Thursday, January 20, 2011

Lesson 24: General Class Exam Course G6B

Here is the 24th lesson in the General Class series.  This lesson goes over the G6B section from the question pool dealing with a wide spectrum of stuff, from batteries to vacuum tubes.  There is no real rhyme or reason behind the questions however, some of them are a bit deep.  You may want to go over the video a couple of times to make sure you've absorbed the information.

Here is the information you need to focus on:

Peak inverse voltage is the maximum voltage a rectifier (diode) can handle before it will allow current in the non-conducting direction.  Remember, a rectifier is, for the most part, a diode that only allows current to flow in one direction.  A rectifier will block current in the non-conductive direction up until the voltage increases to a point where it will overcome the rectifier's ability to block the current.  The maximum voltage a rectifier will block before it breaks down is the peak inverse voltage.

Junction threshold voltage is another one you will need to know.  Semiconductors are not great conductors, but they're not great insulators either.  It takes a little more push than usual to get current to flow across the semiconductor material.  This minimum voltage required to push current across a semiconductor is the junction threshold voltage.  The junction threshold voltage is different for each type of semiconductor material.

The junction threshold voltage for a germanium diode is 0.3 volts.  For a silicon diode it is 0.7 volts.

As we talked about in lesson 23, because of deficiencies in design, an electric component may have characteristics of another component.  Semiconductors and vacuum tubes are victim to this as well.  When AC is applied to these components, unintentional capacitance can build up within the component.  There are a couple different design developments which help disperse this capacitance:

A Schottky diode is a diode that has a piece of gold or platinum added to it which helps disperse the capacitance which builds up across the PN junction.

A vacuum tube has a screen grid placed between its grid and plate to disperse the capacitance that builds up between those two parts.

Vacuum tubes and transistors have some functional parallels.  The example that is brought up in the exam questions is the common functions between a vacuum tube and a Field Effect Transistor (FET).  Each have three leads.  A triode vacuum tube has the cathode, grid, and plate.  A FET has the source, drain, and gate.  The gate on a FET and the grid on a triode vacuum tube serve roughly the same purpose in controlling current flow through the component.

The other deep question in this section is the regions of a bipolar transistor.  The three regions you need to know are the cut-off region, the saturation region, and active region.  The transistor's region is determined by the polarity and amount of voltage applied to the various leads.  What you need to now for the exam is the cut-off region does not allow any current to flow through the transistor.  The saturation region allows current to flow freely through the transistor.  The active region is somewhere in between.  When the transistor is acting as a switch, you only want the transistor to work in the cut-off (off) and the saturation (on) regions.

Finally there's a bit about batteries.

Carbon-zinc batteries are disposable and should never be recharged.
Nickel Metal Hydride (NiMH) batteries are rechargeable.

Like I said, this lesson is all over the place.

If you have any questions, comments, or suggestions, please feel free to leave them in the comments box.

73,
Andy
KE4GKP