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I'm working on this project

https://github.com/diyhue/Devices/tree/master/HueDimmerSwitch

I would ask which characteristics I need to keep in consideration to choose that two NPN and PNP transistors?

Are these one a good choice?

  • NPN: S8050 J3Y
  • PNP: S8550 2TY

EDIT: I forgot to mention I need transistors in SMD package

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There are no special requirements for those transistors- the 8050/8550 in SOT-23 are just fine. There are probably slightly cheaper ones- those have a relatively large die.

You could also use MMBT3904/3906, MMBT4401/4403, 2SC2712/2SA1162 etc. etc.

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S8050 and S8550 are "obscure" transistors in my opinion, why use those?

They will probably just work though, their Hfe is > 100 and usually that's all that matters in such a low power circuit.

But looking at the schematic (of which you should include the picture, not just a link) I think almost any general purpose NPN/PNP will work.

I'd use BC547(NPN) and BC557(PNP) but that's because I have plenty of them (as do many other hobbyists). If you're in US you might choose 2N3905(NPN) and 2N3906(PNP) instead.

But really: it does not matter much, the circuit does not rely on "special properties" of specific transistors.

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  • \$\begingroup\$ The 8050 (NPN) and 8550 (PNP) are extremely common in China, in both through-hole and SMT versions. They're cheap (though maybe not the cheapest) and easily available. The main disadvantage is the relatively low Vceo of 25V. The pinouts of the through-hole versions are not standardized. \$\endgroup\$ – Spehro Pefhany Dec 20 '18 at 17:35
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You want a TUN (transistor universal npn) and a TUP (transistor universal pnp) how they are often called. You can find a Selection here for example: http://pencho.my.contact.bg/start/comp/tuptun/tuptun.htm

BC547 and BC557 are classic examples and very widely used.

According to the specs of the S8050/S8550, they should also work just fine.

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    \$\begingroup\$ I have never heard TUN and TUP. "Transistor universal NPN" is not really English, so I don't understand how they can "often be called" this. \$\endgroup\$ – pipe Dec 20 '18 at 15:01
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    \$\begingroup\$ According to the german wikipedia it was coined by the elektor magazine in 1971. A 1975 Version could bee seen here: 320volt.com/wp-content/uploads/2009/05/TUP-TUN-DUG-DUS-elektor.pdf As far as I understood elektor is mainly popular in Europe. I grew up with those terms here in Germany and it makes perfectly sense to have such default goto transistor categories. \$\endgroup\$ – wiebel Dec 20 '18 at 15:21

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