Electronic – Transistor to drive an ignition coil


I want to drive a 6-volt ignition coil with a DC resistance of 3 ohms, from a power supply of 6.2 volts DC at 2 amps. I propose to use a 555 timer as the PWM, with a 50% duty cycle at about 5 to 10 kHz, with a totem pole amplifier. I don't need full power output from the ignition coil. What would be a good power transistor for the output? (I would like to use a MOSFET, but don't have a grounded working mat or wrist strap.)

Best Answer

Olin is right about the BJT. For power switching a power MOSFET is often chosen because of it's low \$R_{DS(ON)}\$, therefore it's low voltage drop, therefore it's low dissipation. Your requirements are not that high, however. 2A at 50% duty cycle is 1A average, a 200mV saturation voltage will cause only 200mW dissipation. Also, while there are logic MOSFETs that are specified at a few volts \$V_{GS}\$ they usually can only deliver a few hundred mA at that voltage. Most of the time they'll need 10V or higher to get > 1A drain current.

A quick search for low saturation voltage transistors gets us a the OnSemi NSS40301, which has a maximum collector current of 3A continuous.

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The graph shows that \$V_{CE(SAT)}\$ at \$I_C\$ = 2A is less than 200mV, so average power dissipation is less than 200mW, far less than the 2W maximum rating. \$H_{FE}\$ is minimum 200, then you'll need at least 10mA base current.