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I am currently building a soft latch circuit in a CAD editor.

I want to use a PNP transistor, and have 2 choices for this component.

However, I am confused when comparing the datasheets of the components with the symbols and footprints in the CAD editor.

Part 1 looks like this. Pin 1 is base, pin 2 is emitter, pin 3 is collector. This lines up with it's data sheet. Also the footprint numbering lines up with the datasheet.

Part 1 symbol and footprint in CAD software
(Screenshot from EasyEDA)

Part 1 datasheet

(Image source: LCSC - Galaxy Microelectronics MMBT3904 datasheet)

For part 2, Pin 1 is base, pin 2 is collector, pin 3 is emitter in the CAD symbol. Ok, I understand that the pin numbering could be different. Looking at the datasheet, however, pin 1 should be emitter, pin 2 should be base, and pin 3 should be collector. Also, the footprint pin numbering is different then in the datasheet.

I tried to replace the symbol of part 1 in my schematic with part 2, but now I find out that the pins are swapped. Is it true that I can never just replace symbols in the schematic for transistors?

Part 2 symbol and footprint in CAD software
(Screenshot from EasyEDA)

Part 2 datasheet

(Image source: LCSC - Unisonic Technologies MMBT3906 datasheet)

I saw in a video that PNP/NPN transistor symbols are not unified. Does the empty/filled arrow in the symbol represent something different? Is the symbol and footprint for the second part plain wrong? Should I trust the datasheet of the second part, or not use it at all?

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  • \$\begingroup\$ Who are these datasheets from? \$\endgroup\$
    – Tim Williams
    Commented May 6, 2023 at 13:07
  • \$\begingroup\$ Can you explain what you mean by "I saw in a video that PNP/NPN transistor symbols are not unified."? Particularly what "not unified" means. \$\endgroup\$
    – Hearth
    Commented May 6, 2023 at 13:25
  • \$\begingroup\$ Also, what software is this? How replacing symbols works is highly software-dependent; for instance, this all works entirely differently if you use KiCad, to the point that this question would be nigh on nonsensical in a KiCad context. \$\endgroup\$
    – Hearth
    Commented May 6, 2023 at 13:26
  • \$\begingroup\$ To clarify, I am using easyeda editor and these are the datasheets: Part 1 is MMBT3906G from Galaxy and Part 2 is MMBT3906G-AE3-R from UTC. Datasheets are datasheet.lcsc.com/lcsc/1912111437_Galaxy-MMBT3904G_C380901.pdf and datasheet.lcsc.com/szlcsc/… . Sorry, I was not sure if naming specific brands is allowed on this website. By non-unified, I mean to say that there is no single standard for the schematic symbols of PNP transmitters. I could not find the relevant video unfortunately now. \$\endgroup\$
    – ThomasPaper
    Commented May 6, 2023 at 14:08
  • \$\begingroup\$ @ThomasPaper - Hi, Re: "Sorry, I was not sure if naming specific brands is allowed on this website". FYI for material copied from elsewhere onto Stack Exchange (SE), it is required that you include appropriate source references (e.g. a link for online material) in that question/answer. || As you are new here, please read the tour & help center to see how SE rules & etiquette differ from typical forums. Thanks. (I've added the links into the question for you this time, since you kindly provided them in comments.) \$\endgroup\$
    – SamGibson ♦
    Commented May 6, 2023 at 14:14

2 Answers 2

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You appear to have schnipped the datasheet image from the Unisonic (Taiwan) datasheet.

Their numbering of the pins is non-standard, IME. In any case, the transistors are physically interchangeable, only the pin numbering is wonky. I would ignore the Unisonic datasheet for the purposes of creating the symbol/footprint mapping. Use the Onsemi (nee Motorola) datasheet pin numbering.

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  • \$\begingroup\$ I just checked the datasheets for several other MMBT3906 transistor manufacturers, and they all follow the Base, Emitter, Collector for pins 1, 2 and 3. So it seems only the UTC MMBT3906 is different. It seems you are correct about that. I will just avoid the UTC part and go for the \$\endgroup\$
    – ThomasPaper
    Commented May 6, 2023 at 14:19
  • \$\begingroup\$ As I said, there's no need to avoid the part. Assuming the electrical characteristics are the same, they drop in place on the PCB with no changes. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 6, 2023 at 14:59
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The empty / filled arrows are identical in meaning, the transistor's emitter. You are seeing a difference in drafting style. The filled arrow is way more common.

Transistor pin assignments and numbers across manufacturers and packages is a constant headache for any circuit board designer. Note that the recommended surface mount pad dimensions and geometry also vary.

Trust the datasheet for the exact part number and manufacturer you are using, and make no assumptions about anyone else's parts or data.

Welcome to the club.

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  • \$\begingroup\$ Thank you, I guess :) I think the safe way is indeed to always start from the manufacturers datasheet, however, I now checked the datasheet for several other MMBT3906 transistor, and they all follow the Base, Emitter, Collector for pins 1, 2 and 3. So it seems only the UTC MMBT3906 is different. I guess I will avoid that part altogether. \$\endgroup\$
    – ThomasPaper
    Commented May 6, 2023 at 14:17

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