BJT transistor edge saturation vs deep saturation mode - Page 1
Forum section: electronics / beginners

author: sspj9
author_rank: Contributor
timestamp: January 12, 2016, 02:41:00 pm
content: 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

author: SebG
author_rank: Regular Contributor
timestamp: January 12, 2016, 03:31:48 pm
content: 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.

author: sspj9
author_rank: Contributor
timestamp: January 12, 2016, 03:39:48 pm
content: 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

author: SebG
author_rank: Regular Contributor
timestamp: January 12, 2016, 03:45:49 pm
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: January 12, 2016, 03:59:52 pm
content: Use a MOSFET insteadToday there are very few switching applications where a BJT is preferable. Low voltage LED switching isn't among them.

author: matseng
author_rank: Frequent Contributor
timestamp: January 13, 2016, 03:57:38 am
content: Quote from: SebG on January 12, 2016, 03:31:48 pm"A" (one) is the keyword here.... If you hook up many LEDs to the ports then thetotalcurrent must be taken into consideration since also the GND an VCC pins have maximum limits - usually about 200-300 mA.

author: Brumby
author_rank: Unknown
timestamp: January 13, 2016, 05:01:37 am
content: Quote from: dom0 on January 12, 2016, 03:59:52 pmI still have several dozen TIP32's - I find them preferable because they're already paid for and less than a metre away.

author: matseng
author_rank: Frequent Contributor
timestamp: January 13, 2016, 05:43:00 am
content: Quote from: dom0 on January 12, 2016, 03:59:52 pmWell, 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...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2016, 06:03:41 am
content: 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 beundefined!Tim

author: matseng
author_rank: Frequent Contributor
timestamp: January 13, 2016, 06:13:17 am
content: Quote from: T3sl4co1l on January 13, 2016, 06:03:41 amYes, 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...