### Author Topic: electronics  (Read 586 times)

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#### xiaopoodie

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• Posts: 3
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##### electronics
« on: December 30, 2012, 07:13:05 PM »
An electronic timer works by charging up to 100µF capacitor very quickly to 15V and then allowing the capacitor to discharge slowly through a 720k-ohm resistor. The timer operates an electronic switch when the p.d across the capacitor drops to 1.3V. how long is the timer set for?

attempt to solve:
q = CV
q before = 1500µ
q after = 130µ
change in q = 1370µ
q = current x time
have no idea how to find current
7 minutes ago

#### jwatte

• Supreme Robot
• Posts: 1,345
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##### Re: electronics
« Reply #1 on: December 30, 2012, 07:45:03 PM »
Homework questions should generally be answered by your teaching assistant...
In brief, though, the current comes from Ohm's law. U = I * R -> I = U / R.
The trick in this problem is that the voltage changes as the capacitor discharges through current, so you have to solve a differential equation.
The fact that you don't know how to find the current means that you haven't learned what you're supposed to be learning in class. Go back to the book and I'm sure you can figure it out!

#### xiaopoodie

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• Posts: 3
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##### Re: electronics
« Reply #2 on: December 30, 2012, 08:21:11 PM »
thanks mate  i seemed to figure out that the formula is V = Vo (e^-t/rc)
i dont have text book and i only got crappy notes from my lecturer..had tohe i ask someone on tnternet. ~.~

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