Why is an LC filter a better choice than a capacitor/inductor filters when there is a variable load?
Answer:
In many RC and RL filters, the critical frequencies are determined by the resistance values in the circuit, including the variable load resistance. The critical frequencies in an LC filter - or a capacitor/inductor filter, which is another name for the same thing - the critical frequencies are determined by the capacitances and inductances. The variable resistance of the load might change the Q, or selectivity and responsiveness of the filter, but will have only a small effect on the critical frequencies. Capacitive or inductive components in the load will shift these frequencies, but RC and RL filters will be sensitive to reactive components in the load too.
In power circuits, resistors are energetically inefficient. Power filters based on resistors must therefore require more input power to operate than supplies with other kinds of filtering. This wasted power appears as heat in the resistors, which must be removed to prevent damage to the supply and limits the amount of miniaturization these supplies can undergo.
Bleeder resistors are put across the capacitors in power supplies as a safety precaution to ensure that the output of the supply actually goes to zero in a reasonable time after shutting off the supply. Otherwise, if the load has been disconnected before the supply has been shut off, the voltage would sit on the filter capacitors for a long time. I have been nailed a time or two by supplies in which the bleeder resistors were absent or were disconnected.
If I remember correctly, an LC filter contains an Inductor (L) and Capacitor(C).
On schematic diagrams, Inductors are normally labeled L.
The bleeder resistor gets rid of any built up charge left in the capacitor after the input power has been removed. When you cut the input power and the current stops, the inductor's magnetic field collaspes. This collaspe induces the capcitor to charge up again. The bleeder resistor gets rid of this charge.
I'm pretty sure...
An LC filter is just a generalization of
the discrete capacitors and inductors
you find in a storehouse. So it's neither
better or worse.
But you can model transformers,
wet and cell batteries, and even
large parts of you car engine
and transmission as LC filters.
So it's the model that's a better choice,
not the filter.
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