# Capacitor filter question

Discussion in 'Homework Help' started by Sam2002tii, Mar 8, 2014.

1. ### Sam2002tii Thread Starter New Member

Jul 19, 2013
13
0
See the attached pdf schematic

I understand a capacitor can be used as a filter but how does it reduce the ripple from a rectified AC signal?

It's connected in parallel so I understand it won't just charge up and then act as an open like it would in a standard DC circuit. Is it safe to say the ripple current is like a pulsating DC current? Which would be comparable to a very low frequency AC signal?

I understand how capacitive reactance relates to frequency so does it make sense to say the rectified AC signal is like a pulsating DC signal before the capacitor "cleans up" the signal?

Also, what kind of equipment would use this type of schematic? I'm guessing its some sort of small household appliance that requires a DC input.

Any help is greatly appreciated,
Thanks

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2. ### MrChips Moderator

Oct 2, 2009
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Think of a capacitor as a big holding tank that is a reservoir of charge or electrons.
You can take charge from the capacitor without depleting the total charge by too much while the power source tries to top up the charge.

The circuit you have shown is a classic full-wave bridge rectifier circuit that is usually presented at an early lesson in an analog electronics course.

Practically every imaginable household electronic gadget that is powered from AC mains will contain such a circuit.

Apr 5, 2008
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4. ### daviddeakin Active Member

Aug 6, 2009
207
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There are two intuitive ways to think about it.
You can either imagine the capacitor to be a tank like Mr Chips said, which opposes any rapid voltage changes across itself because i = C dV/dt.

Or you can think of the rectified voltage (pulsating DC) from the point of view of Fourier analysis: it contains a DC component, plus a lot of AC components that are progressively shunted or filtered out by the falling reactance of the capacitor, so only the DC is left (mostly).

5. ### #12 Expert

Nov 30, 2010
17,897
9,316
I picture the capacitor as a coffee can. The transformer and rectifiers pump a pulse of electricity into the coffee can every 120th of a second. The load is using up electricity during the pulses and between them. How big the capacitor is determines how much the voltage level drops between pulses. That, "dropping between power pulses" is what makes the output look like AC. It's really a lot of DC with a leak that keeps getting topped up.

6. ### Sam2002tii Thread Starter New Member

Jul 19, 2013
13
0
Thanks for the help!

I now understand how the capacitor "fills in" the low voltage points between the peaks in the ripple current.

Would a device in this configuration still operate with a bad capacitor?

If the pulsating DC was not filtered by the capacitor and the input to the load was just the pulsating voltage, I assume the device could still operate but not very well due to the rising and falling voltage peaks? For example would a light bulb flicker on and off? Would a fan motor continuously run slow/fast due to the rise and fall in voltage?

7. ### #12 Expert

Nov 30, 2010
17,897
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Most devices that use DC will completely fail without a filter capacitor. A light bulb is designed to use AC or DC. It would only become dimmer from lacking the DC component. A fan that is built for DC would also suffer some loss of power because of the lack of DC available. These are simple devices. A more complicated device often fails completely.

8. ### WBahn Moderator

Mar 31, 2012
20,237
5,758
Whether the device could continue to operate, and how well, is very dependent on the device. For instance, a typical lightbulb operates off AC with no filtering of any kind (unless you get really nitpicky about fine details), so is flickering on and off at twice the power-line frequency but we don't notice a difference and perceive a constant light source. The same with many motors that are getting pulses of power and are coasting in-between. Other devices, particularly electronic circuits, are very sensitive to slight deviations in power supply voltage. Consider a digital circuit being clocked at just 600MHz (slow for many of today's gadgets) -- each cycle of the powerline frequency is 10 million clock cycles. So a LOT happens during even a transient drop in voltage lasting only a fraction of a cycle.

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9. ### LvW Active Member

Jun 13, 2013
674
100
Just for clarification, one general remark:
A capacitor alone is NOT a filter. You always need - in addition - a second element (at least). Otherwise no frequency-dependent voltage division is possible. (This "second" element can be a lumped part or the internal resistance of another device (voltage source, diode, transistor,...).