ZTX455STOB Equivalent & Substitute Parts

Part Overview

The ZTX455STOB is an NPN bipolar junction transistor manufactured by Diodes Incorporated, rated for 140 V collector-emitter breakdown voltage and 1 A maximum collector current. This device is packaged in an E-Line-3 (TO-92 compatible) through-hole configuration with a maximum power dissipation of 1 W and transition frequency of 100 MHz.

The ZTX455STOB is classified as an obsolete product. Equivalent and substitute parts are necessary to maintain design continuity and ensure component availability for production, repair, and maintenance applications.

Key Parameters

Parameter	Value	Unit
Transistor Type	NPN	—
Voltage - Collector Emitter Breakdown (Max)	140	V
Current - Collector (Ic) (Max)	1	A
Power - Max	1	W
Frequency - Transition	100	MHz
Mounting Type	Through Hole	—
Package / Case	E-Line-3, Formed Leads (TO-92 compatible)	—
Operating Temperature Range	-55 to 200	°C
RoHS Status	ROHS3 Compliant	—

Substitute Part Grouping Explanation

Substitution of the ZTX455STOB is determined by the following critical parameters:

Primary Substitution Criteria:

Transistor type (NPN)
Collector-emitter breakdown voltage (minimum 140 V)
Maximum collector current (minimum 1 A for direct substitution; lower ratings acceptable for reduced-current applications)
Through-hole mounting configuration
TO-92 compatible package geometry

Secondary Compatibility Factors:

Operating temperature range
Power dissipation capability
DC current gain (hFE)
Saturation voltage characteristics
RoHS and regulatory compliance

Substitute parts are grouped into two categories:

Category 1: Direct Equivalents — Parts with identical or superior electrical ratings and the same packaging standard, suitable for direct board-level replacement without circuit modification.

Category 2: Functional Alternatives — Parts with reduced maximum collector current (600 mA vs. 1 A) or modified electrical characteristics but compatible pinout and package, suitable for applications where the full 1 A rating is not required.

Parameter Comparison

Parameter	ZTX455STOB	ZTX455STZ	2N5550TA	2N5550TAR	2N5550TFR	2N5551TA	2N5551TF	2N5551TFR
Manufacturer	Diodes Inc.	Diodes Inc.	Fairchild Semi.	onsemi	Fairchild Semi.	onsemi	onsemi	onsemi
Product Status	Obsolete	Active	Active	Active	Active	Active	Active	Active
Transistor Type	NPN	NPN	NPN	NPN	NPN	NPN	NPN	NPN
Voltage - Collector Emitter Breakdown (Max)	140 V	140 V	140 V	140 V	140 V	160 V	160 V	160 V
Current - Collector (Ic) (Max)	1 A	1 A	600 mA	600 mA	600 mA	600 mA	600 mA	600 mA
Power - Max	1 W	1 W	625 mW	625 mW	625 mW	625 mW	625 mW	625 mW
Frequency - Transition	100 MHz	100 MHz	300 MHz	300 MHz	300 MHz	100 MHz	100 MHz	100 MHz
Operating Temperature (Max)	200°C	200°C	150°C	150°C	150°C	150°C	150°C	150°C
Package / Case	E-Line-3, TO-92 compatible	E-Line-3, TO-92 compatible	TO-226-3, TO-92-3	TO-226-3, TO-92-3	TO-226-3, TO-92-3	TO-226-3, TO-92-3	TO-226-3, TO-92-3	TO-226-3, TO-92-3
Mounting Type	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole	Through Hole
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	—	ROHS3 Compliant	—	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant
REACH Status	REACH Unaffected	REACH Unaffected	REACH Unaffected	REACH Unaffected	REACH Unaffected	REACH Unaffected	REACH Unaffected	REACH Unaffected

Engineering Selection Recommendations

Primary Recommendation: ZTX455STZ

The ZTX455STZ is the preferred direct substitute for the obsolete ZTX455STOB. Both parts are manufactured by Diodes Incorporated and share identical electrical specifications: 140 V breakdown voltage, 1 A maximum collector current, 1 W power dissipation, and 100 MHz transition frequency. The ZTX455STZ maintains the same operating temperature range (-55°C to 200°C) and is ROHS3 compliant. The only difference is packaging format (Cut Tape versus bulk), which does not affect electrical performance or board-level compatibility. The ZTX455STZ is currently in active production with substantial inventory availability (2210 pcs).

Secondary Recommendations: 2N5551 Series (2N5551TF, 2N5551TA, 2N5551TFR)

The 2N5551 series transistors are suitable substitutes for applications where the full 1 A collector current is not required. These parts provide superior voltage rating (160 V vs. 140 V), matching transition frequency (100 MHz), and are manufactured by onsemi. All 2N5551 variants are ROHS3 compliant and in active production. The trade-off is reduced maximum collector current (600 mA) and power dissipation (625 mW). These parts are appropriate for low-to-medium current applications.

Tertiary Recommendations: 2N5550 Series (2N5550TA, 2N5550TAR, 2N5550TFR)

The 2N5550 series transistors are functional alternatives with 140 V breakdown voltage matching the original part. These devices provide enhanced transition frequency (300 MHz) but reduced collector current (600 mA) and power dissipation (625 mW). The 2N5550 series is suitable for applications prioritizing higher frequency performance over current capacity. Fairchild Semiconductor and onsemi variants are available with active product status and ROHS3 compliance.

Regulatory Compliance:

All recommended substitute parts maintain ROHS3 compliance and REACH unaffected status, ensuring compatibility with current environmental and regulatory requirements.

Frequently Asked Questions (FAQ)

Q: Can the ZTX455STZ directly replace the ZTX455STOB without circuit modification?

A: Yes. The ZTX455STZ is a direct equivalent with identical electrical specifications and through-hole TO-92 compatible packaging. No circuit modification is required for board-level substitution.

Q: What is the difference between ZTX455STOB and ZTX455STZ?

A: The electrical and functional specifications are identical. The difference is in packaging format: ZTX455STOB is supplied in bulk packaging, while ZTX455STZ is supplied in cut tape (CT) format. Both use the same E-Line-3 package geometry.

Q: Can I use a 2N5551 series transistor instead of the ZTX455STOB?

A: The 2N5551 series is compatible for applications where the maximum collector current requirement does not exceed 600 mA. The 2N5551 provides superior voltage rating (160 V) and identical transition frequency (100 MHz). If your circuit requires the full 1 A capability, the 2N5551 is not suitable.

Q: What is the difference between 2N5550 and 2N5551 series transistors?

A: Both series have identical maximum collector current (600 mA) and power dissipation (625 mW). The 2N5550 operates at 140 V breakdown voltage with 300 MHz transition frequency. The 2N5551 operates at 160 V breakdown voltage with 100 MHz transition frequency. Selection depends on whether your application prioritizes voltage margin or frequency performance.

Q: Are all substitute parts RoHS compliant?

A: All recommended substitute parts are ROHS3 compliant and REACH unaffected, ensuring compatibility with current environmental regulations.

Q: What is the operating temperature range for substitute parts?

A: The ZTX455STZ maintains the original operating range of -55°C to 200°C. The 2N5550 and 2N5551 series are rated to -55°C to 150°C maximum junction temperature, which is lower than the original part. Verify that your application does not require operation above 150°C before selecting these alternatives.

Q: Can I use 2N5550 or 2N5551 in a 1 A application?

A: No. Both the 2N5550 and 2N5551 series are rated for maximum 600 mA collector current. Using these parts in a 1 A application will exceed their maximum ratings and result in device failure or degraded performance.

Q: What packaging options are available for substitute parts?

A: ZTX455STZ is available in cut tape (CT) format. The 2N5550 series is available in bulk and cut tape formats. The 2N5551 series is available in cut tape format. All parts use TO-92-3 (TO-226-3) through-hole package geometry compatible with the original ZTX455STOB footprint.

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