RUSBF185 Polymeric PTC Resettable Fuse Equivalent & Substitute Parts

Part Overview

The RUSBF185 is an active polymeric PTC resettable fuse manufactured by Littelfuse Inc. in the PolySwitch® RUSBF series. This component provides overcurrent protection with automatic reset capability in through-hole radial disc configuration. The RUSBF185 is rated for 16V maximum voltage with 1.85A hold current and is designed for applications requiring polymeric PTC protection in compact form factors. Finding equivalent substitute parts is necessary when managing inventory constraints, lead time requirements, or when alternative sourcing from qualified manufacturers is required while maintaining electrical and mechanical compatibility.

Key Parameters

Parameter	RUSBF185 Value
Voltage - Max	16V
Current - Hold (Ih) (Max)	1.85 A
Current - Trip (It)	3.7 A
Current - Max	40 A
Resistance - Initial (Ri) (Min)	30 mOhms
Resistance - Post Trip (R1) (Max)	90 mOhms
Operating Temperature	-40°C ~ 85°C
Mounting Type	Through Hole
Package / Case	Radial, Disc
Lead Spacing	0.199" (5.05mm)
Type	Polymeric
Product Status	Active
RoHS Status	ROHS3 Compliant

Substitute Part Grouping Explanation

Substitution eligibility for polymeric PTC resettable fuses is determined by the following critical parameters:

Mandatory Compatibility Parameters:

Mounting Type: Must be Through Hole to maintain PCB assembly compatibility
Package / Case: Must be Radial, Disc configuration for mechanical fit
Type: Must be Polymeric PTC technology
Lead Spacing: Must be within acceptable tolerance (0.199" ± tolerance) to ensure proper board hole alignment

Electrical Compatibility Parameters:

Current - Hold (Ih): Must match or exceed the application requirement (1.85A maximum)
Voltage - Max: Must equal or exceed the circuit voltage rating (16V minimum)
Operating Temperature: Must encompass the application temperature range (-40°C ~ 85°C)
Current - Trip (It) and Current - Max: Must be compatible with circuit protection requirements

The MF-R185 substitute part meets all mandatory compatibility parameters and maintains electrical performance within acceptable engineering tolerances for polymeric PTC fuse substitution.

Parameter Comparison

Parameter	RUSBF185 (Littelfuse)	MF-R185 (Bourns)	Compatibility Status
Voltage - Max	16V	30V	Compatible - Higher rating acceptable
Current - Hold (Ih) (Max)	1.85 A	1.85 A	Exact Match
Current - Trip (It)	3.7 A	3.7 A	Exact Match
Current - Max	40 A	40 A	Exact Match
Resistance - Initial (Ri) (Min)	30 mOhms	40 mOhms	Compatible - Within tolerance
Resistance - Post Trip (R1) (Max)	90 mOhms	110 mOhms	Compatible - Within tolerance
Operating Temperature	-40°C ~ 85°C	-40°C ~ 85°C	Exact Match
Mounting Type	Through Hole	Through Hole	Exact Match
Package / Case	Radial, Disc	Radial, Disc	Exact Match
Lead Spacing	0.199" (5.05mm)	0.201" (5.10mm)	Compatible - Within tolerance
Type	Polymeric	Polymeric	Exact Match
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	Exact Match

Engineering Selection Recommendations

The MF-R185 (Bourns Multifuse® series) is an active product with ROHS3 compliance and qualified agency approvals (cURus, TUV). This substitute part maintains electrical equivalence with the RUSBF185 across all critical parameters including hold current, trip current, and operating temperature range. The higher voltage rating (30V vs. 16V) provides additional design margin without compromising circuit protection function. Lead spacing tolerance (0.201" vs. 0.199") is within acceptable PCB assembly variation. Both components are polymeric PTC technology in through-hole radial disc configuration, ensuring mechanical and functional interchangeability.

Frequently Asked Questions (FAQ)

Q: Can the MF-R185 be used as a direct replacement for the RUSBF185 in existing designs?

A: Yes. The MF-R185 maintains identical hold current (1.85A), trip current (3.7A), and operating temperature range (-40°C ~ 85°C). Lead spacing tolerance (0.201" vs. 0.199") is within standard PCB assembly specifications. Both components are through-hole radial disc polymeric PTC fuses with ROHS3 compliance.

Q: What is the significance of the voltage rating difference (16V vs. 30V)?

A: The MF-R185 has a higher maximum voltage rating (30V) compared to the RUSBF185 (16V). In circuit applications operating at 16V or below, the higher voltage rating of the substitute part provides additional design margin and does not affect protection function. The hold and trip current characteristics remain identical.

Q: Are there mechanical fit differences between these parts?

A: Both parts are radial disc configuration with through-hole mounting. Lead spacing differs by 0.006" (0.05mm), which is within standard PCB hole tolerance. Physical dimensions vary slightly (RUSBF185: 0.402" L x 0.118" W; MF-R185: 0.472" Dia x 0.118" T), but both fit standard through-hole radial fuse footprints.

Q: What compliance certifications apply to the MF-R185?

A: The MF-R185 holds cURus and TUV approvals and is ROHS3 compliant. The RUSBF185 holds CSA, TUV, and UL approvals and is ROHS3 compliant. Both components meet industrial safety and environmental compliance standards.

Q: How do the resistance characteristics compare?

A: Initial resistance (Ri) differs by 10 mOhms (RUSBF185: 30 mOhms min; MF-R185: 40 mOhms min). Post-trip resistance (R1) differs by 20 mOhms (RUSBF185: 90 mOhms max; MF-R185: 110 mOhms max). These variations are within acceptable tolerance for polymeric PTC fuse substitution and do not affect circuit protection performance.

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