Polymeric Positive Temperature Coefficient (PPTC) devices — commonly called polyfuses or by the trade name PolySwitch — are the unsung heroes of consumer electronics protection. Unlike traditional glass fuses, they reset themselves after the fault clears, making them ideal for USB ports, battery packs, and any interface where a user might short something accidentally.

How they work: A polyfuse is a thin slab of conductive polymer — typically polyethylene loaded with carbon black particles. In its cold state, the carbon particles form continuous chains that conduct electricity with low resistance (often under 0.1 Ω). When excessive current flows, I²R heating warms the polymer until it crosses a phase transition around 125 °C. The polymer expands, the carbon chains break apart, and resistance jumps by 3-4 orders of magnitude — typically from milliohms to tens of kilohms. Current drops to a tiny "hold" value that maintains the hot state. Remove power, let it cool, and the carbon chains reform.

Critical specifications:

• I_hold: Maximum current that won't trip the device at 23 °C
• I_trip: Current that will trip it (typically 2× I_hold)
• V_max: Maximum voltage the device can interrupt safely
• R_min/R_max: Initial resistance range (matters for voltage drop)
• Time-to-trip: At 8× I_hold, often 0.1-1 second — much slower than glass fuses

Real-world example: USB port protection. A USB 2.0 port must deliver 500 mA but survive a dead short. A common choice is the Littelfuse 60R110 (1.1 A hold, 2.2 A trip, 6 V max). At 500 mA normal load, voltage drop is roughly 500 mA × 0.09 Ω = 45 mV — barely noticeable. Short the port and current spikes to perhaps 4 A briefly, then collapses to ~50 mA holding current as the polymer heats up. The host chip survives, the cable doesn't melt, and unplugging the offending device lets the fuse reset within seconds.

Derating rule of thumb: Polyfuses derate badly with temperature — typically 0.6%/°C. At 60 °C ambient inside an enclosure, an "1.1 A" device might only hold 700 mA. Always check the derating curve in the datasheet and design with 30-50% margin.

Watch out for: Polyfuses develop "memory" — repeated trips raise their cold resistance permanently, sometimes 50% after a few hundred cycles. They're not ideal for circuits that fault frequently. Also, their slow response makes them unsuitable for protecting semiconductors against fast surges — pair them with TVS diodes for full protection.