Equivalent & Substitute Parts for 5ST 1.25

Part Overview

The Bel Fuse Inc. 5ST 1.25 is a 1.25 A, 250 V AC/DC glass cartridge fuse with 5mm x 20mm dimensions and slow-blow response characteristics. This part is classified as obsolete. Identifying equivalent and substitute parts is necessary to maintain design continuity, ensure supply chain availability, and support ongoing production requirements for applications utilizing this fuse specification.

Key Parameters

Parameter	Value
Current Rating	1.25 A
Voltage Rating (AC)	250 V
Fuse Type	Cartridge, Glass
Package / Case	5mm x 20mm
Response Time	Slow Blow
Mounting Type	Requires Holder
Breaking Capacity @ Rated Voltage	35 A
Operating Temperature Range	-55°C to 125°C

Substitute Part Grouping Explanation

Substitute parts for the 5ST 1.25 are grouped based on the following critical parameters that determine functional equivalence:

Current Rating: 1.25 A (exact match required)
Voltage Rating: 250 V AC (exact match required)
Package / Case: 5mm x 20mm (exact match required)
Response Time: Slow Blow (exact match required)
Fuse Type: Cartridge, Glass (exact match required)
Mounting Type: Requires Holder (exact match required)

Parts are classified as direct substitutes when all critical parameters match the original specification. Parametric equivalents share the same electrical and mechanical characteristics but may originate from different manufacturers or series lines. Similar parts meet the core functional requirements but may have variations in secondary parameters such as breaking capacity or melting I²t values.

Parameter Comparison

Part Number	Manufacturer	Series	Current (A)	Voltage (V)	Package	Response	Breaking Capacity (A)	Melting I²t	Product Status	RoHS Status
5ST 1.25	Bel Fuse Inc.	5ST	1.25	250	5x20mm	Slow Blow	35	4.1	Obsolete	RoHS non-compliant
5ST 1.25-R	Bel Fuse Inc.	5ST	1.25	250	5x20mm	Slow Blow	35	4.1	Active	ROHS3 Compliant
5ET 1.25-R	Bel Fuse Inc.	5ET	1.25	250	5x20mm	Slow Blow	150	42	Active	ROHS3 Compliant
5TT 1.25-R	Bel Fuse Inc.	5TT	1.25	250	5x20mm	Slow Blow	100	13.1	Active	ROHS3 Compliant
0034.3118	SCHURTER Inc.	FST 5x20	1.25	250	5x20mm	Slow Blow	35	5.5	Active	ROHS3 Compliant
02181.25HXP	Littelfuse Inc.	218	1.25	250	5x20mm	Slow Blow	35	12.65	Active	ROHS3 Compliant
02181.25MXBP	Littelfuse Inc.	218	1.25	250	5x20mm	Slow Blow	35	12.65	Active	ROHS3 Compliant
02181.25MXP	Littelfuse Inc.	218	1.25	250	5x20mm	Slow Blow	35	12.65	Active	ROHS3 Compliant
02181.25VXP	Littelfuse Inc.	218	1.25	250	5x20mm	Slow Blow	35	12.65	Active	ROHS3 Compliant
02131.25MXP	Littelfuse Inc.	213	1.25	250	5x20mm	Slow Blow	35	27.15	Active	ROHS3 Compliant
02181.25TXP	Littelfuse Inc.	218	1.25	250	5x20mm	Slow Blow	35	12.65	Active	ROHS3 Compliant

Engineering Selection Recommendations

Direct Replacement Priority: The 5ST 1.25-R from Bel Fuse Inc. is the primary direct substitute. It maintains identical electrical and mechanical specifications to the original 5ST 1.25 while offering active product status and ROHS3 compliance, addressing obsolescence concerns.

Cross-Manufacturer Alternatives: Littelfuse Inc. 218 series fuses (02181.25HXP, 02181.25MXBP, 02181.25MXP, 02181.25VXP, 02181.25TXP) and the 213 series (02131.25MXP) provide functionally equivalent alternatives with identical current, voltage, and package specifications. These parts carry broader regulatory certifications including CCC, CE, CSA, K-MARK, PSE, SEMKO, UL, and VDE. Melting I²t values vary across these parts (12.65 to 27.15), which may affect circuit protection response characteristics in specific applications.

Secondary Alternatives: SCHURTER Inc. 0034.3118 (FST 5x20 series) and Bel Fuse Inc. 5ET 1.25-R and 5TT 1.25-R provide parametric equivalence with variations in breaking capacity and melting I²t. The 5ET 1.25-R offers significantly higher breaking capacity (150 A) and melting I²t (42), while the 5TT 1.25-R provides intermediate values (100 A breaking capacity, 13.1 melting I²t).

Compliance Considerations: All active substitute parts listed are ROHS3 compliant, addressing environmental regulatory requirements. Regulatory certifications vary by manufacturer; selection should align with target market requirements and end-application compliance standards.

Frequently Asked Questions (FAQ)

Q: Can the 5ST 1.25-R be used as a direct replacement for the obsolete 5ST 1.25? A: Yes. The 5ST 1.25-R maintains identical electrical specifications (1.25 A, 250 V), mechanical package (5x20mm), response time (slow blow), and breaking capacity (35 A). It is the recommended direct substitute and is currently in active production with ROHS3 compliance.

Q: What is the difference between the Littelfuse 218 series and 213 series fuses? A: Both series meet the core electrical and mechanical requirements (1.25 A, 250 V, 5x20mm, slow blow). The primary difference is melting I²t: the 218 series has a melting I²t of 12.65, while the 213 series has 27.15. This affects the thermal response characteristics and circuit protection behavior. Selection depends on specific application requirements.

Q: Are there differences in breaking capacity among the substitute parts? A: Yes. Most substitutes maintain the original 35 A breaking capacity. However, the Bel Fuse Inc. 5ET 1.25-R offers 150 A breaking capacity, and the 5TT 1.25-R offers 100 A. Higher breaking capacity provides enhanced protection in circuits with higher fault currents but does not affect normal operation at rated current.

Q: Do all substitute parts require the same holder as the original 5ST 1.25? A: All listed substitutes are cartridge fuses with 5x20mm dimensions and require a compatible fuse holder. Physical dimensions are consistent across all parts, ensuring mechanical compatibility with standard 5x20mm fuse holders.

Q: Which substitute offers the broadest regulatory certification coverage? A: Littelfuse Inc. 218 and 213 series fuses carry the most comprehensive certification portfolio, including CCC, CE, CSA, K-MARK, PSE, SEMKO, UL, and VDE. These certifications support deployment in diverse geographic markets and regulated applications.

Q: Can the 5ET 1.25-R or 5TT 1.25-R be used interchangeably with the 5ST 1.25-R? A: All three parts meet the core electrical and mechanical specifications. However, they differ in breaking capacity and melting I²t. The 5ET 1.25-R and 5TT 1.25-R are parametric equivalents suitable for applications where higher breaking capacity or different thermal response is acceptable or beneficial. Application-specific circuit analysis is required to confirm suitability.

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