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Home > Products > Discrete Semiconductor Products > Transistors > Bipolar (BJT) > Single Bipolar Transistors

STMicroelectronics STSA851

Name: STSA851
Brand: STMicroelectronics
SKU: STSA851
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: STSA851

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

Package:: TO-226-3, TO-92-3 (TO-226AA)

Datasheet:: STSA851.pdf

Description:: TRANS NPN 60V 5A TO-92-3

Quantity:: RFQ Add To RFQ list

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Inventory:7,457

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Product details

Overview

STSA851 from STMicroelectronics is a low-voltage, fast-switching NPN power transistor in TO-92 and TO-92AP packages, rated for V_CEO = 60 V, I_C = 5 A, and P_tot = 1.1 W at T_amb = 25 °C. It delivers V_CE(sat) = 140 mV at I_C = 2 A / I_B = 50 mA and h_FE = 90 at I_C = 5 A, enabling high-efficiency switching in low-voltage DC control circuits such as relay drivers and emergency lighting power stages.

For engineers reviewing the STSA851 datasheet, STSA851 pinout, STSA851 application, or STSA851 equivalent, key selection criteria include verified saturation voltage under high-current switching, thermal resistance (R_thj-amb = 114 °C/W), DC current gain linearity across 10 mA–5 A, and TO-92 mechanical compatibility with legacy through-hole PCB layouts.

Technical Context

This NPN planar transistor uses a "base island" layout to simultaneously optimize current gain and saturation voltage. Its structure supports fast switching (t_on = 55 ns, t_f = 120 ns) with low storage time (t_s = 1.35 µs) under resistive load conditions (V_CC = 10 V, I_B1 = –I_B2 = 0.1 A).

Electrical behavior is characterized across temperature: I_CBO remains ≤1 µA at T_C = 100 °C, and V_CE(sat) stays below 450 mV even at I_C = 5 A / I_B = 200 mA. The device's f_T = 130 MHz confirms suitability for high-speed digital switching-not RF amplification.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	60 V - Maximum safe collector-emitter voltage with base open; defines upper rail limit in low-voltage switching supplies.
I_C	5 A - Continuous collector current rating; supports relay coils up to ~24 V/2 A and LED arrays requiring sustained 3–5 A drive.
V_CE(sat)	140 mV @ I_C=2 A/I_B=50 mA - Low conduction loss enables >98% efficiency in 5–24 V DC switching nodes.
h_FE	90 @ I_C=5 A/V_CE=1 V - Sustained current gain at full load ensures reliable base drive margin without excessive I_B overhead.
t_f	120 ns - Fast fall time minimizes switching transition losses in PWM-controlled regulators and lamp dimmers.
R_thj-amb	114 °C/W - Thermal resistance dictates heatsink requirement: ΔT = 125 °C at 1.1 W implies no heatsink needed below ~0.7 W dissipation.
f_T	130 MHz - Transition frequency confirms usable bandwidth beyond typical 20–100 kHz switching frequencies in DC-DC and lamp ballasts.

Pinout & Package

STSA851 is packaged in TO-92 (bulk) and TO-92AP (ammopack) variants-both standardized 3-lead through-hole plastic packages with identical pin assignment: Emitter (pin 1), Base (pin 2), Collector (pin 3), viewed from flat side with leads down.

Pin/Terminal	Circuit Role	Design Meaning
Pin 1 (Emitter)	Current sink terminal	Connected to ground or low-side return path; carries full load current with minimal voltage drop due to low R_CE(sat).
Pin 2 (Base)	Control input	Receives TTL- or microcontroller-compatible drive (I_B = 50–200 mA); requires series resistor to limit peak base current during switching.
Pin 3 (Collector)	High-side output node	Switches positive rail (up to 60 V); connects to load anode or relay coil; must be decoupled with flyback diode in inductive applications.

Key Features

Feature	Design Value
Very low V_CE(sat)	140 mV at 2 A enables <0.3 W conduction loss-critical for thermally constrained emergency lighting housings.
High h_FE at full load	90 at 5 A reduces required base drive power by >40% vs. standard general-purpose transistors, easing MCU GPIO sourcing.
Fast-switching capability	55 ns turn-on + 120 ns fall time supports PWM frequencies up to 1 MHz without significant transition loss penalty.
TO-92 mechanical compatibility	Standard footprint allows drop-in replacement in legacy designs using PN2222A, BC337, or MMBT4401 layouts.

Applications

Emergency Lighting Control

Voltage Regulator Pass Element

Use Scenario: Battery-backed LED driver circuit activating during AC mains failure, operating from 12–24 V DC.

IC Role / Device Role / Timing Role: NPN switch controlling current to high-brightness LED strings via PWM or linear regulation.

Use Value: V_CE(sat) ≤ 140 mV at 2 A minimizes heat generation in sealed enclosures, extending battery runtime and LED lifetime.

Use Scenario: Linear post-regulation stage following buck converter, delivering stable 5 V/3 A to sensitive logic.

IC Role / Device Role / Timing Role: Series pass transistor dissipating excess voltage between intermediate rail and final output.

Use Value: h_FE ≥ 90 at 5 A ensures stable bias with minimal base current variation, improving output voltage regulation under load transients.

Relay Driver Interface

Low-Voltage DC Motor Switch

Use Scenario: Microcontroller-driven 12 V/1 A relay coil activation in industrial PLC I/O modules.

IC Role / Device Role / Timing Role: High-gain saturated switch providing full coil current with minimal GPIO loading.

Use Value: I_C = 5 A rating and t_s = 1.35 µs enable clean relay release without contact arcing or coil remanence.

Use Scenario: Bidirectional 24 V brushed DC motor control in portable medical pumps or lab equipment.

IC Role / Device Role / Timing Role: One quadrant of H-bridge (low-side switch) enabling forward-only operation with PWM speed control.

Use Value: Fast t_f = 120 ns and low V_CE(sat) reduce commutation losses, limiting MOSFET gate drive complexity in cost-sensitive designs.

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN power switching applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
BC337-40	V_CEO = 45 V, I_C = 0.8 A, V_CE(sat) = 200 mV @ 0.5 A - lower voltage/current rating, higher saturation voltage.	Limited to sub-1 A loads; unsuitable for 5 A relay coils or high-current LED drivers.	Select only for space-constrained 3.3–5 V logic-level switching where full STSA851 capability is unnecessary.
MMBT4401	V_CEO = 40 V, I_C = 0.6 A, f_T = 250 MHz - higher bandwidth but lower power handling and thermal robustness.	Optimized for signal switching, not power delivery; derates significantly above 0.3 W.	Prefer for high-frequency digital isolation or level-shifting; avoid in >1 A continuous load paths.

Compared with BC337-40 and MMBT4401, STSA851 uniquely combines 5 A current capacity, 60 V blocking, and sub-150 mV saturation in TO-92-making it the only viable choice for thermally demanding, high-current, low-voltage switching where package size and cost constrain SOT-223 or DPAK alternatives.

Availability

STSA851 is available at Aetrix Electronics and suitable for emergency lighting systems, voltage regulators, and relay drivers requiring stable component supply and long-term obsolescence management support.

Supply support for STSA851 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

The STSA851 belongs to ST's legacy bipolar power transistor product line, engineered specifically for high-reliability, low-voltage DC switching in cost-sensitive industrial controls and safety-critical backup systems.

FAQ

Is STSA851 still in active production?

No-STSA851 is marked "Obsolete Product(s)" in the official ST datasheet (Rev 3, March 2008). However, Aetrix Electronics maintains legacy inventory with full traceability and offers extended lifecycle support including cross-reference engineering and last-time-buy planning for ongoing production programs.

What is the maximum safe operating temperature for STSA851?

The absolute maximum junction temperature is 150 °C, and storage temperature range is –65 to +150 °C. With R_thj-amb = 114 °C/W and P_tot = 1.1 W at T_amb = 25 °C, junction temperature reaches ~150 °C at ambient ~55 °C with no heatsink-so derating or minimal copper pour is advised above 0.7 W continuous dissipation.

Can STSA851 replace PN2222A in existing designs?

Yes-with design validation. STSA851 offers higher I_C (5 A vs. 0.8 A), higher V_CEO (60 V vs. 40 V), lower V_CE(sat), and faster switching. Pinout is identical (E-B-C), but base drive requirements differ: STSA851 needs ~200 mA I_B for full 5 A saturation, whereas PN2222A saturates at ~10 mA. Verify driver strength and thermal margin before substitution.

Does STSA851 require a flyback diode when driving relays?

Yes-always. Inductive relay coils generate reverse EMF exceeding V_CEO during turn-off. A 1N4007 or similar diode (cathode to collector, anode to emitter) must be placed across the coil to clamp voltage spikes and prevent junction breakdown, especially given the device's 60 V V_CEO rating and absence of integrated protection.

STSA851 Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

Package/Case:: TO-226-3, TO-92-3 (TO-226AA)

Packaging:: Bulk

Product Status:: Obsolete

Transistor Type:: NPN

Current - Collector (Ic) (Max):: 5 A

Voltage - Collector Emitter Breakdown (Max):: 60 V

Vce Saturation (Max) @ Ib, Ic:: 450mV @ 200mA, 5A

Current - Collector Cutoff (Max):: 50nA (ICBO)

DC Current Gain (hFE) (Min) @ Ic, Vce:: 150 @ 2A, 1V

Power - Max:: 1.1 W

Frequency - Transition:: 130MHz

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-92-3

STSA851 FAQ

1.How can I place an order for STSA851 through Aetrix?

Please submit a Request for Quotation (RFQ) for STSA851 on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for STSA851 reliable?

The price and inventory of STSA851 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for STSA851 is usually 5 days.

3.What payment methods are accepted for STSA851?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for STSA851 transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for STSA851?

STSA851 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your STSA851 order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for STSA851?

For technical support, including STSA851 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your STSA851 requirements.

6.How does Aetrix verify that STSA851 is sourced from the original manufacturer or authorized distributors?

All STSA851 products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that STSA851 meets industry standards.

7.What is the process for return or replacement of STSA851?

All STSA851 units undergo pre-shipment inspection (PSI). If there is an issue with STSA851, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The STSA851 part is unused and in its original packaging.

Return procedure for STSA851:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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