This is my first post. I am trying to use an Arduino Uno to control a 12V 4 WATT solenoid valve to drive a pneumatic cylinder. I am using a TIP102 transistor to control the valve. The transistor base is driven by pin 13, the collector is connected to Vin (the Uno is powered by a 12V barrel jack), the emitter is connected to the solenoid valve. I have a fly back diode in place between the solenoid + and -.
I have 12V at Vin but the solenoid after the transistor is seeing less than 4V, not enough to drive the valve.
I'm new to this and would appreciate any help.
Thanks
You need low side switching:
Positive of solenoid to 12V
Collector to the solenoid -
Emitter to GND
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That's your first problem. The emitter voltage always is lower than the base voltage. Connect collector to load, emitter to Gnd. Consider using a MOSFET instead of the Darlington.
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What you are seeing is correct. You wired it as an emitter follower. The emitter will be about 0.7 or 1.4V below the base depending if it is standard or darlington configuration. In your case it is a darlington. Best to change the configuration.
Here are schematics showing how to use a transistor or MOSFET. You need a 3A diode across the load with either configuration. You can use an appropriately rated logic level MOSFET and skip the diode. In either case I would place a 10K resistor from the output of the Arduino to ground to guarantee the solenoid is off during reset and init.
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I agree with DrDiettrich. MOSFETs are far more efficient at this particular task. Just make sure you chose a "logic-level" compatible one.
If you still want to use the TIP102, you will need to ensure you are using the proper base resistor as you will be drawing current from the output pin of the Uno. Generally speaking, you don't want to draw more than 20ma. You can do the math if you want to get the most efficiency out of it, or just stick a 1K ohm resistor between the output pin and the base of the TIP102, and it should work.
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As TIP102 is a darlington Vbe will be c 1.4V so 1k would limit Ib to around 3mA.
For saturation I'd suggest a 220 ohm would be better. You would need to use the configuration suggested by @gilshultz
Admittedly a MOSFET would have been a better choice, but you can get it working with the parts you already have.
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