capacitorresistors
Is there any use for parallel RC circuits in DC applications? Look at this now full diagram for example. The IC has 3 pins (mosfet maybe).
It seems that the resistor in parallel would only drain unnecessary current out of the source without changing anything on the output, not limiting current and not changing voltage.
Thanks.
The first circuit in your question only exists in badly designed homework problems.
The reason is that an ideal capacitor in parallel with an ideal voltage source does nothing. The ideal voltage source can supply whatever current is needed to drive the capacitor to it's level without sagging, so the capacitor provides no filtering.
but...
In the real world there are no ideal voltage sources. They have series resistance. Or if they're located far from the point of load, there's a series inductance between the source and the load. This is the scenario where a parallel capacitor is useful.
Combined with the voltage source's parasitic resistance, the parallel capacitor provides filtering and reduces noise at the load.
Also, for more complex loads than the simple resistor in your circuit, if the load current varies (for example if it's a digital logic chip with it's outputs changing state), the parallel capacitor can provide the necessary current, which the voltage source may not be able to do because of its parasitic resistance or inductance.
okay, so do you need a resistor in series with your capacitor?
Generally no.
But there are instances where it is used.
One is, when you have a very large capacitor value and you need to limit the inrush current when the voltage source is turned on. In this case you often use a negative-tempco (NTC) resistor to limit resistance during the initial turn-on, but have low loss once it is heated up by current through it.
Another case is that sometimes an RC can be more effective at suppressing noise than a simple capacitor, because the capacitor itself cannot take energy out of the circuit --- it can only store it for later. The series resistor can actually dissipate energy from the circuit.
You are correct — if there's an extra resistor in parallel with a voltage source, the source will have to deliver more current. If there's an extra resistor in series with a current source, the source will have to deliver more voltage. In both cases, the source must supply more power than it would without the extra resistor.
But the point that the book is making is that none of this has any effect whatsoever on the analysis of the rest of the circuit. From its point of view, the source looks exactly the same with or without the resistor.
Best Answer
Sometimes a (fairly large) resistor is placed in parallel to a capacitor to act as a bleeding resistor, i.e. to allow the capacitor to discharge quickly after the power is turned off.
This is a safety measure for circuits with high capacitance capacitors charged at high voltages (for example, high-power switching power supplies). Without the bleeding resistor the capacitor could retain a fairly large amount of energy at high voltage even after minutes (or hours) from power-off. Servicing such a circuit would be very dangerous, without the bleeding resistors, even when disconnected from mains, because it could deliver deadly electrical shocks to the operator inadvertently touching parts of the circuit.