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Topic: Transistor selection (Read 719 times) previous topic - next topic

illbatt

Im puting a project together that includes reading 12v signals form a tachometer and a reed-switch.

Testing the tacho signal I found out that the C549C-transistor works fine without resistors so I think ill use that.
I also have some BC337-transistors so my question is:

What is the difference between those two transistors, can I use either ?


oric_dan

Did you look at the datasheets?

illbatt

Yes, but they dont tell me much since I dont have any real knowledge of electronics.
This is a one time project that is working fine and I just wanted some advice without reading all about transistors.

lax123

you dont have to read all about transistors, how about some basics and when and why to use a base resistor

oric_dan

OP, it would have been nice to include the links to the specific devices you were looking at.
I downloaded datasheets for BC549 and BC337,

http://datasheetz.com/data/Discrete%20Semiconductor%20Products/Transistors%20(BJT)%20-%20Single/BC549C-datasheetz.html

http://www.fairchildsemi.com/ds/BC/BC337.pdf

The most important parameters to consider are probably
BC549, BC337:
NPN, NPN
Ic max = 100 mA, 800 mA
Vceo max = 30V, 45V
hFE = 100-800, 100-250


For general use, they're fairly comparable transistors, and both should work in most ckts.
The BC337 has a better current capacity.

However, your comment about "works fine without resistors" leads one to wonder, as
you basically never see bipolar transistor ckts without some Rs in there.

illbatt

Thanks for a useful answer.
I will look up what hFE and ic max is.

The gadget is for my 91 Harley.
It will include a si4703 radio, a realtimeclock some buttons and a lcd.
It will be connected to the "old" tacho signal so I think thats ok (no resistor between the signal and the transistor).

Im not shure about the reed switch and will check that up when the temperarure outside is better (-10c now).

I will use the arduino mini pro but im testing things on a uno.
Its all built on "boards".

pwillard

Quick Note about Hfe

Hfe is one of those things that often confuses someone just starting to ask the question "what is a transistor anyway?"

Another word for Hfe is ? (beta)... these odd names are related to the mathematical models used to define how a transistor works... It can also be called "gain".

Gain, being the word least associated with a "math" formula,  is essentially how much does this particular device "transfer" what happens at the Base pin to the other 2 pins.  With "transfer" being one of the base words that make up the word "transistor"... maybe this starts to make more sense?


fungus


Yes, but they dont tell me much since I dont have any real knowledge of electronics.
This is a one time project that is working fine and I just wanted some advice without reading all about transistors.


Read this: http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt_theory.html

It has all you need to know and it's only two pages long.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

illbatt

Thanks again for a useful answer.
Thats was an interesting tutorial for me and im goint to read it again tonight.

Docedison

That is also one of the best tutorials I have ever seen in 50 years of looking...
It Should be mandatory reading for all who use the Arduino. IMNSHO

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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