a question about transistors

[neptune]

@Bizzy .The actual voltage on the base depends on how "switched on " you want the transistor to be . A transistor is not like a light switch which is either on or off . Varying the base voltage regulates the current flow between collector and emitter .That is how the transistor works as an amplifier .Hope  this helps .

[Bizzy]

@Bizzy .The actual voltage on the base depends on how "switched on " you want the transistor to be . A transistor is not like a light switch which is either on or off . Varying the base voltage regulates the current flow between collector and emitter .That is how the transistor works as an amplifier .Hope  this helps .

Hi Neptune
So if I put more voltage through the base more voltage will be allowed through the collector to the emitter. IS that correct?
Thanks
and thanks for your patience with my questions.
Bizzy

[Feynman]

It depends on whether it's NPN or PNP transistor, which determines how you should hook it up... what you are trying to drive?

Here is an example of a transistor circuit which can drive a relay (energize 9V or 12V relay coils based on a small input signal like a 555 timer)...

http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

I have built this circuit myself with parts from Radio Shack.  Hope this helps. 



P.S.   R1 going into the base of the transistor should be in the ballpark of 4.7k , depending on your current requirements.

[neptune]

There are guys on here who know much more than I and Feynman is a good example , but having said that one needs to start at the beginning . You stated in your last post , " the more voltage on the base , the more voltage between collector and emitter . " Not quite true . The higher the voltage on the base , the higher the CURRENT between collector and emitter . Think of the collector and emitter as the leads of a variable resistor . the value of this resistor is controlled by the voltage on the base , within certain limits . The name transistor is an abbreviation for TRANSFER RESISTOR . If you are using the transistor to drive a relay , you are really using it as a switch . To switch it on , your base voltage needs to be high enough to pass enough current through the relay to operate it . Build a circuit with a cheap transistor , instead of a relay , use a low voltage bulb with a resistor in series with it to limit current . Apply different low voltages on the base , and watch the results . On a diagram , a NPN transistor has the emitter arrow pointing out ]Not Pointing iN] That's how I remember it .

[fritz]

Maybe I can sum it up in a view word....

Transistor:
Amplifies current.
A transistor has a significant parameter which gives the current amplification.
This parameter is B/Beta B as plain DC amplification and Beta for AC compounds on top of DC bias.
B is in the range of 150 to > 500 for small signal tranistors and <150 for power transistors - can be even <=10
Additional - the B-E thing is a diode.
For a silicon transistor you have a voltage drop of 350-800mV on the B-E path - depending on the current you feed into Base to Emitter.
Lets take a power transitor with B=50.
If you feed a current into the Base with 100mA -> you can drive a C-E current of 5Amps.
If your load - is in away that its not possible to drive more than 5Amps - even a current of 200mA will not cause more C-E current. In that case you speak about "saturation". Transistors are not switches - even if you put a multiple of the necessary current into the Base - there will still be a voltage drop between C-E. This is the saturation voltage VceSat (another important parameter).
Darlington transistors are 2 transistors within one case which helps to get high current amplifications -even for power transistors. The driver transistor has high current amplification - and the power transistor lower one.
The sum current amplification is B1*B2 - results in a sum B of >500 or even > 1000. Because of the nature of a darlington - The B-E path consists of 2 diodes in series - so the minimume base voltage is >700mV

If it comes to switching - mosfets are the better choice.
They are kind of voltage controlled resistors/current sources.
You need no input current - just an input voltage is enough -
and the Drain-Source voltage can be almost zero - depending on the Rds "on" (thats the resistance between drain and source) and your load current- if switched on.
Depending on technology - the voltage needed between gate and source for switched "on" can be 6V down to 1.5Volts.
The gate can be somewhat seen as tiny capacitor. If its loaded the mosfet conducts, if empty the circuit is open.
Power mosfets have a big capacity between Gate and source >10nF, smaller ones as low as 100pf.
To keep switching losses at minimum - the driver has to fill this cap quite fast - otherwise it may take some time until Rds goes down from open to almost zero - which will cause heating.

So you need energy to keep a transistor conducting -
A mosfet needs only control energy on switching on - or off.
If in on state - a mosfet needs no further input current.

To optimize switching with transistors and mosfets - you use driver stages with assure that the proper current/voltage is fed into base/gate to minimize switching losses. Additional there are effects that the load current influences your base or gate impedance - keeping parasitary effects in mind.