JK-nSMD100/16V - -12V Rail PTC Resettable Fuse

Auto-reset overcurrent protection for -12V power rail with 1.0A hold current.

• 🔗 View on JLCPCB: C2830246

• 📄 Datasheet: Contact manufacturer

Overview

The JK-nSMD100/16V is a PTC resettable fuse providing automatic overcurrent protection for the -12V power rail. It protects the CJ7912 negative linear regulator and downstream modules from overcurrent conditions while automatically resetting after cooling.

Part Information

Electrical Specifications

Current Ratings

Key Characteristics

Circuit Integration

Protection Architecture

Layer 1: USB-PD Adapter → Overcurrent protection
Layer 2: ICL7660M (-15V inverter) + LM2596S #3 → Current limiting
Layer 3: LM7912 Linear Regulator → Current limiting ~1A, thermal shutdown
Layer 4: This PTC (1.0A hold / 2.0A trip) → Auto-reset protection
    ↓
-12V Output to Modules

Circuit Placement

LM7912 Output ──┬─── PTC3 (1.0A) ───┬─── TVS3 ─── -12V OUT
                │  JK-nSMD100/16V   │   SMAJ15A
                │                   │   (reversed)
                │                   │      ↑
                │                   └─────GND
                │
         GND ─── LED4 ─── R9 (1kΩ) ─┘
              Red status (reversed polarity)

Note: LED polarity is reversed for negative rail: anode to GND, cathode to -12V.

Protection Behavior

Current vs Protection State

Design Rationale

Why 1.0A hold for 1.0A target?

Hold current:  1.0A
Design target: 1.0A (CJ7912 max)
Note:          PTC and regulator rated equally

Benefits:
✅ Matches CJ7912 regulator maximum (1A)
✅ Provides margin for transients
✅ Typical modular synth -12V usage: 0.6-0.8A
✅ Prevents false trips during normal operation

Negative Voltage Considerations

PTC Polarity

Important: PTCs are non-polarized devices and work equally well with positive or negative voltages:

Positive rail:  +12V ─── [PTC] ─── Output
Negative rail:  -12V ─── [PTC] ─── Output

PTC behavior is identical in both cases!
The PTC responds to current magnitude, not polarity.

Protection on Negative Rails

Current measurement:

• Current flows from GND to -12V rail (conventional direction)

• PTC measures absolute current magnitude

• Trip behavior is identical to positive rails

Example:

Module draws 1.5A from -12V rail:

Current path: GND → Module → -12V → PTC → LM7912
PTC sees: 1.5A (absolute value)
Result: PTC warms up and may trip (same as +12V rail)

Comparison to Other Rails

Note: -12V rail uses CJ7912 which has 1A max output (vs 1.5A for positive rails). This is adequate for typical modular synth applications where -12V draws less current than +12V.

Typical Modular Synth -12V Usage

Why -12V draws less current:

• Analog synth modules use +12V for:

  • Op-amp power (primary)

  • Audio signal processing

  • CV generation

  • LED indicators

• Modules use -12V only for:

  • Op-amp negative rail

  • Bipolar signals

Typical current split:

+12V: 60-70% of total current
-12V: 30-40% of total current
+5V:  10-20% of total current (digital modules)

Example modular system (10 modules):

+12V: 1.0-1.2A
-12V: 0.6-0.8A  ← This rail
+5V:  0.2-0.4A

This is why the -12V PTC has lower current rating (1.0A) compared to +12V (1.5A).

Bill of Materials

Related Components

• Protected circuit: CJ7912 (U8) - -12V Linear Regulator

• Upstream: LM2596S-ADJ (U4) - -13.5V DC-DC Converter (Inverting Buck-Boost)

• Overvoltage protection: SMAJ15A (TVS3, reversed polarity)

• Parallel rails: PTC +12V (C7529589), PTC +5V (C70119)

References

• Protection design: /doc/docs/learning/protection-fuse-strategy.md

• Circuit diagram: Diagram7 - -12V Linear Regulator

• JLCPCB Part Page: https://jlcpcb.com/partdetail/C2830246