full wave rectification smoothing capacitor

[yellowmania]

Hi
have a 12V supply powering a 4 diode bridge circuit containing a load resistor! i was asked to place a capacitor with a calue of 23microfarads across the load and observe the ripple voltage on an oscilloscope. the expected outcome was the oscilloscope to show no ripple and just a straight line. This was observed. i was just wondering why this has occured?????????

 

[Raven Luni]

What value did you use for the load resistor? The lower the value the more current is drawn and the bigger the capacitor you need.

 

[yellowmania]

off the top of my head i believe it was 100Kohm!

 

[Harald Kapp]

Sounds like some homework or assignment, doesn't it?
So if you have to explain the behaviour, here are some lines along which to think:

- AC bridge rectified gives you a Dc signal with a specific waveform. Find out what this waveform is like and why.
- Forget about the load resistor for a moment: add a smoothing capacitor across the +/- poles of the bridge rectifier, what will happen to the voltage across the capacitor? Tip: charge flows into the capacitor from the rectifier's output. What happens to the charge when the voltage at the rectifier's input drops? Is there a way to discharge the capacitor?
- Now add the load resistor (effectively I only reversed the order in which the components were added in your experiment). What current will flow? Where will the current come from when the input voltage to the bridge rectifier drops? What happens to the source of the current during that time? What happens when the voltage at the rectifier's input returns to peak voltage?
- If you saw no drop in voltage across the load resistor? How is the voltage drop related to the current drawn from the voltage source at any moment? How is the current related to the resistance of the resistor?

I hope these hints will allow you to find out for yourself what happens and why.

Regards,
Harald

 

[yellowmania]

yeah its homework, ive done all my answers data wise i just wanted to know the reasons behind them! that last post is brilliant, many thanks!