Transistor Guru's Needed, Help me, Help the community

[burners]

Recently I took on a rather ambitious project which entails controlling some PC fans based on tempurature and other variables.
I broke the project up into sections and have each section done except controlling the fans. What I'm confused about is Transistors in general.

Just to give some background, I'm a level 3 sys admin and am extremely knowledgable in PC's but not electronics. I have some programming background and have always had a love for electronics but only recently got into the hobby seriously.

I have read a few books to help build up my electronics knowledge and since I know VB and not C which is what most Micro's use, I have also been learning C++ and C# for use on .Net Micro controllers like the FEZ. Currently I am playing with a few Arduino's because they appear to be able to do what I want and are easy to program.

As I finish each section of my project I have been detailing the steps to post on my website so others can learn from my experience. I would like to give a lesson on Transistors for the next step in my project but I myself have some questions, so I was hoping I could get some answers from experienced individuals which effectively will be compiled into a nice lesson for others to learn from.

I understand a few things and will rattle them off as I understand them, if I am wrong please correct me. I also have some questions I would like help with.

1- I get the NPN and PNP part, from what I get NPN means a positive Base will connect the input voltage on the Collector to the output line of the Emitter, is this correct?

2- For an NPN Mosfet Transistor a positive gate will switch a negative source input connecting the Source to the Drain output, is this correct?

3- PNP would be oppisite of these, correct?

4- From what I get it seems like a transistor is used as a switch basically but what confuses me is I read something about the gate / base voltage needs to be different by an exact measure when compaired to the source voltage. Can someone explain this better?

5- For Mosfets and transistors I see some have faster switching compabilities which I get, these would be good for PWM control, what I dont get is the Voltage they are rated for. The Amperage makes sense, its the nominal and max Amperage the part can handle but the voltage on most of these are 25 Volt or 100 Volt etc.. Does this mean these can't be used for a 12 Volt source?

6- Based on this link given below what is the difference between what they categorize as a "Power" transistor and a "Mosfet" transistor? I get what each is but I thought Mosfets where best for power switching so why do these "Power" transistors make them good for power applications?
http://www.futurlec.com/Transistors.shtml

7- Based on the part linked below, the specs for this transistor are 50 Volt (Drain & Source), 33 Amp continuous. Does this mean I need 50 Volt or can I use 12 volt? If I can use 12 volt will the part handle more Amperage since its less voltage?
http://www.futurlec.com/Transistors/BUZ11.shtml

7- Based on the same part, the specs for the gate are "Gate Threshold Voltage" "min 2.1" "Typ. 3" "Max 4" "Unit V". I'm guessing this means a minimum of 2.1, typical 3 and maximum of 4 volts is needed for the gate to connect the Source with the Drain, is this correct?

I have some more questions but these are it for now.
Like I said I plan to share what I have learned which is why these questions are so broad. If your interested in my exact project and need for this knowledge I plan to be working with a 12 Volt source and I have a microcontroller which puts out 5 volt. I want to use the 5 volt line to switch on and off fans that will run on 12 volt BUT it goes deeper than that.
First I will take the incoming 12 volt and use a voltage regulator to regulate it down to 9 Volt for the microcontroller (Already built this part) then the 12 volt will branch off to a Voltage Booster which will Boost the voltage to 14 Volt.

The reason for this is because if I worked with 12 volt to the end of the circuit by the time it hit the fans it would be around 11 volt due to voltage drops and effeciency of the electronic parts, I also decided that the math for losing amperage while gaining voltage is exceptable in this case because the maximum amperage draw of the fans is far less than the power supply can put out. In other words I have plenty of amps to spare but need a few more volts.
I would like the voltage to hit the fan at around 12.4 volts max. Most PC fans can be driven with up to 14 volts and minimum 12.5 stable and safe.

From there will come the fast switching high amperage mosfets, these will be driven with PWM to switch the 5 volt or lower signal from the micro to the 12 volt or higher voltage needed for the fans. I also will add water pump support so these mosfets need to handle up to 30 watts each or aproximately 2.5 amps @ 12 volts.

Thanks Everyone

 

[NickS]

I will answer your questions in a simple fashion but I don't think it qualifies you to offer a tutorial to anyone on transistors.

1- I get the NPN and PNP part, from what I get NPN means a positive Base will connect the input voltage on the Collector to the output line of the Emitter, is this correct?

These are BJT type devices. the letters refer to the stack of doped materials. P being positively doped and N being negatively doped. There is a lot to be discussed about energy bands here but for the sake of simple analysis lets say you need both voltage and current flow to turn on a transistor. The voltage difference between C and E is the pressure to make current flow and the current flowing into(NPN) or out of(PNP) the base is what allows the flow between C & E to start. Now the amount of current allowed to flow through the base dictates the proportional amount that can flow from CE by the gain of the device. High base current and you are usually at saturation or fully on. But at a much lower bias current you would see amplification without saturation.

2- For an NPN Mosfet Transistor a positive gate will switch a negative source input connecting the Source to the Drain output, is this correct?

You don't use the NPN/PNP description for MOSFET's. They are classified as N-type or P-type. A MOSFET is fundamentally different from a BJT in that no DC current flows through the gate. Instead a voltage on the gate of a FET creates an electric field that inverts the channel between D & S which allows electrons to flow when there is a voltage diff from D to S. Again just applying a voltage at the gate does not ensure the device is fully on or fully shorting between D & S. The magnitude of the voltages involved plays an important role.

4- From what I get it seems like a transistor is used as a switch basically but what confuses me is I read something about the gate / base voltage needs to be different by an exact measure when compaired to the source voltage. Can someone explain this better?

There is a lot of theory in the answer to this question that deserves more time than I am willing to put forth. I will say transistors are often used as simple switches in which case you want to either turn them all the way off or fully on. Their is a region that all transistors can opperate in called the active region which is neither off nor fully on. Many important functions happen in this region but for switches this is a bad place to be. With a MOSFET here are the basic rules.
OFF: Vgs < Vt
On Soft(active/triode region): Vgs > Vt, BUT Vds < Vgs - Vt
On Hard(saturation region): Vgs > Vt, AND Vds > Vgs - Vt
Once the device is on(in either region) Increasing the Vgs will reduce the internal resistance of the device.

5- For Mosfets and transistors I see some have faster switching compabilities which I get, these would be good for PWM control, what I dont get is the Voltage they are rated for. The Amperage makes sense, its the nominal and max Amperage the part can handle but the voltage on most of these are 25 Volt or 100 Volt etc.. Does this mean these can't be used for a 12 Volt source?

The rated votages are the max the can handel before suffering damage. So a FET with a 50V, 10A rating can at most handel 50V diff from D to S without damage. This voltage is usually different from the max that can be seen from G to S. The current rating is the max current that can flow from D to S but this can be misleading if the designer does not adequately saturate the device or heat sink the package which is why there is also a max Power dissipation rating.

6- Based on this link given below what is the difference between what they categorize as a "Power" transistor and a "Mosfet" transistor? I get what each is but I thought Mosfets where best for power switching so why do these "Power" transistors make them good for power applications?

Power FET's focus on being used in saturation for high power. So they are not meant for use in the triode region and they usually have very low internal resistance(Rds on). A more general purpose FET may have good linearity in the triode region and may be better suited for low power applications. Power transistors that are BJT's go along the same lines they are designed to handle higher currents than a general purpose BJT. Whether you use power BJT or power FET depends on several factors. I think FET's can handle higher voltages than BJT's for one thing. BJT's can be difficult to turn off quickly to when conducting high currents.