Transcript Design a Photoflash Circuitx

Week 11
Design a Photoflash Charging Circuit
New Experiment
• Not in the lab manual. It is posted on the
course Scholar site.
Photoflash Circuits
• 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 Objective
• Design the front-end of a circuit that could be used in
a camera flash.
Design Specifications
• 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.
Design and Simulation
• You have to determine the values for R1, R2,
C1, and RL as well as Vref to meet the design
specifications.
Transient Response
Circuit Construction
• Use a LF356 instead of the LM324 used in the
circuit simulation
• Use a single switch
– The two switches are needed to simulate the
operation of a Photoflash circuit properly in
Pspice.
• Electrolytic capacitors have to be inserted into
a circuit in a particular orientation.
Electrolytic Capacitors
• The negative electrode must always be at a
lower voltage than the positive electrode.
– So in your circuit, the negative electrode must be
grounded.
Data Analysis: Charging t
• 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.
Data Analysis: Discharging t
• 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 expansion is valid when (t-to)/t <<1
Component Measurements
• To determine accuracy of your design and
whether the leakage through your capacitor
affects the charge and discharge time constants,
you must measure R1, R2, and C1.
• WARNING: Do not measure C1 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 your capacitor to
discharge it. The instantaneous current will be very
high.