PTC Fuses

Lithium battery packs have overcurrent protection cutouts, but alkaline cells depend on their internal resistance and may overheat in response to a serious short circuit. So adding a PTC fuse to the circuitry over an alkaline battery case seemed appropriate:

Discrete LM3909 - Darl Q1 - 1X Q2 - blue LED test
Discrete LM3909 – Darl Q1 – 1X Q2 – blue LED test

That’s a test setup for a discrete-transistor version of an LM3909 LED blinker, about which more later. The PTC fuse looks a lot like a ceramic capacitor with one leg caught in an alligator clip.

Two bags of PTC fuses recently arrived from halfway around the planet, rated at 100 mA and 170 mA. One allegedly came from JinKe and the other probably didn’t pass through a Littelfuse factory despite its part number, but the only datasheet I can find is for the Littelfuse RXEF PTC PolySwitch series, which is surely close enough.

I set up a torture test involving a bench power supply and an ammeter, both offscreen and left to your imagination:

PTC Polyfuse test setup
PTC Polyfuse test setup

At 75 °F:

  • 100 mA PTC – 4.75 Ω
  • 170 mA PTC – 2.80 Ω

With a dead short simulated by 3 V from the supply, the current stabilized at:

  • 100 mA PTC – 125 mA
  • 170 mA PTC – 135 mA

Cranked to 5 V for that good old TTL vibe:

  • 100 mA PTC – 70 mA
  • 170 mA PTC – 85 mA

The datasheet says they’re good up to 60 V, but that’s just crazy talk.

The abuse put a shiny gloss on the epoxy coating, sort of like when you overcooked one of those wax-insulated capacitors back in the day.

Despite that, a PTC fuse is better than a dead short, if only because the plastic battery case won’t get all melty with the batteries supplying less than half a watt.

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