MPSA42 — 300V 500mA Ultra-High-Voltage NPN BJT (Nixie Tube Driver, TO-92)

Overview

The MPSA42 (and its SMD variant MMBTA42) is an ultra-high-voltage NPN bipolar junction transistor manufactured by onsemi, Fairchild, STMicroelectronics, and Central Semiconductor. Housed in a through-hole TO-92 plastic package with Emitter - Base - Collector (E-B-C) pinout, it is legendary across the retro-electronics and DIY maker community as the standard discrete driver for Nixie tubes.

Engineered with a massive collector-emitter breakdown voltage ( $V_{CEO}$ ) of $300\text{V}$ and a continuous collector current of $500\text{ mA}$ , the MPSA42 easily tolerates the $170\text{V} \dots 200\text{V}$ DC anode supply rails required to ionize neon gas inside vintage cold-cathode display tubes (IN-12, IN-14, IN-18, Z566M). It is universally used in Nixie tube clocks, neon indicator drivers, vacuum fluorescent display (VFD) filament drivers, electroluminescent (EL) wire inverters, and high-voltage strobe trigger circuits.

Quick reference


Transistor Type	Ultra-High-Voltage Small-Signal NPN BJT
Package	TO-92 (3-pin through-hole) / SOT-23 (MMBTA42) / SOT-223 (PZTA42)
Collector-Emitter Voltage ( $V_{CEO}$ )	$300\text{ V}$ max
Collector-Base Voltage ( $V_{CBO}$ )	$300\text{ V}$ max
Continuous Collector Current ( $I_C$ )	$500\text{ mA}$ max
DC Current Gain ( $h_{FE}$ )	$40$ to $200$ ( $I_C = 10\text{mA}, V_{CE} = 10\text{V}$ )
Transition Frequency ( $f_T$ )	$50\text{ MHz}$ min
Total Power Dissipation ( $P_D$ )	$625\text{ mW}$ ( $T_A = 25^\circ\text{C}$ )
Complementary PNP Pair	MPSA92 (300V PNP)
Operating Temp Range	$-55^\circ\text{C}$ to $+150^\circ\text{C}$

Pinout (TO-92 Package - E-B-C Standard)

Looking at the flat front face with leads pointing downward:

text

        ┌─────────┐
        │  TO-92  │
        │ MPSA42  │
        └─┬───┬───┬─┘
          1   2   3
          E   B   C

Pin	Name	Type	Description
1	`EMITTER`	Emitter Terminal	Emitter (Connect to ground / $0\text{ V}$ )
2	`BASE`	Base Terminal	Base control input (Driven from 3.3V/5V MCU GPIO via series resistor)
3	`COLLECTOR`	Collector Terminal	Collector output (Connected directly to Nixie tube cathode or high-voltage load)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Collector-Emitter Breakdown	$V_{(BR)CEO}$	300	—	—	V	$I_C = 1.0\text{mA}, I_B = 0$
Collector-Base Breakdown	$V_{(BR)CBO}$	300	—	—	V	$I_C = 100\ \mu\text{A}, I_E = 0$
Emitter-Base Breakdown	$V_{(BR)EBO}$	6.0	—	—	V	$I_E = 100\ \mu\text{A}, I_C = 0$
Collector Cutoff Current	$I_{CBO}$	—	—	100	nA	$V_{CB} = 200\text{V}, I_E = 0$
DC Current Gain	$h_{FE}$	40	—	—	—	$I_C = 1.0\text{mA}, V_{CE} = 10\text{V}$
DC Current Gain	$h_{FE}$	40	—	200	—	$I_C = 10\text{mA}, V_{CE} = 10\text{V}$
Collector Saturation Voltage	$V_{CE(sat)}$	—	—	0.50	V	$I_C = 20\text{mA}, I_B = 2.0\text{mA}$
Base Saturation Voltage	$V_{BE(sat)}$	—	—	0.90	V	$I_C = 20\text{mA}, I_B = 2.0\text{mA}$

Typical Nixie Tube Cathode Driver Circuit

In a microcontroller-driven Nixie clock (e.g. ESP32, Arduino, or 74HC595 shift register), ten MPSA42 transistors drive the decimal digit cathodes ( $0 \dots 9$ ):

text

                        +170V to +180V High-Voltage DC Supply
                                          │
                                [ Anode Resistor: 22kΩ 1W ]
                                          │
                                   [ Nixie Tube Anode ]
                                   [   '0' '1' ... '9']
                                        │ (e.g. Digit 7 Cathode)
                                        │
                                 [Pin 3: COLLECTOR]
                                      MPSA42
  MCU GPIO (3.3V / 5.0V)          [Pin 2: BASE]
          │                             │
          ├───[ 4.7kΩ - 10kΩ Resistor ]─┤
          │                             ├───[ 100kΩ Pull-Down Resistor ]──┐
         GND                            │                                 │
                                  [Pin 1: EMITTER]                        │
                                        │                                 │
                                       GND ───────────────────────────────┴─── Common GND

Comparison: MPSA42 vs 2N5551 vs KSP42

Parameter	MPSA42	2N5551	2N3904
$V_{CEO}$ Voltage	$300\text{ V}$	$160\text{ V}$	$40\text{ V}$
Max Current ( $I_C$ )	$500\text{ mA}$	$600\text{ mA}$	$200\text{ mA}$
Nixie (180V) Safe	Yes	No ( $160\text{V}$ breakdown is too close)	No (Destructive Breakdown)
Transition Freq ( $f_T$ )	$50\text{ MHz}$	$100 \dots 300\text{ MHz}$	$300\text{ MHz}$

Common mistakes

Using standard 40V/60V BJTs (2N3904, BC547) for Nixie tubes: Nixie cathodes float up to $100\text{V} \dots 170\text{V}$ when extinguished. Using standard 40V transistors causes avalanche breakdown, causing ghosting numerals and immediate destruction of the transistor. Always use $300\text{V}$ MPSA42 (or K155ID1 / 74141 decoder driver ICs).
Omitting base resistors: Connecting a microcontroller GPIO directly to Pin 2 (Base) without a current-limiting resistor ( $4.7\text{k}\Omega \dots 10\text{k}\Omega$ ) will draw excessive current from the MCU pin and damage the I/O port.

Notes

Complementary Pair: MPSA92 is the exact 300V PNP complementary transistor used for high-side switching in multiplexed Nixie display matrices.

Interface	Current-Driven Base Junction
Package	TO-92 Plastic (through-hole) / SOT-23 (MMBTA42) / SOT-223 (PZTA42)