RC Circuits: Graph Homework - Determine Flash Frequency & Voltage Sketch

[sunflowerzz]

Homework Statement

A simple type of blinking light circuit can be constructed using a neon lamp. The circuit has a 4.0uF capacitor in parallel with a neon lamp. When the voltage is low in the RC portion of the circuit, the lamp does not conduct electricity. Therefore, it is effectively not there from an electrical point of view. The RC circuit will then charge from the 110 V power supply. However, when the voltage across the capacitor reaches 75 V, the neon will ionize very quickly and the neon lamp will become a very good conductor, and will immediately discharge the capacitor. The energy stored in the capacitor will be given off as a flash of orange light, making this a useful circuit. After the flash, the charging process will start once more since the voltage will again be low.

a) Determine the flash frequency with the resistance value shown.
b) Make a sketch of the voltage across the capacitor versus time in such a circuit, showing several periods.

Homework Equations

time constant = RC
V = Vo(1- e^ t/RC)

The Attempt at a Solution

For part a)
time constant = RC = (25 * 10^3 ohm) * (4.0 * 10^-6 F) = 0.1s

t = -ln (1- 75V/110V) * 0.1s
= 0.114s

frequency = 1/t = 8.8 Hz

For part b)
I've attached a sketch but I'm not sure if this is correct. Any help would be great!

 

[technician]

Your sketch is pretty good, if anything it may fall faster than you have shown because it says 'will immediately' discharge the capacitor.

 

[sunflowerzz]

Your sketch is pretty good, if anything it may fall faster than you have shown because it says 'will immediately' discharge the capacitor.

Thanks!