BJT transistor edge saturation vs deep saturation mode

[sspj9]

What is the difference between working in deep saturation vs edge saturation? Specifically to switch on an LED, using a microcontroller to change the state of the led (Voltage high - 5V and voltage low - 0v) which one is the preferred operating point in this particular application?
thanks

[SebG]

Edge saturation or soft saturation is just a region where the Vce nearly saturates but the current is still affected by Ib (base current). It is usually included in the Active region of the BJT at the boundary between both regions. In deep saturation the bjt basically acts as a switch with a low Vce (low on resistance).  So to turn on and off an LED using deep saturation region makes sense although driving the base with a too high a current then necessary is just wasted power which matters if you are designing a battery powered circuit.

Most microcontrollers, on the other hand, can source or sink enough current (15mA - 25mA) to power a standard LED so a bjt is not needed.

[sspj9]

Thank you very much Seb!
One last question? How do you calculate VCE of deep saturation please? Since the emitter is connected directly to ground and Ic remains the same, doesn't Vce also remains the same?
Thanks

[SebG]

You wouldn't necessarily calculate it since it is the saturation voltage which you can obtain from the datasheet for the bjt part. In that region the voltage changes very little for various collector or base currents.  Look for something like Vce(sat) in the datasheet. It could be something like 0.1V or 0.2V or so.

[dom0]

[X] Use a MOSFET instead

Today there are very few switching applications where a BJT is preferable. Low voltage LED switching isn't among them.

[matseng]

Most microcontrollers, on the other hand, can source or sink enough current (15mA - 25mA) to power a standard LED so a bjt is not needed.

"A" (one) is the keyword here.... If you hook up many LEDs to the ports then the total current must be taken into consideration since also the GND an VCC pins have maximum limits - usually about 200-300 mA.

[Brumby]

[X] Use a MOSFET instead

Today there are very few switching applications where a BJT is preferable. Low voltage LED switching isn't among them.

I still have several dozen TIP32's - I find them preferable because they're already paid for and less than a metre away. 

[matseng]

[X] Use a MOSFET instead

Today there are very few switching applications where a BJT is preferable. Low voltage LED switching isn't among them.

Well, since you need to piss away a lot of power anyways it really doesn't matter if it's done 100% in the resistor or shared by the resistor and the BJT.  But using a fet will remove the need for a base resistor so the BOM and pcb area will be smaller...

[T3sl4co1l]

Well... ideally, you should still have a gate damping resistor.  And a pull-down (or pull-up) to set default state, since with no base current, initial state will be undefined!

Tim

[matseng]

Well... ideally, you should still have a gate damping resistor.  And a pull-down (or pull-up) to set default state, since with no base current, initial state will be undefined!

Yes, and that *might* cause a brief millisecond or so flash of all the LEDs during power up if the internal fet capacitor happened to be charged up already.   In most cases that would be completely benign. But of course if your design pulse-drive the LEDs with 50 times the rated continuous current it would be another matter...