Help me pick a power transistor

Thread Starter


Joined Sep 29, 2010
Hi All, new member and looks like a nice and active forum.
Appreciate any help someone could give.

Because of some basic electronics I took over 20 years ago at DeVry, a friend has convinced me to design a valve controller for his new aquarium.

I am using the Arduino platform.
The Arduino will be controlling (3) 24vdc valves, which have a max current pull of ~0.5A.
Duty cycle is open/close every 3-5 minutes, 24x7.

I was initially going to use the standard relays, however they seemed expensive, bulky, and not suitable for the duty cycle. And, would prefer to use an Arduino/AVR 5v output pin to a logic level gate device.

I have the following power transistors available to use, not sure if there is any special considerations that need to be taken into account.

And, I believe I read that a diode is needed to prevent in-rush current?

Appreciate any help.

Rated: 60V 10A
Dynamic dv/dt rating
Logic level gate drive
175°C operating temperature
Fast switching
Ease of paralleling
Simple drive requirements

TIP31B Silicon Plastic NPN Power Transistor (Motorola)
Collector-Emitter Saturation Voltage - VCE(sat) = 1.2 V (Max) @ IC = 3.0 Adc
Collector-Emitter Sustaining Voltage -VCEO(sus) = 80 Vdc (min)
High Current Gain Bandwidth Product fT = 3.0 MHz (min) @ IC = 500 mAdc

MJE802 Power 4A 80V NPND Transistor (Motorola)
High DC Current Gain - hFE = 2000 (Typ) @ IC = 2.0 Adc
Monolithic Construction with Built-in Base-Emitter Resistors to Limit Leakage - Multiplication

John P

Joined Oct 14, 2008
The IRLZ14 or the MJE802 ought to work. Not the TIP31B, because it needs too much base current.

The diode wouldn't be preventing inrush current--that's a different effect that you won't see with magnetic devices. But you do need diodes to carry freewheeling current when the valves turn off. They're installed in parallel with the valve coils but with the cathode pointing to the positive power supply.