We have learned a lot about transistors and how they function. We know that these are bipolar active electronic devices used for switching loads connected across their relevant pin outs in response to a small input signal.
Basically transistors have three leads, namely base, emitter and the collector. The load is always connected at the collector pin of the device, in series with the respective potential.
The emitter is normally connected to the respective reference potential level, either to the ground or the positive, depending on the type of the device (NPN or PNP). The base is the pin where thetrigger signal is applied which is in the form of a small DC potential.
In response to this signal the load voltage from the collector is instantly pulled to the emitter lead and the respective common reference level, switching ON the load. This load is normally a DC load.
However since the current of this DC load will depend on the specs of the particular transistor, becomes quite limited and if the load is heavier will require more current for switching optimally. This is normally achieved by connecting the transistors in parallel.
We discussed the pin outs of transistors and connecting them in parallel is all about connecting the identically assigned pins together; however the base of each device will need a separate individual resistor to be connected. This resistor should be appropriately calculated.
However one big problem with these devices is that when they are connected in parallel, the current through the group may vary due to the uneven specs of each of the device, which in turn starts making a particular device in the group heat up more than the others, this leads to an increase in current dissipation through that transistor which again results more heating up of the device.
The process continues until the device becomes overheated and damaged permanently. This is called current hogging or thermal runway in transistors. To avoid this, normally a small value resistor is connected with the emitter lead of each transistor which helps to stop the transistors from getting into the above situation under any circumstance.