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I'm working on a smart watering system as a home project. I have a water output in my balcony (a tap) and I want to control the water flow using an Arduino Uno and a solenoid valve. I still don't have the valve but the specifications of it are listed here - I'm using Model No. SMOKVW-0516O4.7AC which is the last one on the list (at least that is the plan for now, but I hope to achieve general understanding of how to choose the right components such that even if I'm using different valve at the end, I can adjust the transistor and diode accordingly). My plan is to use a transistor to control the solenoid valve and to use a diode to handle the kickback. My problem is that I'm not sure which specifications of the transistor and the diode I need for this purpose, I've tried looking online for information but I didn't find anything that solved my problem. What I'd like to know is how to adjust the transistor and diode to the solenoid valve I'm using, the things that I'd be grateful if anyone can help me with:

  1. which transistor do I prefer to use? I thought NPN will be suitable since I need an amplification (my Arduino supplies 5V where the solenoid valve expects 12V input signal) although I also need it to behave like a switch, in order to turn on and off the valve.
  2. wich parameters should I check when selecting a specific model of a transistor? i.e. if I chose to use NPN, how do I know that the specification of the transistor will allow it to work as expected? how will I know it won't burn?
  3. I have understood that the solenoid valve has a "kickback" - a current flows in the opposite direction (from the valve to the arduino) when the valve is turned off, and to protect the board I need to add a diode. I'm not sure how can I calculate the current that suppose to be returned or how to choose the right diode?

Thank you very much for your time and attetion

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    \$\begingroup\$ Provide solenoid datasheet. \$\endgroup\$
    – user263983
    Feb 16 at 18:16
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    \$\begingroup\$ Or, as a minimum measure the current taken by the solenoid. \$\endgroup\$
    – Andy aka
    Feb 16 at 18:20
  • \$\begingroup\$ Thank you very much for your reply. I've edited my post and added the datasheet, although what I want is to understand better the physics in the circuit, so that I can choose the transistor and diode for any solenoid that I'll use (in case I won't have a choice but to change the solenoid valve model at the end). Perhaps you could refer me to some reading material about the subject? (I did take few EE courses, but I'm still at the beginning of my studies, so I don't have that much of understanding about those components) Thank you very much once again. \$\endgroup\$ Feb 16 at 18:46
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    \$\begingroup\$ How deep into physics do you want to go? Valve pressure EMF Voltage/current, transistor saturation voltage /current? \$\endgroup\$ Feb 16 at 20:26
  • \$\begingroup\$ The comment below of Justme did help me, but I'm willing to hear about anything that can help me understand even more about what's going on in the circuit, such as the points you've mentioned. \$\endgroup\$ Feb 17 at 5:06
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The solenoid is rated for 12 volts, is rated to have a 20 ohm coil, and is rated to operate at 7.2 watts.

12V / 20ohms is 600mA, and 7.2W/12V is also 600mA.

So you need a transistor that can switch 12V voltage plus some safety margin, and can switch 600mA current plus some safety margin.

This also means that the current is so high that a regular BJT would require too much base current, so you are better off with a darlington or a FET.

Since the current is about 600mA, the flyback transistor never has to pass more than 600mA through it. So any diode that can handle 600mA short peak currents plus some margin is enough, and the DC rating may be smaller than 600mA as there never is continuous current through it.

As the coil resistance has a 7% tolerance rating, and most likely your supply voltage has some tolerance rating as well, the nominal 600mA can at worst case exceed 700mA.

If you are going to prepare for different solenoids, the same applies - simply prepare for higher voltage and higher current and it will handle any valve that does not exceed the voltage and current ratings you chose.

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