SRP420 Polymeric PTC Resettable Fuse Equivalent & Substitute Parts

Part Overview

The SRP420 is a polymeric PTC resettable fuse manufactured by Littelfuse Inc., rated for 30V maximum voltage and 4.2A hold current. This component is classified as obsolete, making equivalent and substitute parts necessary for ongoing system support and new design implementations. The SRP420 operates within the PolySwitch® SRP series and provides overcurrent protection through polymeric positive temperature coefficient technology.

Key Parameters

Parameter	Value
Voltage - Max	30V
Current - Hold (Ih) (Max)	4.2 A
Current - Trip (It)	7.6 A
Current - Max	100 A
Time to Trip	6 s
Resistance - Initial (Ri) (Min)	12 mOhms
Resistance - Post Trip (R1) (Max)	40 mOhms
Operating Temperature	-40°C ~ 85°C
Package / Case	Strap
Size / Dimension	1.240" L x 0.522" W (31.50mm x 13.25mm)
Thickness (Max)	0.043" (1.10mm)
Type	Polymeric

Substitute Part Grouping Explanation

Substitution for the SRP420 is determined by strict electrical and mechanical parameter matching across the polymeric PTC resettable fuse category. The following criteria establish valid equivalence:

Electrical Parameters (Critical):

Voltage - Max: 30V
Current - Hold (Ih) (Max): 4.2 A
Current - Trip (It): 7.6 A
Current - Max: 100 A
Time to Trip: 6 s
Resistance - Initial (Ri) (Min): 12 mOhms
Resistance - Post Trip (R1) (Max): 40 mOhms

Mechanical Parameters (Critical):

Package / Case: Strap
Size / Dimension: 1.240" L x 0.522" W (31.50mm x 13.25mm)
Thickness (Max): 0.043" (1.10mm)

Environmental Parameters:

Operating Temperature: -40°C ~ 85°C

All identified substitute parts match these parameters exactly, ensuring functional and mechanical compatibility with applications originally designed for the SRP420.

Parameter Comparison

Parameter	SRP420 (Littelfuse)	SRP420F (Littelfuse)	MF-S420 (Bourns)	0ZSA0420FF1E (Bel Fuse)
Voltage - Max	30V	30V	30V	30V
Current - Hold (Ih) (Max)	4.2 A	4.2 A	4.2 A	4.2 A
Current - Trip (It)	7.6 A	7.6 A	7.6 A	7.6 A
Current - Max	100 A	100 A	100 A	100 A
Time to Trip	6 s	6 s	6 s	6 s
Resistance - Initial (Ri) (Min)	12 mOhms	12 mOhms	12 mOhms	12 mOhms
Resistance - Post Trip (R1) (Max)	40 mOhms	40 mOhms	40 mOhms	40 mOhms
Operating Temperature	-40°C ~ 85°C	-40°C ~ 85°C	-40°C ~ 85°C	-40°C ~ 85°C
Package / Case	Strap	Strap	Strap	Strap
Size / Dimension	1.240" L x 0.522" W (31.50mm x 13.25mm)	1.240" L x 0.522" W (31.50mm x 13.25mm)	1.240" L x 0.522" W (31.50mm x 13.25mm)	1.240" L x 0.522" W (31.50mm x 13.25mm)
Thickness (Max)	0.043" (1.10mm)	0.043" (1.10mm)	0.043" (1.10mm)	0.043" (1.10mm)
Type	Polymeric	Polymeric	Polymeric	Polymeric
Product Status	Obsolete	Active	Active	Active
RoHS Status	RoHS non-compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant
Moisture Sensitivity Level (MSL)	2A (4 Weeks)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)
Approval Agency	CSA, TUV, UL	CSA, TUV, UL	cURus, TUV	cURus, TUV

Engineering Selection Recommendations

All three substitute parts—SRP420F, MF-S420, and 0ZSA0420FF1E—provide complete electrical and mechanical equivalence to the obsolete SRP420. Selection among these alternatives should be based on the following factors:

SRP420F (Littelfuse): This part maintains manufacturer continuity within the PolySwitch® SRP series. It carries active product status, ROHS3 compliance, and improved moisture sensitivity rating (MSL 1 vs. MSL 2A). Approval agencies include CSA, TUV, and UL. This option is optimal for applications requiring direct series compatibility and maximum regulatory alignment.

MF-S420 (Bourns): This Multifuse® series alternative offers active product status and ROHS3 compliance with MSL 1 rating. Approval agencies include cURus and TUV. This option provides cross-manufacturer equivalence with established supply chain availability.

0ZSA0420FF1E (Bel Fuse): This 0ZSA series part delivers active product status, ROHS3 compliance, and MSL 1 rating. Approval agencies include cURus and TUV. This option extends substitution options across a third qualified manufacturer.

All substitute parts are REACH Unaffected and carry identical ECCN (EAR99) and HTSUS (8533.40.8070) classifications.

Frequently Asked Questions (FAQ)

Q: Can SRP420F directly replace SRP420 in existing designs? A: Yes. SRP420F matches all critical electrical parameters (30V, 4.2A hold current, 7.6A trip current, 6s trip time) and mechanical specifications (strap package, 31.50mm x 13.25mm dimensions, 1.10mm thickness). The primary difference is product status (active vs. obsolete) and improved compliance ratings.

Q: What are the key differences between SRP420 and its substitutes? A: The SRP420 is obsolete with RoHS non-compliance and MSL 2A rating. All substitutes are active products with ROHS3 compliance and MSL 1 rating, providing superior regulatory alignment and moisture handling characteristics. Electrical and mechanical parameters remain identical.

Q: Are MF-S420 and 0ZSA0420FF1E compatible with SRP420 applications? A: Yes. Both parts match all specified electrical parameters and mechanical dimensions. Cross-manufacturer substitution is valid when electrical ratings and package specifications are identical, as they are in this case.

Q: Does packaging type affect substitution eligibility? A: Yes. All qualified substitutes use strap packaging identical to the SRP420. Package type is a critical mechanical parameter that must match exactly for direct substitution.

Q: What certifications should I verify for my application? A: Verify that your application requirements align with the approval agencies listed for your selected substitute. SRP420F includes CSA, TUV, and UL. MF-S420 and 0ZSA0420FF1E include cURus and TUV. Select the part whose certifications match your system requirements.

Q: Is moisture sensitivity a concern when switching from SRP420? A: The SRP420 carries MSL 2A (4 weeks). All substitutes carry MSL 1 (unlimited), representing an improvement in moisture tolerance. This change is beneficial and requires no design modifications.

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