Electrical Faux Pas

[wattsup]

@z_p_e

Well I'm back again and I will keep this one generic, hoping your help will be good for use elsewhere. This is about transistors.

I initially though transistors were miniature on/off devices (like relays) but it seems this is not the case at all. Am I correct to say that depending on the voltage applied to the gate as a percentage of the rated voltage of the transistor, the collector will conduct towards the emitter that percentage of the signal coming from the collector to the emitter. Would this be a correct assumption. Then why would you also call it an amplifier?

So if you wanted to use a transistor as a 100% on/off component, you would still need to pulse the gate 100% on/off, which would still require more control via a timer or oscillator to pulse the gate. Is this right.

My next question will obviously then be about oscillators.

[z_p_e]

Transistors come in a few forms:

BJT
JFET
MOSFET

There is a reason the British call triode tubes "valves", because in terms of electric current, that's exactly how they behave. I will try to be simplistic in the following description.

Think of the above 3-terminal devices as current valves. They all have a pin in which we supply with a voltage/current (the input end of our valve), a pin in which the voltage/current exits (the output end of our valve), and a pin that controls the amount of electron flow from the supply side to exit side (the control valve itself).

Think of the "gain" (amount of amplification) of one of these devices as the ability to control a potentially large amount of electron current (input end to output end) with a much smaller amount of current (or voltage) via the Base or Gate.

BJT's (bipolar junction transistors) are "current controlled current sources". In other words, a small change in current on the Base, will result in a much larger change of current through the device from end to end (Collector to Emitter). This is how they achieve gain or amplification.

JFETs and MOSFETs are "voltage controlled current sources". In other words, a small change in voltage on the Gate, will result in a much larger change in current through the device from end to end (Drain to Source). This too is a form of gain, even though the control and output units are different.

So you can see that all these 3-terminal devices are really current devices (even tubes). Everything is based on electron current flow from one end of the device to the other, and the amount is controlled by the "valve" terminal (Base or Gate), which can be a voltage or current control, depending on the device.

Voltage output is developed by circuit topology, and is generally due to current through a resistor.

In order for there to be control of the electron flow, the control input (Base current or Gate voltage) must be in reference to one of the device's other terminals, and that is via the Emitter (BJT) and Source (JFET/MOSFET).

Each device has a "turn-on" voltage, which means for a BJT, there must be about 0.7V between the Base and Emitter, and about 4V between the Gate and Source for MOSFETs (MOSFETS generally need more like 10V to fully turn them on, despite the 4V spec).

When the devices are fully "ON", they are considered to be in "saturation". What that means is the magnitude of current through the device has reached a maximum, and will not increase any further, even if the control input is increased.

By causing these devices to alternate between fully OFF and ON (cutoff and saturation), they are acting like a switch. If you placed a 20W light bulb in series with this "switch", you could then turn this 20W bulb ON and OFF by using only a few milliamps (BJT) of current to do so.

I hope that sheds some light on how BJT's and MOSFET's work.

Oscillators are a more complicated topic to describe, but post your question and we'll see what comes of it

Cheers.

[wattsup]

@z_p_e

Thanks for your last response as it has cleared up many questions. If ever you get tired of helping please feel comfortable to stop any time. I could go on and on, so again thanks for everything. Your one good explanation is worth hours of mulling through so much info and still getting lost.

1) Can an emitter of one transistor be fed to the base of another transistor?

2) Do zener diodes work like regular diodes but that they only let current pass when it has reached a certain voltage. If yes, is this like an on/off valve that opens when a minimal voltage is reached or do I have this backwards. Do they consume much power?

3) I have an oscillator with four pins on it. How do these work?

4) Toroidal ferrite cores come in a variety of sizes and values. Do the values indicate the amount of magnetic field they can emit and if not, please explain how we can understand ferrite specs.

z_p_e I know I have many question so please answer any you want and when you want. There is no rush.

[duff]

@wattsup

You may be interested in following series of Free books. They are well written from a technicians perspecitve and will probably answer most of your questions :

Lessons In Electric Circuits
A free series of textbooks on the subjects of electricity and electronics

http://www.ibiblio.org/obp/electricCircuits/


"Volume III - Semiconductors"  will answer most of your transistor related questions.


-Duff

[z_p_e]

@z_p_e

1) Can an emitter of one transistor be fed to the base of another transistor?

Generally speaking, yes. A good example utilizing this type of connection is called a "Darlington Pair". Was there something specific you were thinking of doing?

2) Do zener diodes work like regular diodes but that they only let current pass when it has reached a certain voltage. If yes, is this like an on/off valve that opens when a minimal voltage is reached or do I have this backwards. Do they consume much power?

All regular junction diodes are zener diodes. In other words, regular diodes also have a zener voltage, but zener diodes are manufactured to have a specific breakdown voltage. "Zenering" occurs when the diode is reversed-biased to a sufficient voltage to cause a breakdown of the p-n junction, when normally, no current would flow. Regular diodes aren't used for zeners normally, because their reverse breakdown voltage is usually in the thousands of volts. Once a diode is in a "zenering" condition, further increases to the applied reverse voltage does not appreciably increase the voltage across the diode, and this is why they are used in regulator circuits.The amount of power dissipated by the zener diode depends on the excess voltage applied that is over and above its zener voltage, times the current through the diode.

3) I have an oscillator with four pins on it. How do these work?

Not sure what you are referring to here. You'll have to be a lot more specific..i.e. what part number etc.

4) Toroidal ferrite cores come in a variety of sizes and values. Do the values indicate the amount of magnetic field they can emit and if not, please explain how we can understand ferrite specs.

Recommend you download and read this:
http://www.allegromicro.com/en/Products/Design/arnold/coretran.pdf
http://www.arnoldmagnetics.com/mtc/pdf/SoftMag.pdf (same as above)

There are folks here that are more knowledgeable than I on the subject of magnetics and cores, perhaps they can chime in and provide a hand.

In regards to your questions, no problem, keep them coming. I will help out when and where I can. Others, feel free to contribute as well. Thanks duff for the link.

Cheers,
Darren