3AP 125-R Equivalent & Substitute Parts

Part Overview

The 3AP 125-R is a 125 mA, 250 V AC/DC fast-blow glass cartridge fuse manufactured by Bel Fuse Inc. in the 3AB/3AG package format (1/4" x 1-1/4" axial). This component is classified as obsolete, making equivalent substitute parts necessary for ongoing system maintenance and new design implementations. Active alternatives from Littelfuse Inc. and Eaton Electronics Division provide direct functional equivalence while maintaining current product support and availability.

Key Parameters

Parameter	Value
Current Rating	125 mA
Voltage Rating (AC)	250 V
Fuse Type	Cartridge, Glass
Response Time	Fast Blow
Package / Case	3AB, 3AG, 1/4" x 1-1/4" (Axial)
Mounting Type	Through Hole
Operating Temperature Range	-55°C ~ 125°C

Substitute Part Grouping Explanation

Substitution eligibility is determined by strict equivalence across the following parameters:

Current Rating: 125 mA (exact match required)
Voltage Rating: 250 V AC (exact match required)
Fuse Type: Cartridge, Glass (exact match required)
Response Time: Fast Blow (exact match required)
Package / Case: 3AB, 3AG, 1/4" x 1-1/4" (Axial) (exact match required)
Mounting Type: Through Hole (exact match required)
Operating Temperature Range: -55°C ~ 125°C (exact match required)

All substitute parts listed meet these criteria. Variations in breaking capacity, melting I²t, DC cold resistance, and dimensional tolerances are within acceptable engineering ranges for this product category and do not affect functional substitution.

Parameter Comparison

Parameter	3AP 125-R (Bel Fuse)	0318.125HXP (Littelfuse)	0318.125MXBP (Littelfuse)	0318.125MXP (Littelfuse)	0318.125VXP (Littelfuse)	BK/AGC-V-1/8 (Eaton)
Current Rating (Amps)	125 mA	125 mA	125 mA	125 mA	125 mA	125 mA
Voltage Rating - AC	250 V	250 V	250 V	250 V	250 V	250 V
Fuse Type	Cartridge, Glass	Cartridge, Glass	Cartridge, Glass	Cartridge, Glass	Cartridge, Glass	Cartridge, Glass
Response Time	Fast Blow	Fast Blow	Fast Blow	Fast Blow	Fast Blow	Fast Blow
Package / Case	3AB, 3AG, 1/4" x 1-1/4" (Axial)	3AB, 3AG, 1/4" x 1-1/4" (Axial)	3AB, 3AG, 1/4" x 1-1/4" (Axial)	3AB, 3AG, 1/4" x 1-1/4" (Axial)	3AB, 3AG, 1/4" x 1-1/4" (Axial)	3AB, 3AG, 1/4" x 1-1/4" (Axial)
Mounting Type	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole
Operating Temperature	-55°C ~ 125°C	-55°C ~ 125°C	-55°C ~ 125°C	-55°C ~ 125°C	-55°C ~ 125°C	-55°C ~ 125°C
Breaking Capacity @ Rated Voltage	200A	35A	35A	35A	35A	35A
Melting I²t	0.004	0.00289	0.00289	0.00289	0.00289	0.002
DC Cold Resistance (Ohms)	12	7.145	7.145	7.145	7.145	10.825
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	RoHS non-compliant
Approval Agency	CE, CSA, cULus	CE, CSA, UL	CE, CSA, UL	CE, CSA, UL	CE, CSA, UL	CSA, UL
Product Status	Obsolete	Active	Active	Active	Active	Active

Engineering Selection Recommendations

All Littelfuse 318 series parts (0318.125HXP, 0318.125MXBP, 0318.125MXP, 0318.125VXP) are functionally equivalent substitutes for the obsolete 3AP 125-R. These parts maintain active product status, carry ROHS3 compliance, and hold CE, CSA, and UL certifications. The Littelfuse alternatives are preferred for new designs and ongoing production due to current manufacturer support and regulatory compliance.

The Eaton BK/AGC-V-1/8 is a functional equivalent with active product status and CSA/UL certification. However, this part does not meet ROHS3 compliance requirements. Selection of the Eaton part is appropriate only in applications where RoHS non-compliance is acceptable or where existing system specifications explicitly reference AGC series components.

Frequently Asked Questions (FAQ)

Q: Can the Littelfuse 318 series parts be used interchangeably with the 3AP 125-R?

A: Yes. The 0318.125HXP, 0318.125MXBP, 0318.125MXP, and 0318.125VXP are direct functional equivalents. All share identical current rating (125 mA), voltage rating (250 V AC), fuse type (cartridge glass), response time (fast blow), package format (3AB/3AG, 1/4" x 1-1/4"), and operating temperature range (-55°C ~ 125°C).

Q: What is the difference between the four Littelfuse 318 series substitute parts?

A: The four Littelfuse parts (0318.125HXP, 0318.125MXBP, 0318.125MXP, 0318.125VXP) are electrically and mechanically identical. Differences exist only in packaging designation and internal product line categorization. Selection among these parts is based on packaging availability and inventory requirements rather than functional performance.

Q: Why does the 3AP 125-R have a higher breaking capacity (200A) than the substitute parts (35A)?

A: Breaking capacity variation does not affect substitution eligibility for this product category. The 3AP 125-R and all substitute parts are rated for identical current (125 mA), voltage (250 V), and response time (fast blow). Breaking capacity represents the maximum fault current the fuse can safely interrupt; the lower breaking capacity of substitute parts remains adequate for standard circuit protection applications within the specified electrical parameters.

Q: Should the Eaton BK/AGC-V-1/8 be selected over Littelfuse alternatives?

A: The Eaton part is a functional equivalent but does not meet ROHS3 compliance. Selection should be based on system requirements. For applications requiring ROHS3 compliance, Littelfuse 318 series parts are the appropriate choice. The Eaton part is suitable only where RoHS non-compliance is explicitly acceptable or where existing designs mandate AGC series components.

Q: Are there dimensional differences between the 3AP 125-R and substitute parts?

A: Dimensional variations are minimal and within acceptable tolerances. The 3AP 125-R measures 0.272" Dia x 1.280" L (6.90mm x 32.50mm). Littelfuse substitutes measure 0.275" Dia x 1.288" L (6.99mm x 32.72mm). The Eaton part measures 0.266" Dia x 1.292" L (6.76mm x 32.82mm). All parts fit standard 3AB/3AG fuse holders without modification.

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