I am studying this circuit and found these Yellow components with labels with RT on the board. What is the component? I couldn't find the datasheet as well. The words on the components are ("U800",HLUB). Are they thermistor? But what is the purpose?

They connect between relay and positive terminal of battery.

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  • \$\begingroup\$ Negative temperature coefficient varistors for inrush current reduction is my guess. \$\endgroup\$
    – Andy aka
    Commented May 25, 2018 at 10:37
  • 1
    \$\begingroup\$ Or positive temperature coefficient for short circuit protection :) there are resistor networks that may be pull downs to detect ptc going off \$\endgroup\$
    – user76844
    Commented May 25, 2018 at 10:47
  • \$\begingroup\$ For future references here is the wikipedia list of most common designators for parts: en.wikipedia.org/wiki/Reference_designator \$\endgroup\$
    – Remco Vink
    Commented May 25, 2018 at 13:22

1 Answer 1


They look like Resettable PTC fuses I have used before.

R - Resettable T - Thermistor? I am not sure about the abbreviation/references.

If it is the above device, the resistance of the poly fuse (ptc) rises as current goes above the rated limit in order to proctect circuity.

They reset back to normal resistance once overload has disappeared and the part cools down.

  • \$\begingroup\$ Thank you. I thinks this is the one.sg.element14.com/raychem-te-connectivity/rkef500/… \$\endgroup\$
    – Daniel
    Commented May 25, 2018 at 11:13
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    \$\begingroup\$ I don't think the R means "resettable" but is rather just derived from resistor, since it is a manner of specialized resistor. \$\endgroup\$
    – Lundin
    Commented Jun 5, 2020 at 9:29

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