1SMA18AT3 Equivalent & Substitute Parts

Part Overview

The 1SMA18AT3 is a unidirectional Transient Voltage Suppressor (TVS) diode manufactured by onsemi, designed for general-purpose overvoltage protection in surface-mount applications. This component features an 18V reverse standoff voltage with a 29.2V maximum clamping voltage and 13.7A peak pulse current capability in a DO-214AC (SMA) package. The part is currently classified as obsolete, making equivalent substitutes necessary for new designs and ongoing production support.

Key Parameters

Parameter	Value	Unit
Voltage - Reverse Standoff (Typ)	18	V
Voltage - Breakdown (Min)	20	V
Voltage - Clamping (Max) @ Ipp	29.2	V
Current - Peak Pulse (10/1000µs)	13.7	A
Power - Peak Pulse	400	W
Package / Case	DO-214AC, SMA	—
Operating Temperature	-65°C ~ 150°C	TJ
Type	Zener	—
Unidirectional Channels	1	—
Mounting Type	Surface Mount	—

Substitute Part Grouping Explanation

Substitution eligibility for the 1SMA18AT3 is determined by the following critical parameters:

Voltage - Reverse Standoff: 18V (exact match required)
Voltage - Breakdown (Min): 20V (exact match required)
Voltage - Clamping (Max) @ Ipp: 29.2V (exact match required for direct substitution)
Current - Peak Pulse (10/1000µs): 13.7A minimum (equal or greater acceptable)
Power - Peak Pulse: 400W minimum (equal or greater acceptable)
Package / Case: DO-214AC, SMA (exact match required for PCB compatibility)
Type: Zener, Unidirectional (exact match required)
Mounting Type: Surface Mount (exact match required)

Substitute parts are categorized as follows:

Direct Equivalents: Parts matching all critical voltage parameters (18V standoff, 20V breakdown, 29.2V clamping) with identical or superior current and power ratings in the same SMA package.

Similar Alternatives: Parts with matching voltage standoff and breakdown specifications but differing clamping voltages or current ratings, or alternative package types (SOD-123W), suitable only when circuit design permits the parameter variance.

Parameter Comparison

Manufacturer Part Number	Manufacturer	Voltage - Reverse Standoff (V)	Voltage - Breakdown (V)	Voltage - Clamping (V)	Current - Peak Pulse (A)	Power - Peak Pulse (W)	Package	Product Status	RoHS Status
1SMA18AT3	onsemi	18	20	29.2	13.7	400	DO-214AC, SMA	Obsolete	RoHS non-compliant
SMAJ18A	MDD	18	20	29.2	13.7	400	DO-214AC, SMA	Active	ROHS3 Compliant
SMAJ18A-TR	STMicroelectronics	18	20	39.3	59 (8/20µs)	400	DO-214AC, SMA	Active	ROHS3 Compliant
PTVS18VS1UR,115	Nexperia USA Inc.	18	20	29.2	13.7	400	SOD-123W	Active	ROHS3 Compliant
SM4T21AY	STMicroelectronics	18	20	39.3	59 (8/20µs)	400	DO-214AC, SMA	Active	ROHS3 Compliant
SMA6J18A-TP	Micro Commercial Co	18	20	29.2	20.5	600	DO-214AC, SMA	Active	ROHS3 Compliant
SMAJ18A-13-F	Diodes Incorporated	18	20	29.2	13.7	400	DO-214AC, SMA	Active	ROHS3 Compliant
SMAJ18A-TP	Micro Commercial Co	18	20	29.2	13.7	400	DO-214AC, SMA	Active	ROHS3 Compliant
SMAJ18AE3/TR13	Microchip Technology	18	20	29.2	17.2	500	DO-214AC, SMA	Active	ROHS3 Compliant
SMAJ18E3/TR13	Microchip Technology	18	20	32.2	15.5	500	DO-214AC, SMA	Active	ROHS3 Compliant

Engineering Selection Recommendations

Primary Direct Substitutes (Identical Electrical Performance):

SMAJ18A (MDD), SMAJ18A-13-F (Diodes Incorporated), and SMAJ18A-TP (Micro Commercial Co) are direct functional equivalents with matching voltage parameters (18V standoff, 20V breakdown, 29.2V clamping) and identical current/power ratings. All three are active products with ROHS3 compliance, providing superior supply chain availability compared to the obsolete 1SMA18AT3.

Enhanced Performance Alternatives (Same Package, Higher Ratings):

SMA6J18A-TP (Micro Commercial Co) and SMAJ18AE3/TR13 (Microchip Technology) maintain identical voltage specifications while offering increased current and power dissipation capacity (20.5A/600W and 17.2A/500W respectively). These are suitable for applications requiring additional thermal margin or higher transient energy absorption.

Higher Clamping Voltage Alternatives (Same Package):

SMAJ18A-TR (STMicroelectronics) and SM4T21AY (STMicroelectronics) feature elevated clamping voltages (39.3V) with significantly higher peak pulse current (59A at 8/20µs). These are applicable only when circuit design specifications permit the higher clamping voltage.

SMAJ18E3/TR13 (Microchip Technology) offers a 32.2V clamping voltage with 15.5A peak pulse current, representing a moderate increase in both parameters.

Alternative Package Option:

PTVS18VS1UR,115 (Nexperia USA Inc.) provides identical electrical performance in SOD-123W package with automotive-grade qualification (AEC-Q101). This option is suitable only for designs where package footprint change is acceptable and automotive-grade components are required.

All recommended substitutes carry ROHS3 compliance and are currently in active production status, ensuring long-term availability and regulatory alignment.

Frequently Asked Questions (FAQ)

Q: Can SMAJ18A-TR or SM4T21AY be used as direct replacements for 1SMA18AT3?

A: These parts share the same 18V standoff and 20V breakdown voltages but feature higher clamping voltages (39.3V vs. 29.2V). They are electrically compatible only if the circuit design permits the increased clamping voltage. The higher peak pulse current (59A) provides additional protection margin but does not affect basic substitutability.

Q: What is the difference between SMAJ18A-13-F and SMAJ18A-TP?

A: Both parts are direct electrical equivalents with identical voltage and current ratings. The primary differences are manufacturer (Diodes Incorporated vs. Micro Commercial Co) and operating temperature range (-55°C to 150°C vs. -55°C to 175°C). Selection depends on supply chain availability and thermal requirements.

Q: Is PTVS18VS1UR,115 a suitable replacement despite the different package?

A: PTVS18VS1UR,115 matches all electrical parameters (18V standoff, 20V breakdown, 29.2V clamping, 13.7A peak pulse) but uses SOD-123W package instead of DO-214AC (SMA). This substitution requires PCB redesign and is only viable if the smaller footprint is acceptable. The part includes automotive-grade qualification (AEC-Q101).

Q: Why do some substitutes have higher power ratings (500W or 600W vs. 400W)?

A: Higher power ratings indicate greater thermal dissipation capability and energy absorption capacity. These parts remain compatible with the 1SMA18AT3 application and provide additional design margin. The increased rating does not affect circuit operation but improves reliability under transient stress conditions.

Q: Are all recommended substitutes RoHS compliant?

A: All active substitute parts listed are ROHS3 compliant. The original 1SMA18AT3 is RoHS non-compliant. Compliance is mandatory for new designs and many production environments.

Q: What is the significance of the 10/1000µs vs. 8/20µs pulse specification?

A: These represent different test pulse waveforms for measuring peak pulse current. The 10/1000µs waveform (used for 1SMA18AT3 and most substitutes) is a slower transient profile, while 8/20µs (used for SMAJ18A-TR and SM4T21AY) represents faster transients. Both specifications are valid; selection depends on the actual transient profile in the application circuit.

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