SMD1210P150TF/16 - +12V Rail PTC Resettable Fuse
Auto-reset overcurrent protection for +12V power rail with 1.5A hold current and fast trip response.
- π View on JLCPCB: C7529589
- π Datasheet: Contact manufacturer
Overviewβ
The SMD1210P150TF/16 is a Positive Temperature Coefficient (PTC) resettable fuse that provides automatic overcurrent protection for the +12V power rail. Unlike traditional fuses that require replacement, this PTC automatically resets after cooling, making it ideal for modular synthesizer applications where temporary overloads may occur.
Part Informationβ
| Parameter | Value |
|---|---|
| JLCPCB Part Number | C7529589 |
| Manufacturer Part Number | SMD1210P150TF/16 |
| Package | SMD1210 (3.2mm x 2.5mm) |
| Stock | 7,525 units |
| Estimated Price | ~$0.15-0.20 |
| Type | Resettable Polymeric PTC |
Electrical Specificationsβ
Current Ratingsβ
| Parameter | Symbol | Value | Unit | Conditions |
|---|---|---|---|---|
| Hold Current | I_hold | 1.5 | A | Maximum safe continuous current |
| Trip Current | I_trip | 3.0 | A | Current that will cause device to trip |
| Maximum Current | I_max | 35 | A | Can handle during trip transition |
| Voltage Rating | V_max | 16 | V | Maximum voltage |
| Power Dissipation | P_D | 0.6 | W | @ 25Β°C |
Resistance Characteristicsβ
| State | Resistance | Notes |
|---|---|---|
| Initial (cold) | 0.03Ξ© | Minimal voltage drop during normal operation |
| After trip (hot) | >1000Ξ© | High resistance limits current to ~10mA |
| Post-reset | <0.05Ξ© | May increase slightly after trip cycles |
Dynamic Characteristicsβ
| Parameter | Value | Unit | Conditions |
|---|---|---|---|
| Trip Time | 500 | ms | @ 3A (2x hold current) |
| Trip Time | 1-5 | s | @ 1.5-2A (sustained overload) |
| Trip Time | 0.1-0.5 | s | @ >5A (moderate short) |
| Reset Time | 30-60 | s | Time to cool and auto-reset |
| Operating Temp | -40 to +85 | Β°C | Full specification range |
How PTC Resettable Fuses Workβ
Normal Operation (0-1.5A)β
During normal operation below hold current:
Normal State (I < 1.5A):
+12V IN βββ [PTC: 0.03Ξ©] βββ +12V OUT
β
Low resistance
Minimal heating
Voltage drop: ~45mV @ 1.5A
The PTC remains in low-resistance state with negligible heating and voltage drop.
Overload Detection (1.5-3A)β
When current exceeds hold current:
Warming State (I > 1.5A):
+12V IN βββ [PTC: heating up] βββ +12V OUT
β
Polymer heating
Resistance increasing
Temperature rising
The polymer material begins to heat up due to IΒ²R losses, causing resistance to increase.
Trip State (I > 3A)β
When current reaches trip threshold:
Tripped State (I > 3A):
+12V IN βββ [PTC: >1000Ξ©] βββ Output (~10mA)
β
High resistance
Current limited
Hot, but safe
- Rapid heating: Polymer expands, resistance increases dramatically
- Current limiting: High resistance (>1kΞ©) limits current to ~10mA
- Self-heating: Remains hot and tripped until cause is removed
- Protection active: Circuit protected from overcurrent
Auto-Resetβ
After fault is removed and PTC cools:
Reset Sequence:
Remove fault β PTC cools (30-60s) β Resistance drops β Normal operation
β
Temperature decreases
Polymer contracts
Resistance returns to ~0.03Ξ©
The PTC automatically resets to low-resistance state without user intervention.
Circuit Integrationβ
Protection Architectureβ
This PTC is part of a 4-layer protection system:
Layer 1: USB-PD Adapter
β βββ Overcurrent protection (input side)
βΌ
Layer 2: LM2596S DC-DC Converter
β βββ Current limiting ~3-5A
βΌ
Layer 3: LM7812 Linear Regulator
β βββ Current limiting ~1.5-2.2A
β βββ Thermal shutdown @ 150Β°C
βΌ
Layer 4: This PTC (1.5A hold / 3A trip)
β βββ Auto-reset overcurrent protection
βΌ
+12V Output to Modules
Placement in Circuitβ
LM7812 Output βββ¬βββ PTC1 (1.5A) ββββ¬βββ TVS1 βββ +12V OUT
β SMD1210P150TF β SMAJ15A
β β β
β ββββββGND
β
ββββ LED2 βββ R7 (1kΞ©) βββ GND
Green status indicator
Connection:
- Input: +12V from LM7812 regulator output
- Output: +12V rail with TVS overvoltage protection
- LED indicator: Shows when power is present
Protection Behaviorβ
Scenario 1: Normal Operation (0-1.5A)β
Current: 0-1.5A
PTC State: β
Low resistance (0.03Ξ©)
Voltage Drop: ~45mV @ 1.5A
Status: Normal operation
Scenario 2: Moderate Overload (1.5-2.2A)β
Sequence:
1. Current rises to 1.5-2.2A
2. PTC starts heating (warming phase)
3. LM7812 current limiting also active
4. Within 1-5 seconds: PTC trips
5. Current drops to ~10mA
6. After 30-60s: Auto-resets
User Experience:
- LED dims/turns off (indicates overload)
- Auto-recovery after cooling
- No manual intervention needed
Scenario 3: Short Circuit (>2.2A)β
Sequence:
1. Output shorts to GND
2. LM7812 current limiting: IMMEDIATE (limits to ~2.2A)
3. PTC heating: Current limited by regulator
4. Within 1-5 seconds: PTC trips OR regulator thermal shutdown
5. Output current: ~0A
Protection Effectiveness:
β
LM7812 prevents >2.2A (instant)
β
PTC provides backup (1-5s)
β
Thermal shutdown if sustained (5-10s)
β
Auto-recovery when fault cleared
Design Considerationsβ
Why 1.5A Hold Current?β
Design target: 1.5A maximum continuous current (L7812CD2T-TR rated for 1.5A)
Analysis:
Hold current: 1.5A (chosen)
Design target: 1.5A
Margin: PTC and regulator rated equally
Benefits:
β
Matches L7812CD2T-TR maximum output (1.5A)
β
Tolerates brief surges without false trips
β
Trips at 3A provides clear fault indication
β
Regulator current limiting provides first-line protection
Interaction with LM7812β
Key insight: The linear regulator limits current before the PTC trips:
Current Limiting Cascade:
LM7812: Current limit @ ~2.2A max (immediate)
Thermal shutdown @ 150Β°C (1-5s)
β
PTC: Warming @ 1.5A (seconds)
Trip @ 3.0A (0.5-1s)
β
Result: PTC never sees >2.2A in practice!
Both protections complement each other
Protection stages:
| Current | LM7812 State | PTC State | Result |
|---|---|---|---|
| 0-1.5A | β Normal | β Normal | Normal operation |
| 1.5-2.2A | β οΈ Current limiting | β οΈ Warming | Both protections active |
| >2.2A | π Hard limit | π Trips (or warming) | Dual protection |
Voltage Dropβ
Normal operation voltage drop:
V_drop = I Γ R
V_drop = 1.2A Γ 0.03Ξ©
V_drop = 36mV
Impact on output:
Input: 12.00V (from LM7812)
Drop: -0.036V (PTC resistance)
Output: 11.96V β
(within spec)
Negligible impact on +12V rail voltage.
PCB Layout Recommendationsβ
Trace width for 1.5A:
- Minimum: 0.5mm (1oz copper)
- Recommended: 1.0mm (better thermal dissipation)
- Ideal: 1.5mm or copper pour
Thermal considerations:
- Place away from heat-generating components (LM7812)
- Good thermal contact with PCB copper
- Adequate airflow around component
- Keep-out zone: 1mm around component for proper cooling
Failure Modesβ
End-of-Life Behaviorβ
After many trip cycles (thousands):
- Initial resistance may increase slightly (0.03Ξ© β 0.05Ξ©)
- Hold current may decrease slightly
- Trip time may increase
- Still provides protection, just with degraded specs
Indicators of wear:
- Slower reset time
- Higher voltage drop during normal operation
- Earlier trip threshold
Design margin: Selected 1.5A hold matches regulator maximum. Degradation over time may require replacement if voltage drop exceeds spec.
Failure Mode: Stuck Open (Very Rare)β
If PTC fails permanently open:
Result: +12V rail dead
Backup: None on this specific rail
Detection: LED off, no +12V output
Recovery: Replace PTC
Probability: Very low (<0.1% over lifetime)
Failure Mode: Stuck Closed (Extremely Rare)β
If PTC fails permanently closed (doesn't trip):
Result: No PTC protection
Backup 1: LM7812 current limiting (still active)
Backup 2: LM7812 thermal shutdown (still active)
Backup 3: LM2596 DC-DC current limiting (still active)
Backup 4: USB-PD adapter protection (still active)
Probability: Extremely low (<0.01% over lifetime) Impact: Still protected by 3 other layers
Comparison to Alternativesβ
vs Traditional Fuse (1.5A Fast-Blow)β
| Feature | PTC (This Part) | Traditional Fuse |
|---|---|---|
| Auto-reset | β Yes (30-60s) | β No (must replace) |
| Response time | β οΈ 0.5-5s | β <100ms |
| User convenience | β Excellent | β Poor |
| JLCPCB availability | β Yes (7,525 stock) | β No (zero stock) |
| Cost | ~$0.15-0.20 | ~$0.05-0.10 |
| Voltage drop | 36mV @ 1.2A | <1mV |
| Best for | Overloads | Catastrophic shorts |
Verdict: PTC is better for this application due to auto-reset and availability.
vs Electronic Current Limitingβ
| Feature | PTC (This Part) | Active Limiting |
|---|---|---|
| Complexity | β Simple (passive) | β Complex (active) |
| Response time | β οΈ 0.5-5s | β <1Β΅s |
| Cost | β $0.15-0.20 | β $1-5+ |
| Reliability | β High (passive) | β οΈ Lower (active) |
| Auto-reset | β Yes | β Yes |
Verdict: PTC is adequate given LM7812 provides fast current limiting.
Testing and Validationβ
Acceptance Testingβ
Test 1: Normal operation
Apply: 1.2A load
Measure: Voltage drop across PTC
Expected: 30-40mV
Pass if: <50mV
Test 2: Overload trip
Apply: 2.5A load (or short output after regulator)
Expected: PTC trips within 5 seconds
Measure: Output current drops to <20mA
Reset: Remove load, wait 60s, verify recovery
Test 3: Reset functionality
1. Trip PTC with overload
2. Remove overload
3. Wait 60 seconds
4. Measure resistance: Should be <0.1Ξ©
5. Verify normal operation resumes
Long-term Monitoringβ
Monitor for degradation:
- Measure voltage drop periodically
- Check reset time after trip events
- Replace if V_drop >100mV @ 1.2A
Bill of Materialsβ
| Designator | Part | Package | JLCPCB Part # | Qty | Unit Price | Extended |
|---|---|---|---|---|---|---|
| PTC1 | SMD1210P150TF/16 | SMD1210 | C7529589 | 1 | $0.15 | $0.15 |
Related Componentsβ
- Protected circuit: L7812CD2T-TR (U6) - +12V Linear Regulator
- Upstream: LM2596S-ADJ (U2) - +13.5V DC-DC Converter
- Parallel rails: PTC +5V (C70119), PTC -12V (C2830246)
- Overvoltage protection: SMAJ15A (TVS1)
Referencesβ
- Protection design:
/doc/docs/learning/protection-fuse-strategy.md - Circuit diagram: Diagram5 - +12V Linear Regulator
- JLCPCB Part Page: https://jlcpcb.com/partdetail/C7529589