AC Current, Another noob question(s)

I was reading in my electronics book the other day. I just finished up a lesson on DC current and it was all good. But then came the lesson on AC current and I am stuck. I There are a few concepts of AC current as well as some parts that require AC current that I do not completely understand.

1)What the heck is the wave "Shape?" I heard it was like the shape the AC current makes when you put it on a graph or something.
2)Does a crystal take in DC and give off AC at a specific frequency, or does it take in any AC with any frequency and change it to AC at a specific frequency.
3)If it does take in AC, does the crystal change the "shape" of the AC current?

I also heard about this wonderful thing called and oscillator that is suppose to turn DC into AC or something like that. I have not found a simple oscillator design yet(Not sure if one exists) so I was not able to try any experiments. Anyways, If a battery gives of DC at the negative terminal, how can it take in AC at the positive? If anyone could find/make an oscillator schematic that is so simple a caveman can do it, that would be awesome.

1) The AC wave "shape" is normally a plot of the voltage versus time.

AC voltage from a power line looks like a sine-wave.
For 60Hz power it goes to the positive peak and then to the negative peak 60 times per second.

A square-wave oscillator waveform has rapid transitions between two voltages where it stays for 1/2 the period of the waveform. Thus the waveform, when plotted versus time, has square edges and thus the name.

2) A crystal in the appropriate circuit will form an oscillator that takes power from a DC source and generates an AC sine-wave waveform.

3) It does not take in AC

If you Google oscillator you will get a zillion hits.

If you want to build a simple oscillator you might look up "555 astable oscillator". It generates square waves, but it's generally easier to build then a sine-wave oscillator.

The purpose of the crystal is actually to act as a filter. The piezo effect is simple, when there's a voltage difference between two points on the quartz crystal the crystal will deflect slightly, and if that signal is removed it will tend to revert back to it's original shape, it will however bounce a tad bit at whatever it's native frequency, that is fed back into the crystal on the other side out of phase and the crystal will continue to oscilate back and forth at a very precise frequency.

To elaborate on what Sceadwian said, in a circuit the crystal looks like an LC (inductor-capacitor) resonant circuit with very stable characteristics. The crystal oscillates at its mechanical resonant frequency and that appears as a voltage due to the pizeo-electric nature of the crystal.

The prefix "Piezo" is derived from the Greek piezein, which means to squeeze or press.

In the case of a crystal, it is the electrostatic effect on the material. Crystals are mostly made of quartz, which has a crystal lattice that is very stiff and returns most of energy back from a flexing action. Most all materials have some piezo-electric effect but you need a material that has the stiffness and quality factor to 'ring' for a period of time. A string of hair being pulled by a static charged comb is demonstrating piezo-electric effect but a hair does not have sufficient stiffness to spring back and forth for very much time.

Now think of a wine glass being 'twanged' by a flick of a fork hitting the edge of the glass. The glass will vibrate for some period of time, due to the mechanical impulse hit, at a particular audio tone that is the natural 'ringing' frequency (resonance) of the glass. This is very similar to a quartz crystal.
You may have seen demonstations of a wine glass breaking due to an audio tone from an audio loudspeaker (or very loud singer). The audio tone is exciting the wine glass at its natural resonant frequency. If the excitation is strong enough it will excite the glass (flex the glass) beyond its breaking point. The same thing can happen to a quartz crystal in an oscillator circuit.

As to A.C. or D.C., a quartz crystal must have some external excitation to get the oscillation started. This initial excitation can be from the transient ('twang') of initially applying the power supply (D.C) to the oscillator circuit, or low level electrical noise within the amplifier (transistor, etc.). The crystal will begin to ring at its natural resonant frequency which is amplified and reapplied to further increase the crystal's ringing. Eventually the circuit will reach a steady state of constant amplitude, hopefully below the point where crystal fractures.

An answer about a 9V battery being able to make a +- oscillator is all about relativity. If I took two 1K resistors in series and placed them across the battery and I now put my meter negative lead the center point, I'd get +-4.5 volts to the ends. Hence, it's relative. Generally that's not pratical, but it illustrates the concept.

Also AC can ride on top of a a DC voltage, but it usually doesn't.

crutschow said:

For 60Hz power it goes to the positive peak and then to the negative peak 60 times per second.

Click to expand...

For 60 HZ power, from a positive peak to the next negative peak is a half of a cycle and would be 8.33ms. From positive to positive (or negative to negative) it would be one cycle, or 16.66ms or 60 HZ.

Jaguarjoe said:

For 60 HZ power, from a positive peak to the next negative peak is a half of a cycle and would be 8.33ms. From positive to positive (or negative to negative) it would be one cycle, or 16.66ms or 60 HZ.

Click to expand...

That does not contradict what I said.