Alternating Current

Alternating current reverses its direction periodically. The current flows one way for a while and then reverses and flows the other way for a while. The electrons don't move along a wire, they vibrate back and forth at the frequency of the alternations.

Alternating Current

• Most circuits don’t use batteries as a source of voltage and power, but power delivered by the power company

• Alternating current (AC) is what is delivered to outlets in the house

• All communications circuits use AC (signals). Why?

Simple AC circuit: resistor only

More interesting: RC Circuits

An RC circuit can be used to charge a capacitor through a resistor:

Closing switch S results in current flowing, making charge go to both sides of the capacitor

When the voltage across C is = V0, current stops

More interesting: RC Circuits

When the switch in an RC circuit is closed, the current has its maximum value (all the voltage is across the resistor, and the voltage across the capacitor is zero).

As charge builds up on the capacitor, the current decreases and the capacitor voltage increases.

After a time, all the voltage is across the capacitor, no current can flow

RC Circuits

Voltage as a function of time:

Current as a function of time:

The quantity RC has the dimensions of time, and is called the time constant.

RC Circuits

A charged capacitor may be discharged through a resistor; in this case no battery is needed.

RC Circuits

In alternating current, the frequency of voltage swings determines what happens in an RC circuit

If freq = low, capacitor has time to charge, a lot of voltage develops across capacitor on each cycle

If freq = high, before the capacitor has time to charge, the current is already reversing. So current flows freely, very little voltage ever develops across the capacitor

This can be easily used to change the behavior of a circuit that might be exposed to many different frequencies

High Pass Filter

If the input signal has low freq, most of the voltage gets stuck on C. The output (measurement across the resistor), is a small voltage.

But high freqs get transmitted unaffected.

NEED PLOT

Low Pass Filter

If the input signal has low freq, most of the voltage gets stuck on C. The output is the full voltage.

But high freqs don’t cause a voltage being developed on C.

NEED PLOT

frequency

Ou

tpu

t V

/V0

Filters

Radio waves are electromagnetic waves with MHz frequencies. Many different stations operate at special frequencies. Tuning a radio to one station involves changing values of resisters to make a circuit sensitive to only a narrow band of frequencies

We can use high and low pass filters together to make band-pass filters, or band stop filters, or all sorts of filters…

Diodes

• Resistors that don’t follow Ohm’s Law

• Basically 0 resistance in 1 direction, high resistance in opposite direction

Diodes can be used to change AC into DC

Only when current is flowing in the right direction does a voltage develop on resistor

One side is only ever positive

Diodes can be used to change AC into DC

Full wave rectifier circuit:

Positive V get through to + output, negative V gets to - output

Diodes can be used to change AC into DC

If a Capacitor is added in parallel to the output of a Rectifier (to form a simple Low-Pass Filter Circuit), the output of the Rectifier is transformed into a more stable DC Voltage

Uses of diodes and capacitors nearly infinite, one example:

• A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse.

• Marx generators are often used to simulate the effects of lightning on power line gear and aviation equipment.

• A number of capacitors are charged in parallel to a given voltage, V, and then connected in series by spark gap switches, producing a voltage of V multiplied by the number, n, of capacitors (or stages). Very high voltage, very high current is produced (high power but for short time)

Marx Generator

Marx Generator

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