You are asked to design an RC circuit for a camera flash. You want the flash to be ‘on’ for around the same time as the shutter of the camera is open, which is about 60 ms. You are using a bulb that will break if more than 0.5 A flow through it. For the flash to be powered by 2 AA batteries (1.5 V each), what combination of resistors and capacitors should you use? Assume the flash is an ohmic resistor.
Time for which the flash to be on is equal to the time constant of RC circuit.
Time constant of RC circuit, Ꞇ = RC
Where, R-resistance, C- capacitance
Given, Ꞇ = 60ms = 0.060s
RC = 0.060
The maximum current the bulb can withstand is Io = 0.5 A
In a RC circuit, the maximum current, Io = E/R
Where E is the voltage of the battery and E = 1.5 + 1.5 = 3 V
Io = E/R ; 0.5 = 3/R
R = 3/0.5 = 6
R = 6Ω
RC = 0.060
6C = 0.060
C = 0.060/6 = 0.01 F
A resistor of R = 6Ω and capacitor of C = 0.01 F should be used
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