# Homework Help: Blinking light circuit voltage vs. time?

1. Oct 4, 2007

### mikebc

You guys have been so incredibly helpful in this forum I really appreciate it. I am a little bit confused on part of the following question and am hoping to get some help with it also if I can.
I believe I did the first part right, but I am not sure about the graph. If the length of the flash is 0.1s and the flash frequency is 0.114s, then I was thinking that it might take 0.04s for the charge to dissipate before it starts to recharge. Now that I am looking at my graph more closely, I see that my #'s are not the same as my calculation results. Even so, I still do not understand the shape it is supposed to be. Threshold is 75v and when the flash would occur, so charge builds up but once the charge is released does it drop straight down to 0 on the graph or more of a slope?
The question follows and I have sent a scan of my work. Thanks.

Question
A simple type of blinking light circuit can be constructed using a neon lamp. The
circuit shown here has a 4.0 μF 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.

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2. Oct 4, 2007

### mgb_phys

If the flash only lasts for 0.1 secs and it takes .114 secs for the capacitor to recharge the capacitor doesn't drop to zero volts.
The capcitor discharges through the lamp as a normal 1/RC curve, when it gets down to a certain voltage the lamp is no longer ionized and will switch off ( goes open circuit) the capacitor will level off at this voltage and begin to recharge.

3. Oct 5, 2007

### mikebc

Thank you for your help, although I don't think I completely understand. There is 0.114s between each flash and then the flash lasts for 0.1s. Is there a way to determine what voltage it drops to before recharging?

4. Oct 5, 2007

### hagar852

It aslo asks for flash frequency.....