In SX Microcontrollers, SX/B Compiler and SX-Key Tool, davehein wrote:

Shaun,

According to the spec sheet, the ZTX603 has a current gain of 2000 at it's maximum collector current of 1 amp.  A collector current of 1 amp would require only 500 micro-amps of base current at a gain of 2000.  The base voltage is 1.8 volts, and if the SX is running at 5 volts the base resistor would be 6.4 K-ohms.  I would use a lower resistor value in the range of 1K to 3.3K to ensure that the Darlington is fully on.

Even though the Darlington has a high gain, I believe that you can't drive the emitter to ground.  This is because the collector of the first transtor has to ride at the voltage of the base of the second transistor, which must be around 0.9 volts.  If the emitter drops below 0.9 volts the first transitor will turn off.

You should connect one end of the solenoid to 5 volts and the other end to the emitter.  You may need a limiting resistor in series to ensure that you don't drive too much current through the solenoid coil.  What's the resistance of the solenoid coil, and how much current does it need to operate?

Dave

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