still kinda looking to learn to use transistors better, i currently only know how to use NPN as amplifiers... not sure about switching however... something about saturation... ? lol
If you know how transistors work as amplifiers then you should have no trouble understanding how they work as switches.
When transistors are used as switches they operate between saturation and cut-off, which is determined by the base resistor . At saturation the transistor is delivering max current to a connected load, in other words its acting similar to a closed mechanical switch. Cut-off is just the opposite.
But there are a few important things you need to know before you can design a circuit using a transistor switch. The key things are:
1. Power supply voltage
2. load current or resistance
3. the beta of the transistor
4. Max current ratings of the transistor
Its obvious that the data sheet for the particular transistor you are using would really come in handy.
Then you have to do some math to calculate the base resistor required for saturation. As a rough guide: For saturation the Beta of the transistor multiplied by Base current should be equal to collector current.
Beta x base current = collector current
OR
Ic = Beta x Ib
As a design example: Beta of a particular transistor = 100
Power supply = 12 volts
Load resistance = 50 ohms
Base resistor = ?
First find load current, this will also be collector current ( Ic ).
Since at saturation nearly all supply voltage appears across the load ( remember the closed switch analogy) the current thru the load will be:
12/50 = 0.24 amps
Then find Base current ( Ib ) required for saturation using a derivitive of the given formula:
Ib = Ic/Beta
= .24/100
= .0024 amps
Neglecting the small voltage drop of the base-emitter diode, the base resistor will be:
Rb = 12/.0024
= 5000 ohms or 5 K
This is the max value of resistance that will give saturation of this particular transistor, you can use a smaller value and it will still allow for saturation current ( within limits of data sheet ratings ), so a good standard value would be 4.7 K ohms.
Sorry for the long winded reply, but you asked for it . :wink: