After it occured to me a pnp's base voltage is .7v below it's emitter!
So let's say I have 1k on the base of the pnp from the 5v pin on an Arduino and 10v on the emitter... if I supply LOW/gnd the transistor begins conducting... but hold on 9.xV via a 1k resistor to digital out pin...
If to switch it off , it requires bringing the base voltage to the emitter voltage which is not possible and of course, all of this is avoided simply by using 5v on the emitter.
But 10v on the emitter, how's it go for the Arduino end?
cjdelphi:
After it occured to me a pnp's base voltage is .7v below it's emitter!
So let's say I have 1k on the base of the pnp from the 5v pin on an Arduino and 10v on the emitter... if I supply LOW/gnd the transistor begins conducting... but hold on 9.xV via a 1k resistor to digital out pin...
You won't be able to switch it off because Arduino "HIGH" is still enough to make it conduct.
cjdelphi:
But 10v on the emitter, how's it go for the Arduino end?
Badly, unless you add more components (eg. a NPN.to control it).
Because of the 1k resistor on base, there's substantially more current which will keep it tied to 5v or gnd...
I suppose I could try it and find out!
AFAIK the behaviour of a transistor is quite unpredictable as the characteristics do variate (the "beta" parameter among them; I mean different units from the same batch of the same type of transistor) unless you are dealing with a "controlled" series (characteristics "guaranteed").
That means that you can find that one unit (transistor) does (switchs on at LOW arduino output and off at HIGH) but other transistor of the same type does not.
Better if you sofisticate the circuit a bit (adding a NPN).
Regards
(By the way: ALL the bipolar transistors have 0.7 V from emmiter to base)
So let's say I have 1k on the base of the pnp from the 5v pin on an Arduino and 10v on the emitter... if I supply LOW/gnd the transistor begins conducting... but hold on 9.xV via a 1k resistor to digital out pin...
The next time you call me rude names for suggesting that you do some more study, I'm going to point you at this thread.
Yes, the PNP will stay on under those conditions. It is functionally the same as if you had the emitter of an NPN at -5V, and tried to turn it on and off with an Arduino switching between ground and 5V.
If you want to use a PNP to high-side switch a load, use an NPN followed by the PNP. You still need the proper current limiting.
In this circuit, R1 in conjunction with the NPN and the 5V from the Arduino acts as a constant current sink. It is vitally important that R3 be connected to the higher voltage supply, in this case 12V:
It can be a bit strange using PNP BJTs and P channel MOSFETs, until you get used to it.
You could tell me to refer to this thread... however I,ve never used a pnp with a 5v pin, the only time I would is if the emitter was also 5v... it does make me wonder one thing, I get the impression you think you know everything but that's irrelevant. ..
To protect the digital pin (> 5v) on a pnp, what do you think of a zener?
CrossRoads:
Zener on the base of the NPN? Not needed. The Arduino is putting current into the base, it does not see the voltage on the NPN collector at all.
Pnp, current leaves the base pin into the Arduino if the emitter > 5v
