It is common to build a circuit, power it up and -- ouch nothing works. Has it happened to you? It has certainly happened to me. I tested and found 4 BC547s in my stash today to be damaged. That's why this post. After this post you and I do not need to be in that situation. Lets test the transistor. Right now.
The prerequisite of any testing is: knowing what to expect; at least in the ball parks. So let's begin.
Transistors are devices that are called semiconductors. There is a reason they are called that way. Because when not connected to anything they are non-conductors of electricity (to a large extent).
First step to using or testing any transistor is to know its pins. So make sure you have identified the base, collector and emitter correctly. Check and double check before connecting anything to your transistor.
Second step is to know whether it is a PNP or NPN.
Now we are ready to begin.
If you measure the resistance offered by the collector-emitter of any transistor you are going to get a very large value. So large that your meter cannot measure it. It is as good as no connection at all. Remember - semiconductor is a non conductor when no current is applied to it base. So touching the leads of your multimeter to the emitter and collector should read - yes nothing. Thats a good sign. If you are getting some reading on the meter at this point there is a very good chance you have a very dead transistor.
Next measure the resistance between the base and emitter. And then again base and collector. Now the transistor is supposed to behave like a high resistance when touching the positive lead to the base in case of a n-P(ositive)-n [or negative lead to the base of a p-N(egative)-p transistor]. The actual value depends on the characteristics of the specific transistor but is in the range of more than 5 Mega Ohms typically. If your transistor reads in the order of Kilo Ohms then check the data sheet and throw away the transistor (or you can do the reverse as well). So if your meter is reading 5-10 MOhm when positive lead is connected to base of an NPN transistor towards both emitter or collector - It is good! (Negative lead to base for PNP of course)
During that above step you will notice that if you touch the positive lead to the pNp (that is the reverse orientation) the resistance between base and emitter or collector is again very high (meter cannot measure as good as an open connection) - That's also good. If your transistor conducts from base to emitter/collector no matter what the polarity is - again toss it in the bin.
There you have it - complete procedure to understand what to expect of transistors when working / testing them
The prerequisite of any testing is: knowing what to expect; at least in the ball parks. So let's begin.
Transistors are devices that are called semiconductors. There is a reason they are called that way. Because when not connected to anything they are non-conductors of electricity (to a large extent).
First step to using or testing any transistor is to know its pins. So make sure you have identified the base, collector and emitter correctly. Check and double check before connecting anything to your transistor.
Second step is to know whether it is a PNP or NPN.
Now we are ready to begin.
If you measure the resistance offered by the collector-emitter of any transistor you are going to get a very large value. So large that your meter cannot measure it. It is as good as no connection at all. Remember - semiconductor is a non conductor when no current is applied to it base. So touching the leads of your multimeter to the emitter and collector should read - yes nothing. Thats a good sign. If you are getting some reading on the meter at this point there is a very good chance you have a very dead transistor.
Next measure the resistance between the base and emitter. And then again base and collector. Now the transistor is supposed to behave like a high resistance when touching the positive lead to the base in case of a n-P(ositive)-n [or negative lead to the base of a p-N(egative)-p transistor]. The actual value depends on the characteristics of the specific transistor but is in the range of more than 5 Mega Ohms typically. If your transistor reads in the order of Kilo Ohms then check the data sheet and throw away the transistor (or you can do the reverse as well). So if your meter is reading 5-10 MOhm when positive lead is connected to base of an NPN transistor towards both emitter or collector - It is good! (Negative lead to base for PNP of course)
During that above step you will notice that if you touch the positive lead to the pNp (that is the reverse orientation) the resistance between base and emitter or collector is again very high (meter cannot measure as good as an open connection) - That's also good. If your transistor conducts from base to emitter/collector no matter what the polarity is - again toss it in the bin.
There you have it - complete procedure to understand what to expect of transistors when working / testing them
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