For most practical purposes, (though likely few technical ones) bipolar transistors are electrically activated amplifying switches. Allow a certain current to flow into one lead (called the "base") and a proportionally larger current will flow through the remaining two leads. (from the "collector" to the emitter") Limit this current and use it to drive a load (LED, relay... etc) and you've got an electronic switch. This basic concept is good enough for most applications, assuming you can figure out which lead is which.
There are two different types of bipolar transistor, the NPN and PNP. The schematic symbols for each are below. Note that the NPN transistor has an arrow pointing out the emitter, which the PNP has an arrow pointing in.
Both Bipolar transistor types are pictured below. The transistor on the left is an NPN type and the one on the right is a PNP. Being both in the TO-92 package, they look identical. They are not.
Before attempting to explain the differences between NPN and PNP transistors, it may be useful to understand how the NPN transistor is used. Below is a schematic for a simple circuit which uses a NPN transistor to switch a LED.
When SW1 is open R2 and R3 form pull-down which keeps the base a low potential. With the base held "low" the path from collector to emitter does not conduct very well and the LED will not glow. If SW1 is closed, the base is pulled high and the transistor conducts, making the LED glow. Quite simple really, but there are a few points to consider.
PNP transistors function in a similar manner, but the polarity is reversed. While for the NPN transistor "current will flow from the collector to the emitter when a positive voltage is applied to the base (with respect to the emitter)", the PNP transistor "will conduct from emitter to collector when a negative voltage is applied to the base (with respect to the emitter)".
The same circuit can be adapted to make use of a PNP transistor.
Note that the collector is now connected to ground because the pathway from collector to emitter works the other way around in PNP transistors. (this confused me for years) Because the emitter is now more positive, pulling the base "low" (when the switch is closed) will create a negative potential between the base and the emitter and the LED will glow. Confused?
Perhaps the most often used bipolar transistors (in New Zealand anyway) are the BC547, BC548, BC549 (NPN) and the BC557, BC558, BC559. (PNP) These transistors are all similar, although often given an A, B or C suffix on the package or even a different prefix. (like DS548) These transistors are rated at 100mA for the collector to emitter pathway and will occasionally explode if this is exceeded.... They are perfect for driving LED's, small relays and buffering logic signals.
If 100mA is not enough for your needs, look around for a BC337, BC338 (NPN) or BC327, BC328 (PNP). These parts usually come in the same TO-92 package but can handle up to 800mA. Beyond this larger packages (such as TO-220 or TO-3) are used and heatsinks are often required.
Note that you can still destroy transistors (or any component) by exceeding the power rating of the package without actually exceeding the current rating. For example, the BC337 has a collector power dissipation rating of 625mW and a collector current rating of 800mA. The voltage dropped across the collector to emitter pathway (two diode junctions, or at least 1.2V) represents energy lost as heat which must then be dissipated into the surrounding air by the component package. If P = V * I then with a load of 750mA the transistor will have to deal with 900mW, which exceeds the maximum rating of 625mW. If this loading is maintained for extended periods then junction temperature will likely exceed the rating of 150 degrees celsius. Meltdown...
The circuit below is called the "flasher", or a "multi-vibrator" in the absence of the LEDs.
When this circuit is powered on, both capacitors begin to charge (through each each LED, associated resistor and the base-emitter pathway in each transistor. Because no two components are the same the two capacitors will charge at difference rates and therefore one transistor will turn on before the other.
Say that C2 charges slightly quicker than C1, turning on Q1. When Q1 conducts the positive terminal of C1 is pulled to a low potential, just as if Q1 was a switch connecting it to ground. Because C1 was charging it will have a few volts across it, meaning the negative terminal (and therefore the base of Q2) is now at some potential below ground because the positive terminal is connected to ground. (Yes, this is possible, and can even be exploited to generate negative voltages or voltages above the supply rails) Q2 is now well and truly turned off, and will not turn on until C2 discharges through R3, LED2 and R4. Once the potential across C2 drops below ~0.6V Q1 will turn off and the cycle will start again for the other side of the circuit.
The time which each transistor conducts for can be adjusted by tweaking the values of R2 and R3, which is the discharge path of each capacitor. The transistors can switch any load up to their rated current, such as relays are small lamps.
If you have any comments or questions please don't hesitate to contact me.
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