075582814553
The 2N5551 from ON Semiconductor is a high-voltage NPN small-signal transistor in a compact TO-92 package that delivers a 160 V VCEO rating, up to 600 mA collector current, and about 625 mW dissipation. This article will discuss the 2N5551’s key specifications, pinout and CAD models, practical application circuits, verified alternatives, and more.
The 2N5551 from ONSEMI is a high-voltage NPN small-signal transistor designed for reliable amplification and switching in circuits that operate above the capability of common low-voltage BJTs. Housed in a TO-92 package, it supports a collector-emitter voltage of 160 V, a collector current of 600 mA, and a power dissipation of 625 mW, making it suitable for demanding applications without increasing component size. Its stable gain and MHz-level transition frequency also allow clean signal handling in analog and low-power RF stages.
Because of its robust voltage rating, the 2N5551 is widely used in high-voltage amplifiers, audio drivers, power-supply feedback circuits, and general-purpose switching. ONSEMI’s version is known for consistency and durability, even though the device is now considered legacy in their lineup. It remains a dependable choice when you require a compact transistor that tolerates higher voltages than the 2N3904 or BC547.
If you are interested in purchasing the 2N5551, feel free to contact us for pricing and availability.
| Pin Number | Pin Name | Function |
| 1 | Emitter (E) | Releases current from the transistor; reference point of the device. |
| 2 | Base (B) | Controls the transistor’s switching and amplification operation. |
| 3 | Collector (C) | Receives current; main terminal for load connection. |
• 2N5550
• MPSA42
• MPSA44
• KSC3503
• KSC2690A
• BF420
• BF421
• BF422
• NTE194
• 2N5833
• 2SC3503
• 2SC2682
• 2SC2383
• 2SC3953
| Parameter | Specification |
| Manufacturer | onsemi |
| Packaging | Bulk |
| Part Status | Obsolete |
| Transistor Type | NPN |
| Collector Current (IC, Max) | 600 mA |
| Collector-Emitter Breakdown Voltage (VCEO) | 160 V |
| VCE Saturation (Max) @ IB, IC | 200 mV @ 5 mA, 50 mA |
| Collector Cutoff Current (ICBO, Max) | 50 nA |
| DC Current Gain (hFE, Min) @ IC, VCE | 80 @ 10 mA, 5 V |
| Power Dissipation (Max) | 625 mW |
| Transition Frequency (fT) | 300 MHz |
| Operating Temperature Range | –55°C to +150°C |
| Mounting Type | Through-Hole |
| Package / Case | TO-226-3, TO-92-3 Long Body |
| Supplier Device Package | TO-92 (TO-226) |
In the first circuit, the 2N5551 transistors (Q1 and Q2) work together to drive the primary coil of a transformer. Q2 acts as a switching transistor controlled by a 50 kHz clock signal. When the clock pulses go high, Q2 conducts and pulls the base of Q1 toward ground through R1, turning Q1 ON. As Q1 switches, it rapidly connects and disconnects the 12 V supply across the transformer winding. Since the 2N5551 can handle relatively high voltages and fast switching, it efficiently drives the inductive load. The alternating switching action produces an AC-like waveform across the transformer, enabling signal coupling or voltage step-up depending on the transformer design.
2N5551 as a Simple LED Switch
In the second circuit, the 2N5551 functions as a low-side switch that controls current flow through an LED. When SW1 closes, a bias voltage is applied to the transistor’s base through R1. This small current into the base causes Q1 to turn ON, allowing a larger current to flow from the LED (D1), through resistor R2, and finally through the transistor to ground. Because the 2N5551 is designed for small-signal switching, it provides clean and reliable activation of the LED. When SW1 opens, base current disappears, Q1 switches OFF, and the LED turns off. This demonstrates a basic use of the 2N5551 as an electronic switch.
2N5551 as a High-Frequency PWM Switching Element
In the third circuit, the 2N5551 (Q1) is used as a high-speed switch controlled by an MCU-generated PWM signal. Q2 (a PNP transistor) shapes and conditions the control signal, which is then fed to the base of Q1 via R1. When Q1 switches ON, current flows through the inductor L1, charging it and storing energy. During OFF periods, the 1N4148 diode allows current to continue flowing, forming a boost-converter-like topology. The 2N5551 is chosen here because it supports high transition frequency and moderate voltage levels, making it ideal for fast PWM switching. The resulting waveform is filtered and fed into a comparator (LM339), forming a regulated output. This circuit highlights the 2N5551’s ability to operate effectively in high-frequency analog-digital control systems.
• High-voltage signal amplification
• Transformer driver circuits
• LED switching circuits
• High-frequency PWM switching
• Audio preamplifier stages
• Voltage regulation and feedback circuits
• High-voltage pulse generation
• Inductive load drivers
• Comparator interface circuits
• Small-signal switching applications
| Feature | 2N5551 | 2N5550 | MPSA42 | 2N3904 | BC547 |
| Type | NPN | NPN | NPN | NPN | NPN |
| VCEO (Max) | 160 V | 160 V | 300 V | 40 V | 45 V |
| IC (Max) | 600 mA | 600 mA | 500 mA | 200 mA | 100 mA |
| Power Dissipation | 625 mW | 625 mW | 625 mW | 625 mW | 500 mW |
| Gain (hFE) | 80–300 | 80–300 | 40–120 | 100–300 | 110–800 |
| Frequency (fT) | 300 MHz | 300 MHz | 50–100 MHz | 300 MHz | 150 MHz |
| Application Class | High-voltage small signal | High-voltage small signal | Very high-voltage switching | Low-voltage general-purpose | Low-voltage general-purpose |
• Make sure the collector-emitter voltage (VCEO) does not exceed 160 V, and ideally keep your operating voltage well below the limit for added safety margin.
• Avoid running the device close to maximum ratings; keeping voltages 5–10% lower than the datasheet limits helps improve reliability.
• Always use a proper base resistor to limit base current and ensure the transistor does not exceed its 600 mA maximum collector current.
• Maintain safe thermal conditions by ensuring the junction temperature stays within −55 °C to +150 °C, and provide airflow or heatsinking if the circuit dissipates significant power.
• Prevent excessive power dissipation by operating below the 625 mW maximum, adjusting load or switching frequency as necessary.
• Use adequate protection components such as diodes or snubbers when driving inductive loads to prevent voltage spikes that can exceed the transistor’s ratings.
High collector-emitter voltage rating (160 V)
Supports relatively high collector current (600 mA)
High transition frequency (up to 300 MHz)
Suitable for high-voltage small-signal applications
Reliable for fast switching and PWM circuits
Works well in audio preamp and analog stages
Available in compact TO-92 package
Complementary PNP pair available (2N5401)
Not suitable for high-power applications (>1 W dissipation)
Gain (hFE) is moderate compared to low-voltage small-signal BJTs
Requires proper heat management despite small size
Limited current handling compared to larger transistors
Obsolete status from ON Semiconductor may reduce availability
Sensitive to voltage spikes when driving inductive loads
ON Semiconductor (onsemi) has advanced manufacturing capabilities that ensure high reliability and performance for devices like the 2N5551 transistor. The company operates modern wafer fabrication facilities using precise diffusion, doping, and lithography processes to achieve stable high-voltage characteristics and consistent small-signal behavior. onsemi also integrates automated assembly lines, stringent quality control, and comprehensive electrical testing to guarantee device robustness across wide temperature and voltage ranges.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other