Is there a reason for the anvil and post in a LED having the shape they do - other than making it easier to distinguish between the anode and cathode?

  • 2
    \$\begingroup\$ Watch out! The anvil is not a reliable indicator of polarity. Go by the flat spot or lead-length as indicated in the data sheets. I'm working on a product using CREE LEDs where the anvil on the anode for reds and the cathode for blue and green. \$\endgroup\$
    – JimFred
    Apr 10, 2014 at 3:51

2 Answers 2


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Both wires are placed symmetrically with respect to the LED's axis. The die is also placed on the axis. Therefore the cathode wire (which holds the die) has to extend towards the center line.

The anode wire is shaped so that you can't pull it out the plastic housing.

  • \$\begingroup\$ Is the behavior of LEDs related to its geometry in any fashion similar to that of a cats-whisker diode, or is it determined by dopants the anode contact area is small simply to minimize obstruction of the light by the wire? \$\endgroup\$
    – supercat
    Dec 16, 2013 at 18:37
  • \$\begingroup\$ @supercat: I'm not too sure, but IIRC in the cats-whisker diode there was not much control over the doping, so the first diodes were a bit trial and error where the whisker had to be placed. Current technology allows very precise doping. What looks like a whisker is a wire bonded to one (thin) side of the P-N junction, where the photons are generated. Is that what you mean? \$\endgroup\$
    – amadeus
    Dec 16, 2013 at 18:51
  • \$\begingroup\$ My understanding of cats-whisker diodes was that the behavior was in significant measure a consequence of the very small contact area; although the doping of the germanium would likely not be uniform, even an absolutely 100% uniformly doped lump of germanium would behave somewhat as a diode if one connection point were vastly bigger than the other [performance wouldn't be anywhere near as good as could be achieved with selective doping, of course]. \$\endgroup\$
    – supercat
    Dec 16, 2013 at 19:00

The "anvil", as you call it, serves not only as one of the electrical contacts, but also to physically hold the chip, provide some heat-sinking, and provide some directivity to the emitted light (the chip usually sits in a cup-shaped depression). The "post" just needs to have a bonding wire attached to it, so it can be much smaller.

  • 3
    \$\begingroup\$ "anvil" seems to be a common word for it. At least both "anvil" and "post" are mentioned in the drawing in amadeus' answer. \$\endgroup\$
    – flup
    Dec 19, 2013 at 13:12

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