circuit: LED is not working with transistor and some calculation issues


What does "current float" mean?

What type (color) of LED are you using?

LEDs have a voltage drop, between about 1.9 and 4V depending on type. The resistor and transistor emitter also have a voltage drop (0.7 V for the emitter).

After subtracting the voltage drops, there remains only (3.3 - 0.7 - IR2) Volts to light the LED. See

Why do you write all values in kOhm? 0,06kOhm = 60Ohm which is a bit of a weird (non-standard) resistor value.

And why does it need to be 60Ohm?

Also, keep in mind 60Ohm at 5V gives 417mW, which is to much for a normal 1/4W resistor. But if you pulse it this might not be a problem.

And I think you don't have the full current because the transistor isn't fully on because of the drop from the led in line with the base. Let's say it's a red led which has a forward voltage of around 2,0V. 3,3V - 2,0V - 0,7V = 0,6V for the resistor. 0,6V / 500Ohm = 1,2mA. For DC the amplification of the transistor isn't nearly as good. But even if it manages a gain of 100x that's still only 120mA...

I made the circuit wih kiCad and i’ll get an error message when i don’t put a k at the end of the resistor amount. So 600 instead auf 0,06k is not possible - don’t know why.
I use a white led. But when i don’t use an led and just the circuit with the resistors and the transistor, i’ll push it from 0.04A to 0.06A. There is no measurable difference.

White LEDs cannot be lit by 3.3V, and will prevent the transistor from turning on.

i'll push it

What is "it"?

The float from the powerbank. Its 0.04A, but when i trigger the IO-Pin for some ms, it's 0.06A for that time - enough to keep the powerbank (it) alive. I don't know why the white led is working here. It's connected to 3.3V Pin ands the other end to GND. And its working.

I read here, that the voltage drop for white led is about 3.6V. So yes, i see that it should not working. but it works. I also tried a yellow one, and it's still from 0.04A to 0.06A.

See reply #1

I use KiCad all the time but I tried it and I can’t trigger an error. Can you be more precise, what gives the error?

And another way to overcome that is to suffix it with a R (600R) which is also a pretty common notation to leave no doubt about the value.

And a White led at 3,3V and to a base is never going to be lit.

And without resistor it’s still 3,3V - 0,7V = 2,6V. 2,6V / 500Ohm = 5,2mA. And as I said, the gain of the transistor isn’t going to be that big. A common number to use is just 10x. So that’s just 52mA or including base current 57mA. Higher then you see but I have no idea how you measure it. What device? What sketch etc.