HT40RP DIAC Thyristor Equivalent & Substitute Parts

Part Overview

The HT40RP is a DIAC (Diode for Alternating Current) thyristor manufactured by Littelfuse Inc., rated for 35–45V 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 ongoing production requirements. The HT40RP serves as a trigger device in AC control circuits, particularly in triac-based applications requiring precise voltage-triggered switching.

Substiute Parts

HT40RP

Littelfuse Inc.In Stock: 908HT40RP 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 HT40RP is determined by the following critical parameters:

• Voltage - Breakover Range: The substitute must operate within or overlap the 35–45V specification to ensure proper trigger voltage compatibility in the target circuit.
• Current - Peak Output: Minimum 2A capability required to handle the same load switching demands.
• Package / Case: DO-35 axial package is mandatory for mechanical and thermal compatibility.
• Operating Temperature Range: Must support –40°C to +125°C to maintain performance across the same environmental conditions.
• RoHS Compliance: ROHS3 compliance ensures regulatory alignment for modern applications.

Three substitute parts meet these criteria:

1. DB4 (STMicroelectronics): Exact voltage match (35–45V), active product status, full parameter alignment.
2. DB3TG (STMicroelectronics): Slightly lower voltage range (30–34V), active product status, suitable for applications tolerant of lower trigger voltage.
3. DB3 (Fairchild Semiconductor): Lower voltage range (28–36V), obsolete status, acceptable only where voltage tolerance permits.

Parameter Comparison

Engineering Selection Recommendations

Primary Substitute: DB4 (STMicroelectronics)

DB4 is the direct equivalent for HT40RP replacement. It provides identical voltage-breakover specification (35–45V), matching current ratings, and active product status ensuring long-term availability. DB4 is ROHS3 compliant and carries REACH Unaffected status, meeting modern regulatory requirements. This part is the preferred choice for new designs and production continuity.

Secondary Substitute: DB3TG (STMicroelectronics)

DB3TG offers active product status and full compliance certifications. Its lower breakover voltage range (30–34V) makes it suitable for applications where trigger voltage can operate below 35V without circuit malfunction. This part provides an alternative when DB4 availability is constrained.

Tertiary Substitute: DB3 (Fairchild Semiconductor)

DB3 is obsolete and should be considered only when DB4 and DB3TG are unavailable and circuit design permits operation within the 28–36V range. Higher breakover current (50 µA vs. 15 µA) may affect trigger sensitivity in some applications.

Frequently Asked Questions (FAQ)

Q: Can DB4 be used as a direct replacement for HT40RP?

A: Yes. DB4 matches the HT40RP across all critical parameters: 35–45V breakover voltage, 2A peak output current, DO-35 package, and –40°C to +125°C operating range. DB4 is active and ROHS3 compliant, making it the recommended substitute.

Q: What is the difference between DB4 and DB3TG?

A: The primary difference is breakover voltage. DB4 operates at 35–45V (matching HT40RP), while DB3TG operates at 30–34V. Both are active products with identical current ratings and package specifications. DB3TG is suitable only if the circuit tolerates lower trigger voltage.

Q: Why does DB3 have higher breakover current (50 µA) than HT40RP (15 µA)?

A: Breakover current variation reflects manufacturing differences between Fairchild and Littelfuse designs. Higher breakover current may require slightly more gate current to trigger the device, potentially affecting circuit timing or sensitivity. This difference is acceptable in applications with adequate gate drive.

Q: Are all substitutes available in the same packaging?

A: Yes. HT40RP, DB4, DB3TG, and DB3 are all supplied in DO-35 axial package (DO-204AH case style), ensuring mechanical and thermal compatibility in existing PCB layouts.

Q: Which substitute should I choose for a new design?

A: DB4 is the recommended choice for new designs. It provides exact parameter matching, active product status, and guaranteed long-term availability from STMicroelectronics.

Q: Are all substitutes RoHS compliant?

A: Yes. HT40RP, DB4, DB3TG, and DB3 are all ROHS3 compliant, meeting environmental and regulatory requirements for modern electronics manufacturing.