ST3-28B21 Equivalent & Substitute Parts

Part Overview

The ST3-28B21 is a laminated core power transformer manufactured by Pulse Electronics, rated at 2.4VA with dual secondary windings configured for parallel (14V, 170mA) or series (28V, 85mA) operation. This through-hole mounted component is designed for applications requiring isolated AC power conversion with a primary voltage of 115V and maximum isolation of 2500Vrms. The part is currently active in production with RoHS compliance and unlimited moisture sensitivity rating, making it suitable for standard industrial and commercial electronic equipment.

Equivalent parts are identified when manufacturers discontinue production, inventory becomes constrained, or design flexibility requires alternative sourcing from qualified suppliers. The ST3-28B21 has one qualified substitute that maintains electrical and mechanical compatibility within specified tolerances.

Substiute Parts

ST3-28B21

Pulse ElectronicsIn Stock: 1215ST3-28B21 Datasheet

Current PartRFQ

3FS-328

TamuraIn Stock: 20453FS-328 Datasheet

SimilarRFQ

Key Parameters

Substitute Part Grouping Explanation

Substitution eligibility for the ST3-28B21 is determined by strict equivalence across the following critical parameters:

• Electrical Performance: Identical primary voltage (115V), secondary voltage outputs (14V parallel, 28V series), and current ratings (170mA parallel, 85mA series)
• Power Rating: Exact match at 2.4VA
• Core Technology: Laminated core construction
• Winding Architecture: Single primary winding with dual secondary windings, no center tap
• Isolation Rating: 2500Vrms minimum
• Mounting and Termination: Through-hole mounting with PC pin termination
• Compliance: RoHS compliance and EAR99 classification

The 3FS-328 from Tamura meets all substitution criteria and is classified as a direct equivalent.

Parameter Comparison

Engineering Selection Recommendations

Both the ST3-28B21 and 3FS-328 are active production components with full RoHS compliance and identical electrical specifications. Selection between these parts should be based on:

• Inventory Availability: ST3-28B21 has 1128 units in stock; 3FS-328 has 1960 units available
• Packaging Format: ST3-28B21 is supplied in standard packaging; 3FS-328 is supplied in tube packaging
• Physical Dimensions: ST3-28B21 measures 35.05mm L × 28.70mm W × 30.23mm H (seated); 3FS-328 measures 35.56mm L × 31.00mm W × 32.10mm H (seated). Verify PCB layout clearances for the 3FS-328 due to increased height and width
• Regulatory Status: Both parts carry EAR99 classification and are REACH unaffected

Frequently Asked Questions (FAQ)

Q: Can the 3FS-328 be used as a direct replacement for the ST3-28B21?

A: Yes. The 3FS-328 is electrically and functionally equivalent across all critical parameters including power rating, voltage outputs, current capacity, isolation rating, and winding configuration. Both parts are through-hole mounted with PC pin termination and meet identical compliance standards.

Q: What are the physical dimension differences between these parts?

A: The ST3-28B21 is 35.05mm L × 28.70mm W × 30.23mm H (seated). The 3FS-328 is 35.56mm L × 31.00mm W × 32.10mm H (seated). The 3FS-328 is approximately 0.5mm longer, 2.3mm wider, and 1.87mm taller. Confirm PCB footprint and vertical clearance before substitution.

Q: Are both parts RoHS compliant?

A: Yes. Both the ST3-28B21 and 3FS-328 are RoHS compliant with moisture sensitivity level 1 (unlimited exposure).

Q: What is the difference in packaging between these parts?

A: The ST3-28B21 is supplied in standard packaging. The 3FS-328 is supplied in tube packaging, which may affect handling and automated assembly processes.

Q: Can these transformers be configured for both 14V and 28V output?

A: Yes. Both parts feature dual secondary windings that can be connected in parallel for 14V at 170mA or in series for 28V at 85mA output.

Q: Are there any export or regulatory restrictions on these parts?

A: Both parts are classified as EAR99 and are REACH unaffected, indicating standard export treatment and no hazardous substance restrictions beyond RoHS requirements.