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I have the circuit shown below and one of the components (Q2) is a common through-hole transistor. I'd like to replace this through hole device 2N3904 with a common SMD transistor that will do the same job so that my PCB manufacturer can solder these for me.

Does anyone have a recommendation? circuit

Feynman137
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    asking for part recommendations is off topic here, but why aren't you just using an MMBT3904? – Hearth Mar 22 '21 at 15:28
  • I understand my thinking was this transistor is so common that many may have the same issue – Feynman137 Mar 22 '21 at 15:31
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    Surely you can find the SMD equivalent of a specific transistor without asking here! –  Mar 22 '21 at 15:43

1 Answers1

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MMBT3904 is the same die. You should verify that the thermal performance is adequate, because it won't be as good as a TO-92.

The same part in SMD != doing the same job.

Spehro Pefhany
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  • "the thermal performance [...] won't be as good as a TO-92" certainly makes it sound pitiful. Which I suppose it is! – Hearth Mar 22 '21 at 15:38
  • @Hearth About 400°C/W compared to 200°C/W (depending on how much copper you spread around the collector lead it could be better though). – Spehro Pefhany Mar 22 '21 at 15:40
  • I would think an MMBT3904 could be used for anything a sensible designer would use a 2N3904 for, though. I at least would never use a '3904 for anything but switching low currents, due to its extremely poor thermal characteristics. Certainly not anything that would see significant power dissipation. – Hearth Mar 22 '21 at 16:05
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    @Hearth People do strange things like [this](https://electronics.stackexchange.com/questions/174223/class-a-single-transistor-amplifier-with-2n3904) – Spehro Pefhany Mar 22 '21 at 16:07
  • The part is being driven into saturation. Unless the relay is stupidly large, there should be no issue using the SOT-23 part. – hacktastical Mar 22 '21 at 16:13
  • @hacktastical Correct. To make the answer useful for others though, it's worth noting. I would probably put a beefier transistor in there just on principle, for switching an inductive load. The cost delta is pretty negligible (less than 0.2 cent in 50K). – Spehro Pefhany Mar 22 '21 at 16:14