Electronics > Electronics

Question about transistors.

(1/3) > >>

Robotboy86:
I am building a basic H-bridge and was reading around a few places and saw(I believe on this site even) that transitors are more effient if I turn them on/off with higher voltage.  I.E At 5 volts it might turn on, but it would become much hotter then if I did it at say 40 volts?

If that is true, is there a voltage amplifying IC? 

KambeiX:
Hmm you are getting it wrong...

Normal transistors are current amplifiers, so a few mA in the base will give you a max of base current X gain (hFE)

However, all transistors have a saturation point and a cutoff point, so under certain current the transistor will act as an off switch and over certain current the transistor will act as an "on" switch.

BUT, normal transistors can only tolerate up to certain voltage at the base, so if you put too much current they will fail and won't give you any benefit, so you MUST use a current limiter resistor in the base depending on the voltage, gain, etc (you need to calculate).

On the other hand, there are FET transistors (MOSFET, JFET, etc) that are voltage to current amplifiers. In that case, higher voltage will give you a higher gain and 5V won't be enough for driving high power MOSFETS so you will need a driver circuit.


So 5V will be more than enough for some applications and too little for other applications.

 

Robotboy86:
I don't understand..  these mosfet voltage to current amp's work at any voltage up?  or what?

My understanding is that *all* transisitors require a certain base amount of voltage to turn on...    so?

Robotboy86:
"Another note on MOSFETS, they are more efficient with higher Gate voltages. So you probably do not want to just apply a simple binary 5V. Instead you should amplify it with say an operational amplifier or even better, a MOSFET Driver IC. All are quite cheap and easy to implement. I highly recommend amplifying the Gate voltage because it would save on battery power, and significantly reduce overheating. Which reminds me of another point . . . "

That is from this site..  it would appear he is saying it is better to hit it on with higher voltages??


Sorry if I am annoying but this is just an important point and I wanna make sure I understand it

Tsukubadaisei:
There are only 2 things you must understand. You must know that mainly 2 types of transistors exist,i.e., PN channel transistors and MOS-MET. And you must understand their basic principles.
Depending on the task and conditions the kind of transistors you must use is different. Not only being a MOSFET or PN but their sub-categories(like PNP or NPN or nMOS or pMOS or dep or enh or JMOS transistors) and specifications. For that you have to look in their datasheet. All the data you want is in the datasheet. You have to read the datasheet. There are MOS-FET are operational at 1.2V but there are some that would take 40V. There are lots of kinds of circuits including motor-drivers. Depending on your objective NPN transistors may be better than MOS-FET and so on. IF you are just learning the principles then focus on the principles. All those details will just drive you crazy. If you REALLY want to learn the principles then do like me and study some basic qantum physics by yourself, then you can trust me, transistors(and diodes) will never be a problem anymore.

Navigation

[0] Message Index

[#] Next page