Figure 27-64 shows the circuit of a flashing lamp, like those attached to barrel

Question

Figure 27-64 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 = 70.7 V, wired to a 95.3 V ideal battery and a 0.166 mF capacitor, what resistance R is needed for 4 flashes per second?

Explanation / Answer

the voltage across capacitor should reach 70.7 in 1/4=0.25 s Vc(t) = E(1 - e^(-t/RC)) 70.7 = 95.3 ( 1 - e^(-0.25/(R*0.166*10^-3)) ) 0.25*10^3/0.166R = -ln(1 - 70.7/95.3) = 1.3543 R = 1.112*10^3 ohm R = 1.112 Kohm

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