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I have this 12V ball valve which is controlled by three wires. Contaray to the description below, the motor still draws current after the valve opens or closes, so I will not only want to control red/blue is "on" but also power to the circuit.

I'll be using an arduino to control the valve. For power (and other) reasons I don't want to use a relay. I will power the motor with its own 12V battery pack.

If I could figure out how to make a SPDT switch, I would use a transistor to control the valve by using the red/blue and yellow wires. I assume I probably need a diode or two to protect against EMF. I've been driving other 12V motors with Darlington TP120 so hopefully I could use those here as well.

enter image description here

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  • \$\begingroup\$ You could easily do what you want to by breaking either POWER or GND of the supply you've shown. What's the output voltage of the power supply shown and how much current does it have to deliver when SW is connected to RD? To BL? \$\endgroup\$ – EM Fields Feb 11 '16 at 14:24
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The DRV8816 is used quite commonly: -

enter image description here

The power supply range is 8V to 38V and it can supply currents of up to 2.8A (normal recommended levels). There are plenty to choose from and I would recommend this method rather than building from discrete transistors.

See also this for other recommendations and some pointers to devices that may not be suitable. If you are intent on using discrete transistors then google images is your friend. If you go down this route please seek recommendations about a circuit you might choose. This one used MOSFETs and looks OK: -

enter image description here

I'm hinting at MOSFETs because they have much less power loss (and volt drop) compared to BJTs. Darlingtons are particularly hungry on power losses because you cannot properly turn on a darlington to a saturation voltage lower than about 1V. For instance the TIP120 will "drop" about 2V when saturated with 3A flowing according to the Fairchild data sheet. That's potentially a power dissipation of 6 watts per transistor (always 2 used in a H bridge to switch on the motor).

With 3A flowing in an IRF9Z30, the volt drop will be about 0.42 volts and power dissipated about 1.26 watts per transistor. Anyway always read the data sheets for the devices.

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  • \$\begingroup\$ The schematic you have for the DRV is just used to reverse a motor, correct? I'm trying to get an idea of how I would hook this up for my motor. I agree it looks like a better approach than building my own H switch \$\endgroup\$ – WhiskerBiscuit Feb 11 '16 at 17:10
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    \$\begingroup\$ There are enable inputs that can switch each half-bridge bridge off and there are inputs that set the high or low FETs to be active. This easily achieves direction control but you can also use speed control via pwm. If you look at table 3 in the data sheet it explains. \$\endgroup\$ – Andy aka Feb 11 '16 at 17:25
  • \$\begingroup\$ So if I use my arduino to control EN1 and IN1 to "on" it will power on OUT1? I assume IN means input, what does EN mean? It looks like this driver has internal resistors, so I wouldn't need to use one between my arduino and the driver? \$\endgroup\$ – WhiskerBiscuit Feb 11 '16 at 18:24
  • \$\begingroup\$ Can you confirm you have looked at the data sheet? This is all covered in the DS but I suppose if you are unfamiliar with data sheets this might seem a little daunting? There is a table for pin functions and EN does mean enable. \$\endgroup\$ – Andy aka Feb 11 '16 at 18:27
  • \$\begingroup\$ Yes I did look at the DS. I am unfamiliar and it is daunting, but I do try to RTM. I'm going to go ahead and order a few and read up while they are shipped. Mouser has two versions of them. A PWP version and a PWPR version. I can't tell what the difference is other than the cost. Do you know? The datasheets are identical \$\endgroup\$ – WhiskerBiscuit Feb 11 '16 at 18:37
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A simple H bridge would achieve the desired outcome. Circuit below: H bridge

H bridge works by opening the opposing transistor pairs, so the motor turns one way or the other.

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  • \$\begingroup\$ The TIP127 (PNP) transistor is not configured correctly to be used in this circuit - where did you find this erroneous schematic? \$\endgroup\$ – Andy aka Feb 11 '16 at 10:31
  • \$\begingroup\$ Oh, I didn't notice that. Found a correct one. \$\endgroup\$ – Ignas Urba Feb 11 '16 at 10:50
  • \$\begingroup\$ It'll probably work at 12V but the schematic shows 6V. \$\endgroup\$ – Andy aka Feb 11 '16 at 11:14

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