I have been able to grasp some knowledge about Transistors through reading so far:
I been through the theory behind transistors. Helpful in understanding its operation.
Transistors can be used as a switching device.
Transistors can be used to amplify current.
Based on my application can i request some kind help from you guys, in understanding more throroughly the right application to be used for transistors.
CASE 1
I have a small fan with 5V supply and drawing 160mA current. If using a transistor with the fan should be switching type if an external power supply is to used with a high current rating right ?
If i need to use the very same power supply from the arduino itself should i be using the amplification technique ?
CASE 2
2 Brushless dc motor 24Vdc and current rating 160mA. Same here should i be using the transistor as a switch because definitely i should be having an external power supply ?
Another question, can i connect both Fans through only 1 output pin from the duino board using 1 transistor and 1 reverse biased diode ?
Since your application is to switch the fans, then that is how you should choose your transistor. You won't care too much about linearity, but you will be concerned about your Rds(on) values so that the transistor doesn't end up being a space heater. I suggest using some kind of logic-level FET to do your work for you. While being overkill for this situation, something like an IRL530 would easily be able to control several fans from one I/O pin. It's a decent general purpose FET to have in your junk box. 160mA is just a bit large to be switching with a 2n3904 or 2n7000, but those are good transistors to have in your junk box too.
When a transistor "amplifies", it is only able to supply as much "power" as the supply has. There is no magic. Think of it like a faucet valve; with a minimum of effort you can regulate a much larger "pressure". A small current thru the base creates a (hopefully) proportionately larger current flow thru the collector and emitter. When used as an amplifier, linearity and distortion often become the primary factors you are concerned about when picking a transistor. It goes without saying that power capacity is the first consideration.
In addition to what af said, whenever controlling currents greater than 100 or 200 mA or so,
you should definitely think about MOSFETs rather than bipolar transistors. Also, be certain to
get the "logic-level" devices, such as the IRL530 he mentions, as they will work with 5V input
on the gate, and not the IRF530/etc devices (ie, no "L" in p/n), as those don't turn on fully
with < 10V on the gate.
Transistors can be used to amplify current.
In regards this statement, for bipolar transistors used for switches, it may help to think more
along the lines of "using a small current [Ib = base current] to control a large current [Ic =
collector current]. Arduino I/O pins can only source about 40 mA, so you cannot drive
high-current devices directly.
Also, MOSFETs and other fets work very differently, and use the input voltage rather than
current for control. You might do some background research on the differences between
bipolar and MOSFET.
taz3m:
Another question, can i connect both Fans through only 1 output pin from the duino board using 1 transistor and 1 reverse biased diode ?
No, because one is 5V and one is 24V. You can have one pin drive two different transistors / fans / diodes though (for NPN
use separate base resistors, for MOSFETs a base resistor can be shared - this is the difference between
current and voltage control).
Transistor as an amplifier is the most common usage, imho. Even the young generation of EE thinks a transistor just switches something on/off (ie. within a CMOS processor)
