HT35 Equivalent & Substitute Parts

Part Overview

The HT35 is a DIAC thyristor manufactured by Littelfuse Inc., rated for 30–40V breakover voltage with 2A peak output current in a DO-35 axial package. This component is classified as obsolete, making equivalent substitutes necessary for new designs and production continuity. The HT35 operates across the industrial temperature range of –40°C to 125°C and complies with ROHS3 standards.

Substiute Parts

HT35

Littelfuse Inc.In Stock: 779HT35 Datasheet

Current PartRFQ

DB3

Fairchild SemiconductorIn Stock: 770983DB3 Datasheet

MFR RecommendedRFQ

DB3TG

STMicroelectronicsIn Stock: 2484DB3TG Datasheet

MFR RecommendedRFQ

DB4

STMicroelectronicsIn Stock: 20892DB4 Datasheet

MFR RecommendedRFQ

Key Parameters

Substitute Part Grouping Explanation

Substitution eligibility for the HT35 is determined by the following critical parameters:

• Package compatibility: All substitutes must use DO-204AH / DO-35 axial packaging
• Voltage range overlap: Breakover voltage must fall within or closely align with the 30–40V specification
• Current rating: Peak output current must meet or exceed 2A
• Temperature range: Operating range must cover –40°C to 125°C
• Compliance: ROHS3 compliance required for modern supply chains

Three substitute parts meet these criteria:

• DB3 (Fairchild Semiconductor): Voltage range 28–36V overlaps the lower portion of HT35 specification
• DB3TG (STMicroelectronics): Voltage range 30–34V falls entirely within HT35 range; active product status
• DB4 (STMicroelectronics): Voltage range 35–45V overlaps the upper portion of HT35 specification

Parameter Comparison

Engineering Selection Recommendations

DB3TG is the primary substitute for new designs. It maintains identical breakover current (15 µA), operates within the HT35 voltage specification (30–34V), and carries active product status with full ROHS3 compliance. This part ensures long-term supply availability and eliminates obsolescence risk.

DB3 serves as an alternative where the lower voltage range (28–36V) is acceptable. Fairchild Semiconductor's DB3 provides voltage overlap but carries obsolete status, limiting its suitability for new production.

DB4 is applicable only in applications where higher breakover voltage (35–45V) is tolerable. The upper voltage range of DB4 extends beyond the HT35 specification, making it suitable for circuits designed to accommodate this shift.

All three substitutes maintain identical package geometry, current rating, and temperature range, ensuring mechanical and thermal compatibility with existing designs.

Frequently Asked Questions (FAQ)

Q: Can DB3 directly replace HT35 in all applications? A: DB3 operates within a voltage range (28–36V) that overlaps HT35 (30–40V) but does not fully contain it. Substitution is valid only if the circuit tolerates the lower minimum voltage (28V) and the higher breakover current (50 µA vs. 15 µA). DB3 is also obsolete, limiting supply continuity.

Q: Why is DB3TG recommended over DB3? A: DB3TG maintains the identical breakover current specification (15 µA) and operates entirely within the HT35 voltage range (30–34V). Critically, DB3TG carries active product status, ensuring ongoing availability and supply chain stability. DB3 is obsolete.

Q: Is DB4 a suitable substitute? A: DB4 is suitable only if the application circuit can tolerate a higher breakover voltage range (35–45V). The upper limit of DB4 (45V) exceeds HT35 (40V), which may affect circuit triggering behavior. DB4 is active and ROHS3 compliant but represents a voltage shift rather than a direct equivalent.

Q: Are all substitutes available in the same package? A: Yes. HT35, DB3, DB3TG, and DB4 all use DO-204AH / DO-35 axial packaging, ensuring identical mechanical fit and PCB compatibility.

Q: What compliance certifications apply to these parts? A: All four parts (HT35, DB3, DB3TG, DB4) are ROHS3 compliant. DB3TG and DB4 carry REACH Unaffected status. All parts are classified under ECCN EAR99 and HTSUS 8541.30.0080.

Q: Which substitute should I use for new production? A: DB3TG is the recommended choice for new designs due to its active product status, matching breakover current, and voltage specification alignment with HT35.