Emitter follower suitability question. - Page 1
Forum section: electronics / projects

author: DonnieBrodan
author_rank: Contributor
timestamp: January 30, 2021, 12:04:56 pm
content: I have an LED connected directly to ground, which I want to power with 3v3 from an ESP32 GPIO. I can only use 16mA on the IO, so I will need a transistor of some kind to help with this. Need to switch 10-50mA. Can I use aDTC114YET1Gin emitter follower config?

author: mikerj
author_rank: Super Contributor
timestamp: January 30, 2021, 12:44:55 pm
content: An emitter follower will drop ~0.6v, leaving you with only 3.3-0.6=2.7 volts to drop across the LED and the current limiting resistor. If you are using a white or blue LED this probably won't even be sufficient to light it.You could use a PNP (e.g. DBT114EK if you want a 'digital' biploar) in a common emitter configuration which would drop a much smaller voltage (~0.1v) but the logic on your GPIO pin will be inverted i.e. high for off, low for on. You could also use a P channel mosfet with a low enough Vgs e.g. BSS84.

author: gnuarm
author_rank: Super Contributor
timestamp: January 30, 2021, 08:42:49 pm
content: Quote from: DonnieBrodan on January 30, 2021, 12:04:56 pmA bipolar will require 0.6V or so across the BE junction. The CE junction can be less. You can put the current limiting resistor in the collector leg so it has more voltage across it and drive the base with enough current to keep it in saturation. That means a pretty small base resistor. You will need to do some calcs on your LED type to set resistor values.Oh, wait, that part has the resistors built in. 10K is pretty certainly too large to work. With a minimum hFE of 80 you will need close to 1 mA into the base with very little voltge depending on the voltage of the LED. You will need to leave plenty of margin for the LED voltage to vary over temperature, etc. So shoot for 2 mA into the base at the max voltage. It will be tight, so make sure you don't draw too much current from the output when the LED voltage is minimum, but that likely won't be an issue because the output voltage will drop with current, equalizing everything. Do they specify an output voltage at 5 or 10 mA or even 16 mA?If the max/min numbers don't work put part of the current limiting resistance in the emitter leg. This will help stabilize the base current.

author: DonnieBrodan
author_rank: Contributor
timestamp: January 31, 2021, 12:08:19 am
content: Perhaps easiest would be to just add an P Channel MOSFET, something like this? R2 and R3 are on a remote board, so not so easy to modify their config..Rather than add anew partto the BoM, maybe could useSI2301BDS. Cost a little more but saves adding a new part.

author: Peabody
author_rank: Super Contributor
timestamp: January 31, 2021, 05:26:04 am
content: In that last drawing, the P-channel BSS84AK mosfet is upside down. The source should be at the top.

author: DonnieBrodan
author_rank: Contributor
timestamp: February 01, 2021, 05:00:49 am
content: oops yep.I have gone with R4 = 330 Ohms. Not really sure if this is required..

author: Peabody
author_rank: Super Contributor
timestamp: February 01, 2021, 02:55:08 pm
content: Quote from: DonnieBrodan on February 01, 2021, 05:00:49 amThat's a subject of discussion here quite often. When you turn the LEDs on or off, the gate capacitance has to be charged or discharged, and if there's no resistor, the IO pin may briefly see a pretty low impednce load. So something like a 100-ohm resistor helps with that, at the cost of slowing down the transitions a bit. If you aren't doing PWM or other high-speed switching of the LEDs, then it probably doesn't matter. However, note that many here with more experience than I have will always use an R4, while others rarely do.If you have any choice on the mosfet, you might look for one with a lower RDSon. Something like the DMP1045U is more expensive, but more typical of modern mosfet performance - lower ON resistance, higher current. At these current levels, a few ohms of resistance isn't a deal killer, but why not use a better mosfet if you can.

author: DonnieBrodan
author_rank: Contributor
timestamp: February 02, 2021, 01:42:16 pm
content: Thanks for the advice Peabody, was suggested to go with R4 = 1k.. I guess this will work as well?

author: Peabody
author_rank: Super Contributor
timestamp: February 02, 2021, 03:13:48 pm
content: I suggest you Google on 'mosfet gate resistor calculator' and see what comes up. You'll probably find more than you ever wanted to know about gate resistors. But I suspect 1K would work fine.