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Resistors to stop shorts?

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Screech

New Member
Must you always use a resistor between most ic's out pin and a transistor?
Eg:pin 3 of 555 to an npn transistor.

Say I need 200 milliamps going to base of tansistor mj150013.
The 555's output can supply 200 milliamps.

Can I run the 555's output to the transistor without a resistor?
Will that cause the ic to short (as current flows from base to emitter(earth)).

does the tranny act as a load from base to emitter?
Think I'm going on and on.

Thanks
 

ljcox

Well-Known Member
Resistors to stop shorts

Yes, you need a resistor in order to limit the base current to a safe level.

What collector current do you want? The base current only needs to be about one tenth of the collector current in order to saturate the transistor.

200mA seems a bit much. I don't know whether the 555 can source this much.

Len
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, you need a current limiting resistor - otherwise you are effectively connecting the base emitter junction of the transistor straight across the supply rails - shorting them together.
 

Screech

New Member
That transistor is a 5 amp one.
Its hFE is 25

so 5000mA/25 =200mAmps.

I only nead 4amps:
4000/25=160milliamps
so I'm going to supply it with a bit more then 160milliamps, with a resistor.

Thanks for answering my question guys.
 
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