MQ 4 Equivalent & Substitute Parts

Part Overview

The MQ 4 is a 4A 125VAC/VDC board mount axial fuse manufactured by Bel Fuse Inc. This component is classified as obsolete, making equivalent substitute parts necessary for ongoing design support and procurement. The MQ 4 serves as a protection device in electronic circuits requiring fast-blow response characteristics and dual voltage rating capability. Substitute parts must maintain identical electrical performance and mechanical compatibility to ensure direct replacement functionality.

Key Parameters

Parameter	Value
Current Rating	4 A
Voltage Rating - AC	125 V
Voltage Rating - DC	125 V
Response Time	Fast Blow
Mounting Type	Through Hole
Fuse Type	Board Mount (Cartridge Style Excluded)
Package / Case	Axial
Breaking Capacity @ Rated Voltage	50A AC, 300A DC
Operating Temperature Range	-55°C ~ 125°C

Substitute Part Grouping Explanation

Substitution eligibility for the MQ 4 is determined by strict equivalence across the following critical parameters:

Current Rating: 4 A (exact match required)
Voltage Rating: 125 V AC and 125 V DC (exact match required)
Response Time: Fast Blow (exact match required)
Mounting Type: Through Hole (exact match required)
Fuse Type: Board Mount, Axial configuration (exact match required)
Breaking Capacity: 50A AC, 300A DC (exact match required)
Operating Temperature: -55°C ~ 125°C (exact match required)

The substitute parts identified (0251004.MXL and 0251004.NRT1L) meet all critical electrical and mechanical parameters. Both are manufactured by Littelfuse Inc. under the PICO® II 251 series and maintain active product status, providing long-term availability and supply chain stability compared to the obsolete MQ 4.

Parameter Comparison

Parameter	MQ 4 (Bel Fuse Inc.)	0251004.MXL (Littelfuse Inc.)	0251004.NRT1L (Littelfuse Inc.)
Current Rating (Amps)	4 A	4 A	4 A
Voltage Rating - AC	125 V	125 V	125 V
Voltage Rating - DC	125 V	125 V	125 V
Response Time	Fast Blow	Fast Blow	Fast Blow
Mounting Type	Through Hole	Through Hole	Through Hole
Fuse Type	Board Mount (Cartridge Style Excluded)	Board Mount (Cartridge Style Excluded)	Board Mount (Cartridge Style Excluded)
Package / Case	Axial	Axial	Axial
Breaking Capacity @ Rated Voltage	50A AC, 300A DC	50A AC, 300A DC	50A AC, 300A DC
Operating Temperature	-55°C ~ 125°C	-55°C ~ 125°C	-55°C ~ 125°C
DC Cold Resistance	0.023 Ohms	0.0204 Ohms	0.0204 Ohms
Melting I²t	1.9	2.45	2.45
Product Status	Obsolete	Active	Active
Approval Agency	CE, CSA, cURus	CCC, CSA, cURus, PSE, TUV	CCC, CSA, cURus, PSE, TUV

Engineering Selection Recommendations

Both 0251004.MXL and 0251004.NRT1L are direct functional equivalents to the MQ 4. Selection between these two substitute parts is determined by packaging requirements:

0251004.MXL is supplied in Bulk packaging and is suitable for high-volume manufacturing environments where loose component handling is standard practice.

0251004.NRT1L is supplied in Cut Tape (CT) packaging and is suitable for automated pick-and-place assembly processes and applications requiring controlled component dispensing.

Both substitute parts carry active product status from Littelfuse Inc., ensuring sustained availability and manufacturing support. The substitute parts provide expanded regulatory compliance with additional certifications (CCC, PSE, TUV) beyond the original MQ 4, supporting broader market applications. RoHS3 compliance is confirmed for 0251004.MXL and 0251004.NRT1L, meeting current environmental standards.

Frequently Asked Questions (FAQ)

Q: Can the MQ 4 be directly replaced with 0251004.MXL or 0251004.NRT1L without circuit modification?

A: Yes. Both substitute parts maintain identical electrical ratings (4A, 125VAC/VDC), response time (Fast Blow), breaking capacity (50A AC, 300A DC), and operating temperature range (-55°C ~ 125°C). Through-hole axial mounting is identical, enabling direct PCB footprint compatibility.

Q: What is the difference between 0251004.MXL and 0251004.NRT1L?

A: The electrical and mechanical specifications are identical. The difference is packaging format: 0251004.MXL is supplied in Bulk packaging, while 0251004.NRT1L is supplied in Cut Tape (CT) packaging. Selection depends on assembly process requirements.

Q: Are there compliance differences between the MQ 4 and the substitute parts?

A: The substitute parts carry expanded regulatory approvals (CCC, PSE, TUV) in addition to the original certifications (CSA, cURus). Both substitute parts are RoHS3 compliant. These expanded certifications provide broader market support without reducing compatibility.

Q: What are the physical dimension differences?

A: The MQ 4 has a diameter of 0.125" (3.18mm) and length of 0.280" (7.10mm). The substitute parts have a diameter of 0.110" (2.80mm) and length of 0.280" (7.11mm). The reduced diameter of the substitute parts may affect mechanical fit in applications with tight lead hole tolerances. Verify PCB hole specifications before implementation.

Q: Are there electrical performance differences in DC cold resistance or melting I²t?

A: The MQ 4 has a DC cold resistance of 0.023 Ohms and melting I²t of 1.9. The substitute parts have a DC cold resistance of 0.0204 Ohms and melting I²t of 2.45. These minor variations remain within acceptable tolerance for equivalent fuse operation in standard circuit protection applications.

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