13. PPTC Fuse for USB Port Protection
1. Component Purpose & Calculation
If a faulty USB device creates a short circuit, the full supply current rushes through the port. This can burn PCB traces, damage the USB host IC, or even melt the connector.
A PTC (Positive Temperature Coefficient) resettable fuse in series with the 5V output changes from low resistance to very high resistance when current exceeds its trip threshold. This effectively disconnects the faulty port. When the fault is removed, the PTC cools down and resets automatically.
Hold Current Selection:
USB port supplies 500mA. PTC fuses derate with temperature (typically 25–50% at +85°C). Hold current range: 500mA to 750mA. Below 500mA = nuisance tripping. Above 750mA = insufficient protection.
Required: Polymeric · Ih 500–750mA · ≥6V · Imax ≥5A · 1210 · SMD · Littelfuse
2. How to Find It on DigiKey
Go to DigiKey.com → Circuit Protection → PTC Resettable Fuses
Apply these filters:
- Current - Hold (Ih) → 500mA to 750mA range
- Voltage - Max → 6V or higher
- Package / Case → 1210
- Mounting Type → Surface Mount
- Manufacturer → Littelfuse Inc.
- Part Status → Active
Tip: PTC fuses have two key currents: Hold current (Ih) = max current where it stays conducting. Trip current (It) = min current where it trips. It is typically ~2× Ih.
3. Key Specifications & What They Mean
| Specification | Required | Why |
|---|---|---|
| Type | Polymeric | Standard resettable PTC technology. |
| Hold Current | 500–750mA | Holds at 500mA; trips on overload. |
| Voltage Max | ≥6V | Must exceed 5V USB rail. |
| Current Max | ≥5A | Survives short-circuit fault current. |
| Package | 1210 SMD | Compact for USB port PCB area. |
| Manufacturer | Littelfuse | Company AVL requirement. |
| Temp Range | −40°C to +85°C | Standard operating range. |
| RoHS | Yes | Compliance required. |
4. Selecting a Safe, Production-Ready Part
The filtered list will show matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?
Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide