be cemented together so that their temperatures never differ
by any substantial amount.
This circuit works just as well at high
frequencies as it does at low frequencies. Replace the 470
�F capacitor with a 0.1 �F capacitor, and use an audio
detector to sense the voltage waveform at the 555's output
terminal. The detector should produce an audio tone that is
easy to hear. The capacitor's voltage will now be changing
much too fast to view with a voltmeter in the DC mode, but
we can still measure capacitor current with an ammeter.
With the ammeter connected between TP1 and
TP3 (RC mode), measure both DC microamps and AC microamps.
Record these current figures on paper. Now, connect the
ammeter between TP1 and TP2 (constant-current mode). Measure
both DC microamps and AC microamps, noting any differences
in current readings between this circuit configuration and
the last one. Measuring AC current in addition to DC current
is an easy way to determine which circuit configuration
gives the most stable charging current. If the current
mirror circuit were perfect -- the capacitor charging
current absolutely constant -- there would be zero AC
current measured by the meter. |