mSMD110-33V - +5V Rail PTC Resettable Fuse

Auto-reset overcurrent protection for +5V power rail with 1.1A hold current.

🔗 View on JLCPCB: C70119
📄 Datasheet: Contact manufacturer

Overview

The mSMD110-33V is a PTC resettable fuse providing automatic overcurrent protection for the +5V power rail. It protects the L7805 linear regulator and downstream modules from overcurrent conditions while automatically resetting after cooling.

Part Information

Parameter	Value
JLCPCB Part Number	C70119
Manufacturer Part Number	mSMD110-33V
Package	1812 (4.5mm x 3.2mm)
Stock	44,459 units (excellent availability)
Estimated Price	~$0.10-0.15
Type	Resettable Polymeric PTC

Electrical Specifications

Current Ratings

Parameter	Value	Unit	Conditions
Hold Current	1.1	A	Maximum safe continuous current
Trip Current	~2.2	A	Typical (2x hold current)
Voltage Rating	33	V	Maximum voltage

Key Characteristics

Parameter	Value	Unit
Initial Resistance	<0.05Ω	(typical)
Trip Time @ 2x Hold	1-5	s
Reset Time	30-60	s
Operating Temp	-40 to +85	°C

Circuit Integration

Protection Architecture

Layer 1: USB-PD Adapter → Overcurrent protection
Layer 2: LM2596S #2 (+7.5V) → Current limiting ~3-5A
Layer 3: LM7805 Linear Regulator → Current limiting ~1A, thermal shutdown
Layer 4: This PTC (1.1A hold / 2.2A trip) → Auto-reset protection
    ↓
+5V Output to Modules

Circuit Placement

LM7805 Output ──┬─── PTC2 (1.1A) ───┬─── TVS2 ─── +5V OUT
                │   mSMD110-33V     │    SD05
                │                   │      ↕
                │                   └─────GND
                │
                └─── LED3 ─── R8 (1kΩ) ─── GND
                     Blue status indicator

Protection Behavior

Current vs Protection State

Current	L7805 State	PTC State	Result
0-1.0A	✅ Normal	✅ Normal	Normal operation
1.0-1.5A	⚠️ Current limiting	⚠️ Warming	Both protections activating
>1.5A	🛑 Hard limit (~1A max)	🛑 Trips	Dual protection active

Design Rationale

Why 1.1A hold for 1.5A target?

Hold current:  1.1A
Design target: 1.5A (L7805ABD2T-TR max)
Note:          PTC trips before reaching regulator max

Behavior:
⚠️ PTC will trip if load exceeds 1.1A sustained
✅ Prevents false trips during power-on surge
✅ Allows brief current spikes
✅ Provides additional protection layer
✅ L7805ABD2T-TR handles 1.5A but PTC limits to 1.1A

Note: The 1.1A PTC hold current is lower than the 1.5A regulator capacity. If 1.5A sustained current is needed, consider upgrading to a 1.5A PTC. For typical modular synth applications, 1.1A is usually sufficient.

Comparison to +12V Rail

Feature	+5V PTC (This)	+12V PTC
Hold current	1.1A	1.5A
Package	1812	SMD1210
Regulator max	1.5A	1.5A
PTC limits to	1.1A	1.5A
Stock	44,459	7,525

Note: The +5V PTC limits current to 1.1A, which is lower than the regulator's 1.5A capacity. For typical modular synth applications using the +5V rail (digital modules, some LEDs), 1.1A is usually sufficient.

Bill of Materials

Designator	Part	Package	JLCPCB Part #	Qty	Unit Price	Extended
PTC2	mSMD110-33V	1812	C70119	1	$0.10	$0.10

Protected circuit: L7805ABD2T-TR (U7) - +5V Linear Regulator
Upstream: LM2596S-ADJ #2 (U3) - +7.5V DC-DC Converter
Overvoltage protection: SD05 (TVS2)
Parallel rails: PTC +12V (C7529589), PTC -12V (C2830246)

References

Protection design: /doc/docs/learning/protection-fuse-strategy.md
Circuit diagram: Diagram6 - +5V Linear Regulator
JLCPCB Part Page: https://jlcpcb.com/partdetail/C70119