Transistor selection - Page 1
Forum section: electronics / beginners

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 12:32:43 pm
content: I have a 3V3 switch key when the transistor's gate is 0. Which transistor should I choose here?

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 29, 2020, 12:48:28 pm
content: Quote from: Maciek12 on April 29, 2020, 12:32:43 pmThat is a bipolar transistor. The terminals are Emitter, Base, Collector.If it were a MOSFET, the terminals would be Source, Gate, Drain.What are you trying to accomplish here?

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 01:01:33 pm
content: There is 0V or 3.3V at the thistor base (3V3_BAT)I want the 3V3 (3V3) voltage from the collector to turn on when the base is 0V and power the processor (3V3_Out)I do not know how to prepare such a key and which one to choose.

author: ebclr
author_rank: Super Contributor
timestamp: April 29, 2020, 01:07:26 pm
content: This transistor on the schematic does not have a gate, has a base. This application is so simple that any Bipolar transistor ( NPN) will work. Note that your output 3.3 V will go via a 10K resistor, This means your processor must push very low current, otherwise, your 3.3V will be consumed entirely on the 10K resistor. If that is the case you will need to decrease the 10K resistor a lot, Or even add a second PnP transistor to have a low impedance path

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 01:09:58 pm
content: If it will be ok?In datasheet Ic=100mA, so I choose R14=33RIb=5mA, so I choose R13=660RIt will be correct?I only want simple transistor keyVb=0, so Vout=3V3ifVb=3V3 then Vout=0

author: ebclr
author_rank: Super Contributor
timestamp: April 29, 2020, 01:19:15 pm
content: Rbase must Be (3.3 - 07) / ( 0.1 A * transistor beta )You output tension will never be 3.3V , Will be 3.3 - ( emitter resistor * 0.1 A + you output current consumption )Also will never be 0, Will be VCE saturate topically + 0.25V

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 01:22:00 pm
content: What and how to apply to get 3V3 output?

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 29, 2020, 01:46:37 pm
content: See fig

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 01:52:31 pm
content: Can I use BC857BLT1G in this configuration?

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 29, 2020, 01:59:16 pm
content: Quote from: Maciek12 on April 29, 2020, 01:52:31 pmNo. That is an NPN. You need a PNP in the circuit I drew.

author: Maciek12
author_rank: Newbie
timestamp: April 29, 2020, 02:11:17 pm
content: https://www.tme.eu/Document/1a1e3af2539579d004bcdd8b7742a187/BC856ALT1-D.PDFHere is information that it is pnp

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 29, 2020, 02:39:50 pm
content: Quote from: Maciek12 on April 29, 2020, 02:11:17 pmThat will work. Maximum collector current continuous is 100mA.

author: StillTrying
author_rank: Super Contributor
timestamp: April 29, 2020, 04:04:27 pm
content: Quote from: Maciek12 on April 29, 2020, 01:01:33 pmIf the 3V3 out really is topowera processor rather than just provide a logic level input you could do with knowing how many mA the processor needs, it might only need a few mA, but it could also be a lot more than 100mA.