Hello Mr. Bruce Leung,

You are exceeding the current capabilities of the device.

The TIP31C is Designed for general-purpose power amplifier and switching applications.

Features:
Collector-Emitter Saturation Voltage
- VCE(sat) = 1.2 V (Max) @ IC = 3.0 Adc

Collector-Emitter Sustaining Voltage
-VCEO(sus) = 60 Vdc (min) - TIP31A, TIP32A
=80 Vdc (min) - TIP31B, TIP32B
=100 Vdc (min) - TIP31C, TIP32C

High Current Gain Bandwidth Product
fT = 3.0 MHz (min) @ IC = 500 mAdc
Compact TO-220 AB Package

Data Sheet can be seen at the following web site.
http://www.onsemi.com/productSu...1C,00.html

I would recommend you go with an Integrated circuit meant for this application. National Semiconductor has some you can use. I think the part number is L293

Good luck,

Charles Bannister