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This probably has been asked a bunch of times but I couldn't find anything with image search. The component is designated as R1 and the only other piece of info is 500mA written on top of it. It is placed right after the power input. Due to the position I thought it was either a fuse or a ferrite bead, but as far as I can tell this is not a standard symbol for either. Any help?


The schematic is the ReSpeaker Raspberry Pi hat from seeed: https://files.seeedstudio.com/wiki/ReSpeaker-4-Mic-Array-for-Raspberry-Pi/src/ReSpeaker%204-Mic%20Array%20for%20Raspberry%20Pi%20%20v1.0.pdf

The current flows to a reverse polarity protection circuit and a stack of LEDs afterwards.

  • \$\begingroup\$ I've looked at a lot of schematics over the years and I'm pretty sure this is the first I've seen of it, can you share where you found it? Ferrit bead is a good guess, especially if it is a squished inductor symbol. Would an NTC inrush current limiter make sense here? Or a PTC resettable fuse? \$\endgroup\$ Jun 4, 2020 at 22:24
  • \$\begingroup\$ What is the context? Is this part of a Turbo Encabulator or a Carbon Tachometer? \$\endgroup\$
    – JYelton
    Jun 4, 2020 at 22:25
  • 1
    \$\begingroup\$ If the 500mA is written on the component itself, I'm leaning towards PTC resettable fuse. \$\endgroup\$ Jun 4, 2020 at 22:26
  • \$\begingroup\$ looks like a thermal breaker symbol that someone decided to split in half and overlap the two halves \$\endgroup\$
    – jsotola
    Jun 4, 2020 at 22:27
  • \$\begingroup\$ I will add the source in the question. It is a ReSpeaker Raspberry Pi hat schematic. files.seeedstudio.com/wiki/… \$\endgroup\$ Jun 4, 2020 at 23:04

2 Answers 2


I've not seen that symbol before. My best guess is that someone or something got messed up in the library (Altium/Eagle/whatever) and it's been drawn incorrectly.

Since it's on a voltage input and specifies a current amount, it's most likely a fuse. Because it's designated as a resistor, it may well be a resettable fuse, such as a PTC.

Here are some similar symbols for fuses. It may be that the symbol you show is a badly drawn version of one of these:

Fuse Symbols


  • 1
    \$\begingroup\$ Very possible. A PTC was my first thought but the most common symbol for it is so different I doubted it. I did notice many small mistakes on the schematic so it's possible the designer could mess up the symbol. \$\endgroup\$ Jun 4, 2020 at 23:10

I believe someone may simply have messed up the part's symbol for this part in the CAD's parts library. I have never seen anything like this, and I've seen lots of different schematics over the years,the very old and the newer.

The "PTC" suggestion sounds good, or it can be a thermal cut-off breaker - as in it acts as a temporary circuit breaker if the rated load ( 500 mA ) is exceeded. I've seen a few applications that use them. It takes the place of the usual fuse that many older projects used to use to protect from overloads and short-circuits.


I think the schematic for the part may not be correct in the library though.


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