PNP base voltage

[dynek]

Hello everyone,

I'm trying to turn on/off the backlight of an LCD display with the following.
Backlight is the GPIO used to turn on the backlight (PWM), 1kΩ resistor to base of PNP, +3.3V on emitter, load on collector:


1 and 2 correspond to oscilloscope probe.
How come 2 goes from 0V to +3.3V while 1 (after resistor, on PNP base) is not going down to 0V ?


Anything logical I am not yet aware of ?

Thank you for any hint

[magic]

Everything is fine, that red triangle inside the circle between pins 1 and 2 is a diode

With the emitter at 3V you would need to pull many amps from the base to drag it all the way down to zero. The 1kΩ resistor prevents such pathology and limits base current to a few mA, which is exactly how things should be.

(If a MOSFET behaved like that, then it would be a reason to worry, but for BJT it's normal operation.)

[Zero999]

Doesn't the backlight require a current limiting resistor?

[golden_labels]

Looking from another perspective: since there is current through R15, there must be voltage drop on it. The base-emitter junction is a diode and in this situation is the primary element determining voltage at point #2, keeping it at 0.6–0.8V below +3.3V supply voltage, even without that the R15 would not allow the voltage to go to whatever is at point #1.

Here are some simulations in Falstad’s webapp to depict the situation. First, the BE diode being the primary factor determining voltage at #2: simulation. Second, the voltage drop on the resistor: simulation. The results of both in the attachments.

The corresponding sources for the simulations:

Code: [Select]

$ 1 0.000005 10.20027730826997 50 5 50 5e-11
d 288 160 256 192 2 default
R 288 160 288 128 0 0 40 3.3 0 0 0.5
w 256 192 192 192 2
i 192 192 144 192 0 0.001
g 144 192 144 208 0 0
g 144 272 144 288 0 0
i 192 272 144 272 0 0.01
w 256 272 192 272 2
R 288 240 288 208 0 0 40 3.3 0 0 0.5
d 288 240 256 272 2 default
g 144 352 144 368 0 0
i 192 352 144 352 0 0.1
w 256 352 192 352 2
R 288 320 288 288 0 0 40 3.3 0 0 0.5
d 288 320 256 352 2 default
g 144 432 144 448 0 0
i 192 432 144 432 0 1
w 256 432 192 432 2
R 288 400 288 368 0 0 40 3.3 0 0 0.5
d 288 400 256 432 2 default
d 288 480 256 512 2 default
R 288 480 288 448 0 0 40 3.3 0 0 0.5
w 256 512 192 512 2
i 192 512 144 512 0 10
g 144 512 144 528 0 0


Code: [Select]

$ 1 0.000005 10.20027730826997 50 5 50 5e-11
R 368 160 368 128 0 0 40 3.3 0 0 0.5
w 304 192 240 192 2
i 336 192 304 192 0 0.000001
g 144 192 144 208 0 0
r 192 192 240 192 0 1500
w 368 160 336 192 0
w 192 192 144 192 0
w 192 272 144 272 0
w 368 240 336 272 0
r 192 272 240 272 0 1500
g 144 272 144 288 0 0
i 336 272 304 272 0 0.000009999999999999999
w 304 272 240 272 2
R 368 240 368 208 0 0 40 3.3 0 0 0.5
R 368 320 368 288 0 0 40 3.3 0 0 0.5
w 304 352 240 352 2
i 336 352 304 352 0 0.00009999999999999999
g 144 352 144 368 0 0
r 192 352 240 352 0 1500
w 368 320 336 352 0
w 192 352 144 352 0
w 192 432 144 432 0
w 368 400 336 432 0
r 192 432 240 432 0 1500
g 144 432 144 448 0 0
i 336 432 304 432 0 0.001
w 304 432 240 432 2
R 368 400 368 368 0 0 40 3.3 0 0 0.5
R 368 480 368 448 0 0 40 3.3 0 0 0.5
w 304 512 240 512 2
i 336 512 304 512 0 0.002
g 144 512 144 528 0 0
r 192 512 240 512 0 1500
w 368 480 336 512 0
w 192 512 144 512 0


[CaptDon]

Why are you driving it with a square wave? Are you using PWM to control brightness? If the backlight is LED where are the current limiting resistors? In fact given 3.3vdc minus the E-C drop in the transistor it is possible the backlight will only light dimly if at all depending on the LED voltage requirement....Some need over 3.5 volts to light.

[nigelwright7557]

The transistor needs 0.7 volts across emitter base to turn on.

[dynek]

Thank you all for your answers.

Yes, I'd like to drive the brightness using PWM and it seems to work.
I'm not quite sure if the backlight requires a current limiting resistor, I bought it off AliExpress and it's not mentioned: https://www.aliexpress.com/item/32340141990.html
I'd guess the circuit already has one? I'll check quickly.