# A Level Physics Question HELP

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Thread starter 1 year ago
#1
Hi, my Physics teacher has given us a load of questions on capacitance and rectifier circuits and I have no idea how to even attempt question one!

I haven't got a clue how to do question one, and I'm not 100% sure about my answers for question two and three. Any help at all would be much appreciated.

"For a conducting silicon diode with a threshold voltage of 0.7 V, the diode will allow the capacitor to charge up every time the alternating input voltage is greater than 0.7 V. As soon as the input voltage starts to decrease, the capacitor starts to discharge through the resistor. The rate of discharge depends on the time constant CR. [...]

1. Explain what would happen to output voltage from the recitifer circuit if the time constant were much smaller than the period of the input alternating voltage.

2. The period of the mains voltage is 20ms and it has a peak voltage 340 V. Calculate the ripple voltage when a 1000µF smoothing capacitor is used across a resistor of resistance 220Ω.

3. Suggest suitable values for C and R for a ripple voltage that is 0.10% of the peak voltage for the mains voltage."
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1 year ago
#2
It's a question about ripple voltage in power supplies.

The rectifier will output pulsating dc which rapidly charges the capacitor every 20ms - the capacitor doesn't discharge through the rectifier... it's like a one way valve.

the capacitor will discharge through the resistor depending on the value of RC according to the formula you've probably been taught
V=V0e(-t/RC)

There is an extra constraint - the voltage across the capacitor can't fall below the voltage from the rectifier - if RC is negligably small the voltage across the capacitor will just follow the output from the rectifier... if you get RC large enough the voltage on the capacitor will be an approximately smooth DC voltage
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