Turning on a PNP transistor

[Experientia_docet]

I need advice as to how to turn on a transistor. The power supply is 5V ,
the base is connected to ground with a resistor. The transistor is turning on a LED with a 470 Ohmn resistor. So the load is about 7mA.
How do i calulate the base resistor ?

 

[MPep]

5 - 0.7 = 4.3V. This is from Base to GND.
If the Gain is about 10 (for instance) then you need a current of 0.7mA.
4.3 / 0.0007 = 6142 Ohms.

If the gain was 100, then you need 61,423 Ohms.
Use a resistor close these values, dependent on the Gain (hFE) value.

Hope this helps.

MPep.

 

[premelec]

Note that with a PNP transistor the base must be NEGATIVE with regard to the emitter to turn on - usually NPN is used... with emitter to negative and a positive base signal.

That said a PNP type can be used with the emitter tied to the POSITIVE rail and the base switched to negative with a reisistor - say 1K [depends on what you are trying to do]. Then the collector of the PNP transistor goes to the LED and the LED goes through a current limiting resistor [commonly 330 with 5volt supply] to Negative supply. OK?

 

[boriz]

Ib should be at least Ic/hfe.

Without knowing the transistor type or the hfe, any answer would be a guess.

I guess 2k2.

 

[boriz]

oops. Crosspost.

 

[boriz]

Some reading: http://openbookproject.net//electricCircuits/Semi/SEMI_4.html