Computing stuff tied to the physical world

Thermo Murphy

In Hardware on Jan 4, 2011 at 00:01

Mr. Murphy strikes again in the new year…

A post a while back, mentioned that the NPN transistor footprint on the Thermo Plug was wrong. I should have tracked this down earlier, but due to all the supply issues I only found some time just now (with thanks to Lennart for helping figure this one out).

Here’s the board footprint:

Screen Shot 2011 01 03 at 12.08.50

It’s not hard to see that from left to right, the pinout is emitter-base-collector.

Guess what… NPN TO-92 transistor footprints are not standardized!

Here’s the one from the BC549 I’ve been shipping in a few recent units:

Screen Shot 2011 01 03 at 12.06.16

Bzzzt… you have to mount it the other way around to make it work!

To give you an example of the mess w.r.t. orientation, here’s a diagram I found on Solarbotics:


And here’s the one from the 2N4401 I’m going to use for the Thermo Plug from now on:

Screen Shot 2011 01 03 at 12.05.37

Well, at least it’s just mirrored, with the base always in the middle. Not quite as bad as with voltage regulators (such as with MCP1702 vs LM2940, where the order is completely different).

If you’ve got a kit with the BC549, please make sure to mount that transistor 180° rotated w.r.t. what’s shown on the pcb. All units sent out from now on will have an NPN transistor which matches what’s on the board.

  1. Standards, don’t you just love them? Well you never know, you might… If you ever found one!

    Sometimes I really do think these manufacturers are out there trying to find the most annoying things they can do with a 3 pin package to drive us all slowly insane!

  2. Hi Jean

    In fact the situation where you learn the most about electronics, is when the circuit does not work. This said, I have to thank you for this bug, I learned a lot about testing a circuit with a transistor.

    Cheers Rubi

  3. It’s even worse than what you describe here. Take a look at for instance the datasheet of a BC547 from ON Semiconductor and compare it to the datasheet of a BC547 from Philips. You’ll see that the footprints are not the same. The same holds true for 2N3904. (In my experience Philips always seems to have mirrored footprints compared to most other manufacturers).

    In addition, manufacturers sometimes produce special versions of a common part number using a completely different footprint. I have a few transistors marked as 2N3904 in a TO-92 case, for which pin 1 is base.

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