BZX55C18 Equivalent & Substitute Parts

Part Overview

The BZX55C18 is a Zener diode manufactured by onsemi, rated at 18 V nominal with 500 mW power dissipation and ±6% tolerance. This component is classified as obsolete, necessitating identification of equivalent substitute parts for ongoing design requirements and procurement needs. The through-hole DO-35 package configuration and specified electrical parameters define the substitution criteria for this device category.

Key Parameters

Parameter	Value	Unit
Voltage - Zener (Nom)	18	V
Tolerance	±6%	—
Power - Max	500	mW
Impedance (Max)	50	Ohms
Current - Reverse Leakage @ Vr	100 nA @ 14 V	—
Voltage - Forward (Max) @ If	1.3 V @ 100 mA	—
Operating Temperature Range	-65 to 200	°C
Mounting Type	Through Hole	—
Package / Case	DO-35 (DO-204AH)	—

Substitute Part Grouping Explanation

Substitution of the BZX55C18 is determined by the following critical parameters:

Primary Matching Criteria:

Zener voltage: 18 V nominal
Power rating: 500 mW minimum
Package type: DO-35 through-hole configuration
Operating temperature range: -65°C to 200°C minimum
Mounting type: Through hole axial

Secondary Compatibility Factors:

Tolerance specification (±5% to ±6%)
Impedance characteristics
Reverse leakage current specifications
Forward voltage characteristics

Substitute parts must maintain the 18 V zener voltage specification and support through-hole mounting. Power rating and thermal operating range are critical for circuit performance and reliability.

Parameter Comparison

Parameter	BZX55C18 (Main)	BZX79C18-T50A	1N5073
Manufacturer	onsemi	Fairchild Semiconductor	Microchip Technology
Voltage - Zener (Nom)	18 V	18 V	18 V
Tolerance	±6%	±5%	±5%
Power - Max	500 mW	500 mW	3 W
Impedance (Max)	50 Ohms	45 Ohms	8 Ohms
Current - Reverse Leakage @ Vr	100 nA @ 14 V	50 nA @ 12.6 V	5 µA @ 13.7 V
Voltage - Forward (Max) @ If	1.3 V @ 100 mA	1.5 V @ 100 mA	Not specified
Operating Temperature Range	-65 to 200°C	-65 to 200°C	-65 to 175°C
Mounting Type	Through Hole	Through Hole	Through Hole
Package / Case	DO-35 (DO-204AH)	DO-35 (DO-204AH)	Axial
Product Status	Obsolete	Active	Active

Engineering Selection Recommendations

BZX79C18-T50A (Fairchild Semiconductor): This substitute maintains electrical and mechanical compatibility with the BZX55C18. Both devices share identical 18 V zener voltage, 500 mW power rating, DO-35 package configuration, and -65°C to 200°C operating temperature range. The BZX79C18-T50A features improved tolerance (±5% versus ±6%) and lower impedance (45 Ohms versus 50 Ohms). This part is currently in active production status with substantial inventory availability. REACH compliance and EAR99 export classification are equivalent to the original part.

1N5073 (Microchip Technology): This substitute provides 18 V zener voltage and through-hole mounting compatibility. The 1N5073 offers significantly higher power dissipation capability (3 W versus 500 mW) and substantially lower impedance (8 Ohms versus 50 Ohms), making it suitable for applications requiring enhanced thermal performance. The operating temperature range extends to -65°C to 175°C, which is 25°C lower than the original specification. This part is currently in active production status. REACH compliance and EAR99 export classification are equivalent to the original part.

Frequently Asked Questions (FAQ)

Q: Can the BZX79C18-T50A directly replace the BZX55C18?

A: Yes. Both devices share the same 18 V zener voltage, 500 mW power rating, DO-35 package, and -65°C to 200°C operating temperature range. The BZX79C18-T50A is electrically and mechanically compatible for direct substitution.

Q: What are the differences between the BZX79C18-T50A and 1N5073?

A: The primary differences are power rating and impedance. The BZX79C18-T50A maintains the 500 mW specification of the original BZX55C18, while the 1N5073 provides 3 W capability. The 1N5073 has significantly lower impedance (8 Ohms versus 45 Ohms). The 1N5073 operating temperature range is limited to -65°C to 175°C, compared to -65°C to 200°C for both the BZX55C18 and BZX79C18-T50A.

Q: Is the 1N5073 suitable for all applications requiring the BZX55C18?

A: The 1N5073 is suitable for applications where the lower maximum operating temperature (-65°C to 175°C) does not conflict with circuit requirements. The higher power rating and lower impedance may alter circuit behavior in precision voltage regulation applications. Verify that the maximum operating temperature specification meets application requirements.

Q: Are all substitute parts available in the same package configuration?

A: The BZX79C18-T50A and BZX55C18 both use the DO-35 (DO-204AH) package. The 1N5073 uses an axial package configuration, which is mechanically compatible with through-hole mounting but may require board layout adjustment due to different physical dimensions.

Q: What is the significance of the tolerance specification differences?

A: The BZX55C18 has ±6% tolerance, while both substitute parts have ±5% tolerance. The tighter tolerance of the substitutes provides improved voltage regulation accuracy. This represents an enhancement over the original specification and does not create compatibility issues.

Q: Are there compliance or regulatory differences between these parts?

A: All three parts share identical REACH compliance status (REACH Unaffected) and export classification (EAR99). No regulatory or compliance barriers exist for substitution.

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