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  • Created:
  • Updated:
  • Author:
    Takeshi Takatsudo

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

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

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​

ParameterValue
JLCPCB Part NumberC2830246
Manufacturer Part NumberJK-nSMD100/16V
Package1206 (3.2mm x 1.6mm)
Stock99,339 units (excellent availability)
Estimated Price~$0.10-0.15
TypeResettable Polymeric PTC

Electrical Specifications​

Current Ratings​

ParameterValueUnitConditions
Hold Current1.0AMaximum safe continuous current
Trip Current~2.0ATypical (2x hold current)
Voltage Rating16VMaximum voltage

Key Characteristics​

ParameterValueUnit
Initial Resistance<0.05Ξ©(typical)
Trip Time @ 2x Hold1-5s
Reset Time30-60s
Operating Temp-40 to +85Β°C

Circuit Integration​

Protection Architecture​

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

Circuit Placement​

LM7912 Output ──┬─── PTC3 (1.0A) ───┬─── TVS3 ─── -12V OUT
                β”‚  JK-nSMD100/16V   β”‚   SMAJ15A
                β”‚                   β”‚   (reversed)
                β”‚                   β”‚      ↑
                β”‚                   └─────GND
                β”‚
         GND ─── LED4 ─── R9 (1kΞ©) β”€β”˜
              Red status (reversed polarity)

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

Protection Behavior​

Current vs Protection State​

CurrentLM7912 StatePTC StateResult
0-1.0Aβœ… Normalβœ… NormalNormal operation
1.0-1.5A⚠️ Current limiting⚠️ WarmingBoth protections activating
>1.5AπŸ›‘ Hard limit (~1A max)πŸ›‘ TripsDual protection active

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:
βœ… Matches CJ7912 regulator maximum (1A)
βœ… Provides margin for transients
βœ… Typical modular synth -12V usage: 0.6-0.8A
βœ… 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 β†’ Module β†’ -12V β†’ PTC β†’ LM7912
PTC sees: 1.5A (absolute value)
Result: PTC warms up and may trip (same as +12V rail)

Comparison to Other Rails​

Feature-12V PTC (This)+12V PTC+5V PTC
Hold current1.0A1.5A1.1A
Package1206SMD12101812
Regulator max1.0A (CJ7912)1.5A1.5A
PTC limits to1.0A1.5A1.1A
Stock99,3397,52544,459

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​

DesignatorPartPackageJLCPCB Part #QtyUnit PriceExtended
PTC3JK-nSMD100/16V1206C28302461$0.10$0.10
  • 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​

Last updated on by Takeshi Takatsudo