I am trying to repurpose a scavenged motor, however the wiring diagram that's on it has a schematic symbol I have never seen and could not find on the Internet.

Anyone know what this is and what it means?

Mystery symbol

For more context, here's a reproduction of the entire schematic:

Whole schematic


The symbol is for a thermal switch/contact. This particular kind is normally closed and opens on temperature rise (http://www.industrial-electronics.com/Electricity-Refrigeration-Heating-Air-Conditioning_5b.html)

  • 1
    \$\begingroup\$ It reminds me of a temperature switch. It looks like the device is built into your motor, can you possibly identify it physically? \$\endgroup\$
    – PlasmaHH
    Oct 6, 2016 at 10:59
  • \$\begingroup\$ From the context I would guess that it indicates a switch controlled by a digital input on the yellow wire. \$\endgroup\$
    – Andrew
    Oct 6, 2016 at 11:00
  • \$\begingroup\$ "PWM controlled switch", perhaps? (also, just guessing) \$\endgroup\$ Oct 6, 2016 at 11:01
  • \$\begingroup\$ It's likely a Negative Temperature Coefficient (NTC) switch, designed to allow starting torque to be generated in a single-phase motor. Initially it's very low resistance, allowing current to flow through a capacitor (the starting cap), causing a phase shift of the AC... causing torque in the 'stalled' motor, starting the spin. As the 'switch' heats up due to current flow, it's resistance rises, eventually removing the starting cap's phase shift, since it's not needed after the motor is rotating. Look up single-phase AC motors and starting capacitors. \$\endgroup\$
    – lornix
    Oct 7, 2016 at 8:30
  • \$\begingroup\$ @DerStrom8 -- I could be wrong, but all of the images I've found of this symbol with the squiggle away from the switch refer to this as being open normally. \$\endgroup\$
    – conan
    Oct 7, 2016 at 12:00

1 Answer 1


It's a thermal contact (Normally closed - thermal trip).

If OP Googles "contact switch symbols" and jumps to the Images, he/she can find a lot of symbol tables. Here's one OP could see: ibiblio.org/kuphaldt/electricCircuits/Ref/01053.png

  • \$\begingroup\$ Yes! Looks like it's a thermal switch. \$\endgroup\$
    – conan
    Oct 6, 2016 at 11:11
  • \$\begingroup\$ If I'm not mistaken, another term for this kind of device thermal cutoff (here and here). Some of these devices are self-resetting, some of them need manual reset, some of them are sacrificial (like glass fuses). \$\endgroup\$ Oct 6, 2016 at 15:44

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