Electronics > Beginners

transistors

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ledtester:
Here's how to perform a "sanity check" on your transistors with the diode test function of your multimeter:

https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07

One of the "$20 component testers" (which you can get now for ~ $10) is also a convenient tool to have. Search ebay/aliexpress for "component tester".

Even the much poo-pooed "hfe transistor tester" port found on many cheap multimeters is useful:



When the transistor is good and plugged in correctly you should get a number >50 for a small signal transistor like the BC547.


magic:

--- Quote from: Circlotron on August 15, 2019, 12:51:05 am ---But if the inductive motor load is in the emitter circuit it doesn't need any clamp diode for over voltage protection. When the transistor turns off, the motor will try and pull the emitter below earth (base) potential, partially turning the transistor back on and clamping the emitter voltage to 0.6V or so below base voltage.

--- End quote ---
Yeah, the 10k resistor to ground saves the day when emitter falls below Ic/β*10kΩ, a few V negative. Actually, it could be a clever way to quickly brake the motor by applying maximum reverse voltage the transistor can withstand if 10k is replaced with higher resistance or simply a 30V zener. What's the pulsed SOA of BC547? :D

Jwillis:
Transistors functionality is dependent on application.What are you using it for ? What voltages are applied and how much current is pass through the transistor. For example your BC547 has a maximum voltage rating of 50V and a maximum current that can pass through  ("Drawn by the load ")  is 100mA.
The Hfe (AKA beta) is the gain of the transistor .Many factors can alter the gain but for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base. Sometimes there will be a letter following the BC547 like "A" ,"B" or "C" This is the hfe classification .This means for example that a BC547A will have a hfe "Gain" of between 110 ~ 220.
So in "Common Collector" mode  if  1 mA is applied to the base then the transistor will pass current across the collector  to the emitter between 110ma - 220 ma which would ,of course, exceed its maximum current capability.As you can see it doesn't take much current at the base to drive current across the collector and emitter. So it won't take much to burn out a transistor quickly.You also need to remember that the current at the emitter is the current at the collector Plus the current at the base . An easy solution is to place a resistor at the base that limits the current to the base.So as in Ohms law Voltage divided by the Resistance equals  the current that will pass across the resistor to the base of the transistor.
You have to consider that current requires a Load.Load "draws" current.And the transistor can also be considered a load at the base.So by using a limiting resistor, the base will only "draw" as much as the resistor will allow.So if your supply is 10 volts and you place a resistor of say 100K ohms at the base then there will be 0.1 mA drawn across the resistor to the base. So 0.1mA times a "gain" of 110 equals 11mA from the collector. And 0.1mA from the base plus 11mA from the collector equals 11.1 ma at the emitter. No matter how much current is available from your power supply that transistor will only pass 11.1mA.

Other modes include Common Base and Common Emitter which have different functions .

Playing with transistors can be very interesting. Even blowing them up is educational because you can see limitations and what can happen when limitations are exceeded .Failure can be as informative as success.Just wear eye protection.

Without getting in a conflict about components from Ebay .I know from experience that they're just fine and have always matched design parameters.The only time they fail is when I've done something wrong .Digikey is a good source as well ,but the shipping charges is what puts me off.Maybe it's better in the States I don't know.

If your interested ,as ledtester mentioned,there are inexpensive transistor testers from Ebay  available that are good starter tools for the novice .They will tell you what pins are what and the "gain" .Plus they will test other components such as diodes ,capacitors and resistors .Very handy tools.

Circlotron:

--- Quote from: Jwillis on August 15, 2019, 05:24:12 am ---for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base.

--- End quote ---
Not trying to be pedantic, but with a gain of 110-800 a *change* of base current causes a *change* of collector current that is 110-800 times as great as the base current change. Or was that allowed for in your simplification?

queennikki1972:
So I tried the flashing light circuit on page 104 of Getting started in electronics. Unfortunately the only 100k variable resistor i had was used pull from old stuff and was a little flaky. It did flash properly for a minute so it must have been ok. I will order more parts and keep doing the same circuit until i figure out what im doing wrong. I will also try more expensive parts from digikey as well.
I do have one of the component testers as well. And the parts test ok before using.

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