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    Understanding capacitors and inductors

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    1) If caps block dc what is the purpose of a cap in a dc circuit?

    2) What does an inductor do in a dc circuit?

    3)What is the point of having a cap to ground in a dc circuit?

    4)What is the purpose of a null modem?

    5) How do you design a circuit to use caps or inductors in a dc circuit? An ac circuit?

    6) What is a biasing cap?

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    SOLUTION This solution is FREE courtesy of BrainMass!

    1.In practice, most digital circuits such as microcontroller circuits are designed as direct current (DC) circuits. It turns out that variations in the voltages of these circuits can cause problems. If the voltages swing too much, the circuit may operate incorrectly. For most practical purposes, a voltage that fluctuates is considered an AC component. The function of the bypass capacitor is to dampen the AC, or the noise. Another term used for the bypass capacitor is a filter capacitor.

    2. If a DC current is flowing through an inductor, there is no induced voltage. Only the winding resistance voltage drop is present . When an inductor is connected across a dc supply, the current increases in a nonlinear fashion. If a switch is used to connect the inductor to the source, the instant the switch is closed the current is zero. The induced voltage across the coil is maximum. After the switch is closed, the current increases until the magnetic field is completely built up. The time constant (T) in an RL circuit is L/Rt. The formula is T = L/Irt. The time constant is the time for all voltages and currents to change to 63.2% of the maximum change. This is similar to capacitive circuits. When first energized, an inductor appears as an open circuit because the current is zero.

    This will prevent large currents from flowing when a switch is closed. Current will reach to a steady value slowly.

    3.Answer to this is given in Q1. The capacitor you specify is a filter capacitor. It bypasses unwanted AC components in DC circuits.

    4. The purpose of a null-modem cable is to permit two RS-232 "DTE" devices to communicate with each other without modems or other communication devices (i.e., "DCE"s) between them.

    5. Inductors are used in tuned circuits with capacitors to provide frequency selection, as in a radio. They ae also used to reject a particular frequency.

    In AC circuits, inductors can be used as chokes by connecting them in series

    6. The term "biasing" is used a lot when discussing transistor circuits. It simply means applying the relevant voltages and/or currents to make the transistor work the way we want it to.
    This capacitor allows to stabilize the bias voltage. The value is usually high for a good stabilization and a lower bandwidth because the AC part of the current flowing through the transistor will pass through this cap.

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