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Old 21st July 2006, 03:58 PM   #11
mr.duck is offline mr.duck  United Kingdom
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I have one more question.. if the low pass filter was used in a power supply, would the capacitor only be involved in filtering or would it also be able to supply power when it is needed?

For example, if the resistor was not there and you just had the capacitor smoothing out the DC, the capacitor would also be able to provide a large amount of current on demand. But when you put the resistor in to make the filter, does the capacitor loose this dual funtion and only be involved with filtering?


Does that make sence??
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Old 21st July 2006, 04:20 PM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
your RC filter does both jobs.

With or without the R the cap will try to keep the voltage at the same level until it's charge changes.

i.e. when the current demand increases the input voltage falls, but as soon as the voltage has fallen below the voltage level of the cap, the cap starts to discharge into the increased current demand.
Similarly if the current demand reduces then the input voltage will increase and the capacitor will absorb current (charge) until it's stored voltage matches the new voltage supply.
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Old 22nd July 2006, 01:48 AM   #13
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Simple RC networks like this a extremely easy to simulate using spice. You might consider downloading SwitcherCAD from Linear Technology (FREE!) and learn how to use it. You will be able to answer all your own questions in a matter of seconds, in far greater detail than you can imagine.

Example:
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Old 22nd July 2006, 02:16 AM   #14
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Quote:
Originally posted by mr.duck
I have one more question.. if the low pass filter was used in a power supply, would the capacitor only be involved in filtering or would it also be able to supply power when it is needed?

For example, if the resistor was not there and you just had the capacitor smoothing out the DC, the capacitor would also be able to provide a large amount of current on demand. But when you put the resistor in to make the filter, does the capacitor loose this dual funtion and only be involved with filtering?
The cap can supply current far beyond the regulated supply's nominal capability. I recently made a stroboscopic illuminator for a microscope that uses a 5W LED. I am driving the LED with 12A pulses even though the power supply in the strobe is a 15V, 5A regulated supply. I simply added a 10,000 uF cap on the supply rail at the FET that switches the current through the LED. This works fine for this application because the duty cycle is kept below 1/15. The high current discharge is brief (variable from 5-250 us) and the recharge time relatively long, so I can flash the LED up to about 250 Hz and still maintain the pulses at 12A.

I_F
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