This is to test the PNP transistor .. disconnect (for a while) 3.9kΩ resistor or shorten (for a while) the B(ase) with the E(mitter) --> LED should go OFF !!! ..Assuming Vs=5V and that you have a pull-up (anyting between 1kΩ and 10kΩ) in place, disconnect 3.9kΩ --> LED should go OFF. If it doesn't, you have faulty PNP, if it does, PNP is OK ..
banh said:now the circuit is:
R57 is still 3k9.
across E and B there is 330 Ω.
when MCU pin is 0V : current flowing into mcu pin= \[ \frac{12-0.75}{3K9}=2.8mA\]
when MCU pin is 5V: current flowing into mcu pin = \[ \frac{12-0.75-5}{3K9}=1.6mA\]
from the spec, mcu pin is still ok sinking these current
the mcu is driving around 20 pnp like this.
. but sometimes mcu misbehave.. i think due to these current sinking problems..
is there any way to reduce the current sinkings?
Added after 15 minutes:
well i think increasing R57 to around 10K while across B and E is 1K might reduce a bit..
banh said:one more thing: my MCU output can be configured as open-drain output.
so Gorgon, if i'm using open-drain output, i wont need your extra 6K8 any more rite? and i wont be worried about the high voltage 11.25V any more.
Added after 17 minutes:
so this is my latest circuit
sp said:... refer the datasheet, Vbe = 4V
this is the voltage to turn ON the PNP....
let make something simple as not to confuse on negative number...
Vbe state there is 4V tht means tht the voltage diff between the base n the emitter atleast 4V to turn on(say it another way, is tht more than 4V diff is required to turn the PNP off)
to turn OFF:
emitter voltage - base voltage = ????
12V - 5V = 7V ... still ON
12V - 6V = 6V ... still ON
12V - 7V = 5V ... still ON
12V - 8V = 4V ... OK OK just enuff to turn it OFF!!!
the easiest way to turn on the PNP is to cascade another NPN..
regards,
sp
do you really mean it?Vbe=4V this is the voltage to turn ON the PNP....
...
12V - 7V = 5V ... still ON
12V - 8V = 4V ... OK OK just enuff to turn it OFF!!!
...
...now it becomes troublesome to use such PNP.
Using NPN in the first place will help avoid all these things...
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