BJTs as Transistor Switches | AddOhms #10

switches like this allow you to safely turn lights on and off electronic switches look like this and this and this transistors are used to turn on lights control digital information in your smartphone and let you crank your audio all the way to 11 this Adams tutorial we're going to take a look at BJ transistors and how to use them as a switch this video is part of a two part series on transistors this one will cover bipolar Junction transistors or known as bjts while the next video will cover metal-oxide-semiconductor field-effect transistors more commonly nice fans generally you'll see a BJT for small loads say things less than one amp of current while MOSFETs are well suited for applications with much higher current check the show notes for a link to part two when it becomes available here's an example of ATO 92 package and the symbol for a NPN BJT there are three terminals which have the names emitter base and collector and when talking about voltages and currents at each of these terminals we use subscripts like BC which means the voltage at the collector or IB the current at the base to describe voltage between terminals we'll use two letters like vbe by the way this is where the common term VCC comes from it indicates the positive supply at the collectors of all of the transistors inside of an integrated circuit or IC back to the transistor itself let's say you have this transistor and apply a voltage at the base while connecting the emitter to ground which allows one milli amp of current to flow the key to this transistor is that when this happens 100 milliamps will be allowed to flow from the collector to the emitter the relationship between these two currents is known as the beta of the transistor if we take a look at an example data sheet like this one for the 3904 we'll see that the beta stated here as HFE is about 100 you'll also notice the beta varies under different conditions meaning it should be treated more as a guideline and not a rule using a simple formula we can estimate the maximum current the transistor will allow through its collector to the emitter the collector current is going to be equal to the base current times the beta of the transistor so how do you know for sure what that base current will be well you might have noticed there is a diode in the transistor symbol and that's what is actually inside of a transistor a couple of diodes remember from video number eight we learned that diodes need current limiting to work correctly since this is a standard diode the forward voltage is about 0.7 volts if we want to use a BeagleBone or Raspberry Pi GPIO pin the supply voltage would be 3.3 volts so this means the current limiting resistor will have to drop about two point six volts using Ohm's law we can calculate that we would need two thousand six hundred ohms so I'm going to pick a more reasonable value like two point seven kilohms as the resistor now that we've got this switch all set up let's add a small motor which requires six volts to operate so we'll use a battery this example demonstrates two capabilities of a BJT first it amplifies current by turning our one milliamp into 100 milliamps and second it allows us to operate as an electronic switch okay two quick notes first when using inductive devices like a motor or relay with a transistor you really need to use a suppression diode across that load this will protect the transistor from voltage spikes for a good general purpose diode look for a one-in-four zero zero one second this circuit as drawn only works with an NPN BJT a PNP type BJT works similar to the NPN except that the diode Junction is backwards which means the circuit will behave differently but that's a subject for another video check out the show notes at Adams comm slash EP 10 for suggestions on what parts to use more information on bjts and a link to in the episode on MOSFETs is available remember if you've got questions you can send them to us by Twitter Facebook element14 or by leaving a question or comment with this video below

Channel: AddOhms

Views: 703,080

Keywords: Electronics, Transistor (Invention), Bipolar Junction Transistor, Switch, AddOhms, 2n3904, BJT, NPN, 2n3904 led circuit, electronics 101, electronics tutorial, bipolar junction transistor tutorial, bipolar junction transistor example, 2n3904 transistor, 2n3904 circuit, bjt transistor, bjt transistor tutorial, npn transistor

Id: sRVvUkK0U80

Length: 4min 48sec (288 seconds)

Published: Thu May 08 2014