The figure shows the circuit of a flashing lamp, like those attached to barrels

Question

The figure shows the circuit of a flashing lamp, like those attached to barrels at highway construction sites. The fluorescent lamp L (of negligible capacitance) is connected in parallel across the capacitor C of an RC circuit. There is a current through the lamp only when the potential difference across it reaches the breakdown voltage VL; then the capacitor discharges completely through the lamp and the lamp flashes briefly. For a lamp with breakdown voltage VL = 78.5 V, wired to a 96.1 V ideal battery and a 0.124 mF capacitor, what resistance R is needed for 3 flashes per second?

Explanation / Answer

VL = 78.5 V

E = 96.1 V

C = 0.124 mF

t = 1/3 s

let the resistance is R

as VL = E * (1 - e^(-t/(R * C)))

78.5 = 96.1 * (1 - e^((-1/3)/(R * 0.124 *10^-3) ))

solving for R

R = 1584 Ohm

esistance R is needed for 3 flashes per second is 1584 Ohm

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