I have recently started studying BJTs with an hands-on approach.
I am trying to make a simple switch by working in the cut-off and saturation regions and, while the equations seem quite clear to me now, I still can't wrap my head around WHICH values of the datasheet should be actually used in said equations.
For example, let's look at this datasheet by Onsemi, I want to switch ON a fake load (a resistor) to feed it 90mA, so I went straight to this graph:
and estimated a VCEsat of 105mV, a VBEsat of 900mV and an hfe of 10 (as shown in the graph!!).
So what I did then, I've calculated my resistances:
Rc = (Vcc-VfLED-VCEsat)/Ic= (5-0.105)/0.09 = 54ohm
Ib = Ic/hfe = 0.09/10 = 9mA
Rb = (Vcc-VBE)/Ib = (5-0.9)/0.009 = 457ohm
So I was expecting to measure around 90mA going into my collector. But that didn't happen.
Here are the actual measured values:
VCE=0.126V (in my expected range)
VBE=0.88V (in my expected range)
Ic=76.4mA (what??)
Ib=7.6mA (ok it's 10% of Ic but it's still not right)
As you can see, I'm getting capped at 7.6mA, using even smaller resistances at the Base doesn't change much... The voltages and the hfe on the other hand are normal...
A person on another forum told me to make the same circuit but they gave me values of the resistors: 1kohm on the base and 44ohm on the collector, and it actually worked: I've got Ib=3.6mA and Ic=94.4mA. This means that the actual hfe needed to switch my load should've been 26, not 10, while at the same time the measured VCE was 0.22V so I'm well within the saturation region.
Other people also recommended to do the same thing, but everyone basically refused to explain to me why stuff suddenly started working, or why it doesn't work in my (textbook) way.
Honestly, I can see by myself that, on another table, the datasheet states that I can switch 100mA with an Ib of 5mA.
However, I do not understand then WHY is there also a graph (FIGURE 4) that explicitly shows an hfe of 10 (so it's a graph about the saturation region) and VCEsat/VBEsat values that aren't the ones I'm supposed to use in this case.
Of course datasheets are made like this for a reason, I want to understand which reason is it though, and I also want to understand in which case I'm supposed to use the graph and in which the table.
It's not about not knowing the equations, but about understanding what to put inside.
P.s. Before you ask:
- Yes my PSU is stable, and has a max output of 2A.
- Yes I'm taking the measurements in the right positions.
- Yes I have an high quality DMM that isn't playing tricks on me.
- No I don' think temperature is involved that much, I might be wrong tho.
- Yes the pins are oriented correctly.