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Design a Photoflash Charging Circuit

Not in the lab manual. It is posted on the
course Scholar site.
•
The circuit charges a large capacitor using a
relatively low time constant so that the capacitor
current doesn’t exceed the amount that a typical
alkaline battery or set of batteries can supply.
– An LED display is used to show that the capacitor is a)
charging and b) charged sufficiently.
•
A switch in the time constant of the circuit is
then implemented so that the energy stored in
the capacitor is quickly discharged through the
flash bulb, enabling a large amount of current to
flow in a short period of time.
– The amount of light given off by the flash bulb and the
color of the light are dependent on the square of the
current.
• Design the front-end of a circuit that could be used
in a camera flash.
•
Construct a circuit such that:
– the capacitor charges to ~7V in 5 seconds when a switch
is closed between the 9V supply and the rest of the
circuit,
– the capacitor discharges to 0V in 4 minutes when a
switch is closed after the capacitor has been fully
charged (this is done to insure that there is no residual
charge left on the capacitor if the flash is not used),
– a red LED is lit only while the capacitor is charging and
the current flowing through the red LED is ~ 3-4 mA,
– a green LED is lit when the voltage on the capacitor has
reached 80% of its maximum value and the current
flowing through the green LED is ~ 10 mA.

You have to determine the values for R1, R2,
C1, and RL as well as Vref to meet the design
specifications.

Use a LF356 instead of the LM324 used in the
circuit simulation
◦ The reason to use the LF356 is because it has a
high input impedance.

Use a single switch
◦ The two switches are needed to simulate the
operation of a Photoflash circuit properly in Pspice.

Using the cursor on the software oscilloscope
◦ Measure 5 data points as the capacitor charges,
 Data should include the initial voltage across the
capacitor (should be 0V), the time at which the switch
is closed, the maximum voltage across the capacitor,
and three voltage vs. time measurements in between
the initial and final conditions.
◦ Fit the data to the appropriate equation to
determine the time constant of the charging circuit.
•
Using the cursor on the software oscilloscope
– Measure 5 data points as the capacitor slowly
discharges
• Data should include the initial voltage across the
capacitor (~7V), the time at which the switch opened
and the capacitor begins to discharge, and four other
points.
– Do not wait until the capacitor fully discharges to obtain
the final condition of the capacitor.
– Fit the data to appropriate equation, but use a
Taylor series expansion for
• Remember that the Taylor series expansion is valid
when (t-to)/t <<1
•
To determine accuracy of your design and
whether the leakage through your capacitor
affects the charge and discharge time constants,
you must measure R1 and R2.
• Unfortunately, you can’t measure the value of C1
because it is beyond the range of the DMM. See the
specifications on the tolerance for the capacitor that are
listed in the Appendix of the lab manual and in the file
about the Parts Kit posted on the OpEL website.
• WARNING: Do not measure the value of any capacitor using
your DMM unless you sure that there is no charge stored on
the capacitor or you may damage your DMM.
– Do not place a wire directly across a capacitor to discharge
it. The instantaneous current will be very high. Please a
resistor across the terminals of a capacitor to limit the peak
inrush current.