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.