Electronic Basics #22: Transistor (BJT) as a Switch

in my previous basics video I showed you that many ICS consist of at least one op-amp but if we go even deeper and inspect the schematic of an op-amp itself we can see familiar components like resistors capacitors and diodes but there are two symbols which seem unfamiliar so far those are so-called bipolar Junction transistors or BJT for short which come in two variations as NP n type or PNP type in general those can be used to either act as an electronic switch to for example turn your high-power idea on and off through your Arduino or to amplify an analog signal to for example drive a loudspeaker so in this video I will show you the basics on how to use such a BJT as a switch in your circuits without releasing the magic smoke let’s get started first off I grabbed my soon-to-be electric switch the BCE 6 3 7 & PM BJT and the load I want to turn on and off in this case a white 1 watt high power LED warper forward voltage of 3.1 volts the three terminals of the transistor are called a metal collector and base and since they come in a variety of different packages you should always check beforehand which pin is which in the datasheet for my symbol switch circuits I connected the emitter to ground and the load between the supply voltage of 3.1 volts and the collector but you should always be careful that your supply voltage does not exceed the maximum rated collector emitter voltage of the transistor next I powered up the circuits and notice that no collector current was flowing so far because the bjts collector current is the product of its base current and the value Bella which can be found in the datasheet as current gain so let’s fix that by connecting the base straight to the supply voltage which let’s indeed a rather big base current flow but also destroys the transistor along the way the reason for that is that the base emitter path basically consists of a diode bubble forward voltage of around 1 volts thus every voltage above this one volt value will increase the current drastically so we need a current limiting resistor to keep that under control to calculate the necessary base current though we possibly need the worst-case value of Bedard to keep the circuit conservative then we can calculate the necessary base resistor and choose a real life one which is the closest to the calculated value and just like that the switch circuit seems to work fine but not perfect because our collector current is smaller than what we estimated the reason is the voltage drop across the collector emitter path of the transistor which exists in the saturated States and becomes even bigger in its active region as an amplifier and in order to control the circuits whoopi arduino you simply need to recalculate the base resistor value with a 5 volt control voltage but let’s assume now that the load is not connected to the supply voltage but to ground instead this way the NPN transistor is not a decent choice for such a switching application thankfully though there exist the PNP BJT which is placed in the circuit similar to the NPN but has all of the polarities reversed in a nutshell that means that a ground potential needs to get applied to the base resistor in order to switch on the loads instead of a positive voltage okay moving on now if I want to control a bigger loads like the six volts 21 Watts light bulb it is pretty obvious that the small PC is 637 Wooper maximum collector current of 1 amp cannot handle this job we need something ppm like this BD of 555 above a maximum collector current of a damns the calculations were pretty much the same as before but it is noticeable that even by calculating what the low is better value mentioned in the data sheets I did not reach the collector emitter saturation voltage to do that I need to get closer to the maximum rated base current of 1 amp because the current gain always drops quite a bit whoop I hire collector currents but nevertheless the circuit did work in the end which was quite hot literally the transistor easily reach a temperature of around 70 degrees Celsius after only a couple of seconds the reason is the power loss through the base current and the current through the collector emitter path which is combined around 6 watts and that is actually the biggest disadvantage of bjts as high current switches the power loss and thus the lower efficiency of the circuits and since my base current requires around 420 milliamps I can also not easily use my Arduino to controller directly but luckily they exist so called Darlington transistors like this TI p 142 which mainly consists of two bipolar transistors this way its current gain is a lot bigger and thus the transistor only needs a small base current of in this case eight point five milliamps in order to switch on and off our three point eight and loads which is even small enough to sync the current directly from the arduino that is why they’re often used invariably bench power suppliers to regulate the output currents but on the other hand they feature a bigger collector emitter and base emitter voltage which can lead to more power loss and with that being said you already know quite a bit about bipolar Junction transistors and how to use them as switches I hope you like this video if so consider supporting me yo through patreon to keep such videos coming stay creative and now we’ll see you next time


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