HT34B DIAC Thyristor Equivalent & Substitute Parts

Part Overview

The HT34B is a DIAC (Diode for Alternating Current) thyristor manufactured by Littelfuse Inc., rated for 32–36V 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 HT34B serves as a trigger device in AC switching applications, particularly in triac-based circuits for phase control and dimming applications.

Substiute Parts

HT34B

Littelfuse Inc.In Stock: 1153HT34B 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 for the HT34B is determined by the following critical parameters:

• Breakover Voltage Range: The substitute must operate within or overlap the 32–36V specification of the HT34B
• Peak Output Current: Must support minimum 2A peak current capability
• Package Type: DO-35 axial package is mandatory for mechanical and thermal compatibility
• Operating Temperature: Must maintain -40°C to 125°C operating range
• Compliance: RoHS3 compliance required for regulatory alignment

Three substitute parts meet these criteria with varying voltage specifications:

• DB3TG (STMicroelectronics): 30–34V breakover, closest voltage match with active product status
• DB3 (Fairchild Semiconductor): 28–36V breakover, overlaps HT34B range but obsolete status
• DB4 (STMicroelectronics): 35–45V breakover, higher voltage tier for applications requiring elevated trigger thresholds

Parameter Comparison

Engineering Selection Recommendations

Primary Substitute: DB3TG (STMicroelectronics)

DB3TG is the preferred substitute for HT34B applications. It maintains identical breakover current (15 µA), matching trigger sensitivity, and operates within the 30–34V range that overlaps the HT34B specification. Critically, DB3TG carries active product status, ensuring long-term availability and supply chain stability. All compliance certifications (ROHS3, REACH Unaffected) align with HT34B requirements.

Secondary Substitute: DB3 (Fairchild Semiconductor)

DB3 provides the widest voltage overlap (28–36V) with HT34B, fully encompassing the original specification. However, obsolete product status limits its viability for new production. This part is suitable only for legacy system maintenance where existing inventory exists.

Alternative Substitute: DB4 (STMicroelectronics)

DB4 operates at a higher voltage tier (35–45V) and exhibits higher breakover current (50 µA). This part is applicable only to applications where the circuit design can accommodate elevated trigger thresholds. DB4 maintains active status and full compliance certifications.

Frequently Asked Questions (FAQ)

Q: Can DB3TG directly replace HT34B in existing circuits?

A: DB3TG is mechanically and electrically compatible with HT34B. Both use DO-35 axial packaging and support 2A peak output current. The 30–34V breakover range of DB3TG overlaps the HT34B specification (32–36V), making it suitable for circuits designed around the 32–36V trigger point. Verify circuit tolerance to the lower voltage boundary (30V minimum vs. 32V minimum for HT34B).

Q: What is the difference between DB3 and DB3TG?

A: Both parts share the same base product number (DB3) and identical breakover current (15 µA). DB3TG is manufactured by STMicroelectronics with active product status, while DB3 is a Fairchild Semiconductor variant with obsolete status. DB3TG is preferred for new designs due to supply availability.

Q: Why does DB4 have a higher breakover voltage?

A: DB4 is a separate product line designed for applications requiring higher trigger thresholds. The 35–45V range positions it for circuits operating at elevated AC line voltages or requiring delayed trigger response. DB4 is not a direct substitute unless the circuit design explicitly accommodates this voltage tier.

Q: Are all substitutes RoHS3 compliant?

A: Yes. HT34B, DB3TG, DB3, and DB4 all carry ROHS3 Compliant certification, meeting regulatory requirements for hazardous substance restrictions.

Q: What is the significance of breakover current (15 µA vs. 50 µA)?

A: Breakover current determines the gate trigger sensitivity. HT34B and DB3TG both specify 15 µA, ensuring identical trigger behavior. DB3 and DB4 specify 50 µA, requiring higher gate current for activation. This difference affects circuit design and may necessitate gate resistor adjustments if substituting with higher breakover current parts.