Electronics > Beginners
Picking a transistor.
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hamster_nz:
I've used the now obsolete si4944dy and sy4946dy for switching a few amps with 5V logic.

Si9925DY seem to be is the 3V equivilent for switching a couple of amps.

I was using an optoisolated mosfet driver to isolate the micro, so haven't look if a simple micro pin would turn it on fast enough if you intend to use PWM
magic:
Zetex (now Diodes Inc) makes good BJTs for saturated switching. ZTX851 might fit the bill with β>100 at 1A, 200 typical. It also coincidentally has some of the lowest voltage noise available in any BJT, supposedly at 200pV/rtHz, but that's another story.

Also check if they make something in SMD, may be cheaper if SMD is okay with you.

Worse saturation voltage but more beta and much cheaper: 2SD965, but I can only find it at TME and Reichelt and some small shops in Poland. Perhaps it's out of production now :-//

edit
CDIL still has CD965 on their website, but good luck finding distributors of CDIL in the West.
MagicSmoker:

--- Quote from: paulca on August 10, 2019, 07:15:56 pm ---Basically this.
https://randomnerdtutorials.com/10-diy-wifi-rgb-led-mood-light-with-esp8266-step-by-step/

But with 2 x 1 meter of strips of LEDs.

A thought did occur to me.  These are to light a fish tank that contains plants.  I could need it to power 4 strips.  60W, 20W per colour.

--- End quote ---

Skipping the video and scrolling down the page to the schematic, it shows the expected tiny small-signal NPN BJTs being driven directly by the MCU, but that ain't gonna cut it if you need to switch 20W at 12V or so.

The easiest solutions are either use an n-channel MOSFET driven by a MOSFET gate driver IC, or a "smart" low-side switch IC (which will also likely be protected against overvoltage and shorts, so even better).

An example of a smart low-side switch is: https://uk.rs-online.com/web/p/mosfet-drivers/1630759/

A Zetex (nee Diodes, Inc.) BJT as suggested by @magic above might just work, but it would be marginal, in my opinion. You really don't want to demand more than about 5-10mA per output pin on an MCU.
electrode:
I like the IRLB8721PbF. Its on resistance is around 10 mΩ @ 5 V (Vgs). However, the datasheet doesn't specify RDson at 3.3 V.

You could get a couple and run some measurements on them – at only 1 A of current, they're likely to handle your requirements without requiring MOSFET drivers (bleh), but no guarantees...
MagicSmoker:

--- Quote from: electrode on August 11, 2019, 10:01:50 am ---I like the IRLB8721PbF. Its on resistance is around 10 mΩ @ 5 V (Vgs). However, the datasheet doesn't specify RDson at 3.3 V.
--- End quote ---

There's always a graph of Id vs. Vgs (ie - transconductance), and from that you can extrapolate Rds[on]. In this case, Fig. 3 shows Id is around 7A at around 3.3V Vgs and with Vds = 15V, so Rds[on] is approximately 2Ω...

That said, upon actually going through the datasheet of the the low-side switch I suggested above it really appears to be nothing more than a MOSFET with a bit of protection circuitry thrown in, so it has a similar transconductance curve... A proper smart low- (or high-) side switch would still be the easiest to implement, if not the least expensive, option. Generally for beginners I err on the side of easier rather than cheaper.
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