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I've opened up the battery box of a string of fairy lights (like pictured at bottom) and found a resistor, plus what looks like a capacitor in series with the lights. Is this really a capacitor? If so, how does it work in a DC circuit like this?

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

My aim is to convert these to run on 12v. Does this circuit look ok? How can I determine the right value for the current limiting resistor?

schematic

simulate this circuit

Fairy Lights

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    \$\begingroup\$ A capacitor there would cause the led to shine for a very short time until the circuit is turned off. Maybe it's a resetable fuse. I don't know. To make a 12V circuit, the total of the series must make 12V. If there are 4V LEDs, it's OK with a 10 ohms resistor, if they are 4.5V LEDs then it could be a bit dim, but also OK. with no resistor. As they are powered by 3x1.5V batteries they can be 4 or 4.5V. \$\endgroup\$
    – Fredled
    May 24, 2020 at 23:11
  • \$\begingroup\$ That's what I thought about the capacitor too but they don't dim at all. Maybe it's not a capacitor? \$\endgroup\$
    – Jake
    May 26, 2020 at 3:12
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    \$\begingroup\$ It can be a resetable fuse. \$\endgroup\$
    – Fredled
    May 26, 2020 at 17:04

2 Answers 2

Reset to default
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It is a PTC resettable fuse, rated at 250mA and 6V maximum voltage.

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Thanks to @Fredled I've discovered that it is indeed a resettable fuse! https://www.diytrade.com/china/pd/9092091/PPTC_resettable_fuse_polymer_switch.html

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  • \$\begingroup\$ Once you find/post the answer to your question you can mark it as "best answer" this resolves the question and makes it not appear on the unanswered tab. You can mark your own answer as 'best', or there is another answer with similar information. \$\endgroup\$
    – Michael
    May 27, 2020 at 23:02

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