BZX79C18 Zener Diode Equivalent & Substitute Parts

Part Overview

The BZX79C18 is an 18 V, 500 mW Zener diode manufactured by onsemi in a through-hole DO-35 package. This component is classified as obsolete, making equivalent and substitute parts necessary for ongoing design support and procurement. The BZX79C18 operates across a temperature range of -65°C to 200°C with ±5% voltage tolerance and is suitable for voltage regulation and protection applications in electronic circuits.

Key Parameters

Parameter	Value
Voltage - Zener (Nom) (Vz)	18 V
Tolerance	±5%
Power - Max	500 mW
Impedance (Max) (Zzt)	45 Ohms
Current - Reverse Leakage @ Vr	50 nA @ 12.6 V
Voltage - Forward (Vf) (Max) @ If	1.5 V @ 100 mA
Operating Temperature	-65°C ~ 200°C
Mounting Type	Through Hole
Package / Case	DO-204AH, DO-35, Axial
Moisture Sensitivity Level (MSL)	1 (Unlimited)

Substitute Part Grouping Explanation

Substitution of the BZX79C18 is determined by strict electrical and mechanical parameter matching. The following criteria establish valid substitution relationships:

Primary Substitution Criteria:

Voltage - Zener (Nom): 18 V (exact match required)
Power - Max: 500 mW (exact match required)
Mounting Type: Through Hole (exact match required)
Package / Case: DO-35 or DO-204AH (exact match required)

Secondary Compatibility Parameters:

Tolerance: ±5% or tighter (±1% acceptable)
Operating Temperature: Must support -65°C minimum; upper limit may vary
Impedance (Max): Variations acceptable within application requirements
Current - Reverse Leakage: Variations acceptable within application requirements

Substitute parts are classified into two categories:

Direct Substitutes: Parts with identical electrical specifications across all primary and secondary parameters. These parts provide drop-in replacement capability.

Similar Substitutes: Parts meeting all primary criteria with variations in secondary parameters such as impedance, reverse leakage current, or tolerance. These parts are functionally equivalent for the specified 18 V, 500 mW application but may exhibit different performance characteristics in specific circuit conditions.

Parameter Comparison

Part Number	Manufacturer	Vz (V)	Tolerance	Power (mW)	Zzt (Ohms)	Vr Leakage (nA)	Vf (Max) @ If	Temp Range (°C)	Package	Status
BZX79C18	onsemi	18	±5%	500	45	50 @ 12.6 V	1.5 V @ 100 mA	-65 ~ 200	DO-35	Obsolete
BZX79C18-T50A	Fairchild Semiconductor	18	±5%	500	45	50 @ 12.6 V	1.5 V @ 100 mA	-65 ~ 200	DO-35	Active
1N5248B	Microchip Technology	18	±5%	500	21	100 @ 14 V	1.5 V @ 200 mA	-65 ~ 175	DO-35	Active
1N3524A	Microchip Technology	18	±5%	500	18	Not specified	1.1 V @ 200 mA	-65 ~ 175	DO-35	Active
1N4705 (DO35)	Microsemi Corporation	18	±5%	500	Not specified	50 @ 13.6 V	1.1 V @ 200 mA	-65 ~ 175	DO-35	Active
1N5248B-TR	Vishay General Semiconductor - Diodes Division	18	±5%	500	21	100 @ 14 V	1.1 V @ 200 mA	Not specified	DO-35	Active
1N5538B	Microchip Technology	18	±5%	500	105	10 @ 16.2 V	1.1 V @ 200 mA	-65 ~ 175	DO-35	Active
1N5742C	Microchip Technology	18	±1%	500	45	100 @ 12 V	0.9 V @ 10 mA	-65 ~ 175	DO-35	Active
1N5742D	Microchip Technology	18	±1%	500	45	100 @ 12 V	0.9 V @ 10 mA	-65 ~ 175	DO-35	Active
1N6006A	Microchip Technology	18	±10%	500	55	100 @ 11 V	1.1 V @ 200 mA	-65 ~ 175	DO-35	Active

Engineering Selection Recommendations

Direct Substitute (Preferred):

The BZX79C18-T50A manufactured by Fairchild Semiconductor is the direct equivalent to the BZX79C18. This part maintains identical electrical specifications including 18 V nominal voltage, ±5% tolerance, 500 mW power rating, 45 Ohms maximum impedance, and 50 nA reverse leakage current. The operating temperature range extends to 200°C, matching the original specification. The BZX79C18-T50A is in active production status with substantial inventory availability (45,987 units), ensuring long-term procurement viability.

Similar Substitutes (Application-Dependent):

The following parts meet all primary substitution criteria (18 V, 500 mW, DO-35 package, through-hole mounting) and are suitable for applications where secondary parameter variations are acceptable:

1N5248B and 1N5248B-TR (Microchip Technology / Vishay): These parts feature lower impedance (21 Ohms) and higher reverse leakage current (100 nA @ 14 V) compared to the original. The 1N5248B-TR variant is ROHS3 compliant. Both are in active production with high inventory levels.

1N3524A (Microchip Technology): Offers the lowest impedance (18 Ohms) among available substitutes with forward voltage of 1.1 V @ 200 mA. Active production status with 913 units in stock.

1N4705 (Microsemi Corporation): Maintains 50 nA reverse leakage current matching the original specification. Operating temperature range limited to -65°C to 175°C. Active production with 714 units available.

1N5538B (Microchip Technology): Features the highest impedance (105 Ohms) and lowest reverse leakage current (10 nA @ 16.2 V). Suitable for applications requiring minimal leakage. Active production with 979 units in stock.

1N5742C and 1N5742D (Microchip Technology): Provide tighter voltage tolerance (±1% versus ±5%) with impedance matching the original (45 Ohms). Forward voltage reduced to 0.9 V @ 10 mA. Both in active production with 831 and 727 units respectively.

1N6006A (Microchip Technology): Offers wider voltage tolerance (±10%) with impedance of 55 Ohms. Suitable for less stringent tolerance applications. Active production with 1,078 units in stock.

Compliance Considerations:

The original BZX79C18 is REACH unaffected and classified as EAR99 for export control purposes. Most substitute parts maintain these compliance designations. The 1N5248B-TR variant is ROHS3 compliant, providing enhanced environmental compliance for new designs.

Frequently Asked Questions (FAQ)

Q: Can the BZX79C18-T50A be used as a direct replacement for the BZX79C18?

A: Yes. The BZX79C18-T50A is a direct equivalent with identical electrical specifications across all parameters including voltage, tolerance, power rating, impedance, and operating temperature range. No circuit modifications are required.

Q: What is the primary difference between the BZX79C18 and the 1N5248B?

A: Both parts share the same 18 V nominal voltage, ±5% tolerance, and 500 mW power rating in DO-35 packages. The 1N5248B features lower impedance (21 Ohms versus 45 Ohms) and higher reverse leakage current (100 nA versus 50 nA). The operating temperature range is limited to -65°C to 175°C on the 1N5248B compared to 200°C on the original.

Q: Are the 1N5742C and 1N5742D suitable substitutes for the BZX79C18?

A: These parts are functionally equivalent for 18 V, 500 mW applications. The primary advantage is tighter voltage tolerance (±1% versus ±5%), making them suitable for applications requiring higher precision. The operating temperature range is limited to 175°C. Forward voltage characteristics differ, with lower values at reduced current levels.

Q: What does the impedance (Zzt) parameter indicate, and why does it vary among substitutes?

A: Impedance (Zzt) represents the dynamic impedance of the Zener diode at the specified test current. Lower impedance values indicate better voltage regulation characteristics with reduced voltage variation under load changes. Higher impedance values may result in greater voltage ripple in regulation circuits. Selection depends on specific circuit requirements for voltage stability.

Q: Is the 1N6006A suitable for applications requiring tight voltage tolerance?

A: No. The 1N6006A features ±10% voltage tolerance, which is wider than the original BZX79C18 (±5%). This part is suitable for applications with relaxed tolerance requirements. For applications requiring ±5% or tighter tolerance, select the BZX79C18-T50A, 1N5248B, 1N3524A, 1N4705, 1N5248B-TR, 1N5538B, 1N5742C, or 1N5742D.

Q: What is the significance of the DO-35 package designation?

A: DO-35 (also designated DO-204AH) is a through-hole axial package for discrete diodes. All listed substitute parts use this identical package, ensuring mechanical compatibility with existing PCB layouts and component placement equipment. No board redesign is required when substituting between parts with the same package designation.

Q: Which substitute part offers the best reverse leakage characteristics?

A: The 1N5538B features the lowest reverse leakage current at 10 nA @ 16.2 V, compared to 50 nA @ 12.6 V for the original BZX79C18. This characteristic is beneficial in applications where minimal leakage current is critical for circuit performance.

Q: Are there RoHS compliance differences among the substitute parts?

A: Most substitute parts are RoHS non-compliant, matching the original BZX79C18. The 1N5248B-TR variant is ROHS3 compliant, making it suitable for designs requiring environmental compliance with current regulations. Verify compliance requirements for your specific application before selection.

Q: Can I use the 1N5248B-TR in place of the BZX79C18 in a high-temperature application?

A: The 1N5248B-TR operating temperature range is -65°C to 175°C, which is lower than the original BZX79C18 range of -65°C to 200°C. If your application requires operation above 175°C, use the BZX79C18-T50A, which maintains the full -65°C to 200°C range.

Q: What inventory considerations should guide my substitute part selection?

A: The BZX79C18-T50A offers the highest inventory availability (45,987 units), ensuring reliable long-term procurement. The 1N5248B and 1N5248B-TR also maintain substantial stock levels (62,638 and 30,061 units respectively). Parts with lower inventory (1N3524A, 1N4705, 1N5538B, 1N5742C, 1N5742D, 1N6006A) range from 713 to 1,078 units. For critical applications, prioritize parts with higher inventory levels.

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