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I'm working on an automotive project. I have an electric water pump and fan, both of which will be powered through relays. I also have an on-off-on dpdt switch. What I'd like to achieve is activating the pump at both poles and just the fan at the other. This seems doable, but in all my attempts I've only been able to have both on or both off. How do I wire the switch to achieve my goal?

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Just use a diode.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Do you have a 3 position switch ON-OFF-ON and two diodes? \$\endgroup\$ – Sunnyskyguy EE75 Oct 19 '16 at 0:59
  • \$\begingroup\$ Not one of the available schematic symbols. It isn't significant in any case, the switch is DPDT when in one of the two "on" positions. \$\endgroup\$ – pgvoorhees Oct 19 '16 at 1:01
  • \$\begingroup\$ drive both coils with 1 diode drop for each coil from ON-1 and drive one coil from ON-2. Then add reverse diodes across each coil for snubbers. so 4 diodes. centre pole=12V fused. only need SPDT \$\endgroup\$ – Sunnyskyguy EE75 Oct 19 '16 at 1:04
  • \$\begingroup\$ Ah, okay. I was thinking to myself, "I feel like there's something out there that only allows the current to flow in one direction." Now I know my hunch was right, but more importantly what it is I need. \$\endgroup\$ – geraddavis Oct 19 '16 at 1:07
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    \$\begingroup\$ Sounds like I could use a DPDT switch with one diode, or a SPDT with four diodes. I'd like to keep it as simple as possible, so I think I'll try the single diode approach first. How do I know which diode I need for this project? It's just your typical 12v DC automotive electrical system. \$\endgroup\$ – geraddavis Oct 19 '16 at 1:18
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schematic

simulate this circuit – Schematic created using CircuitLab

This was what I had in mind for EMI control of Pump glitches which are probably worse than relay coil glitches.

  • no transistor protection needed if battery nearby.
  • Battery cables are twisted as well for EMI control.
  • Avoid large current loop area with paired wires or twisted wires

    • (Pwr/Return)

Although it may not matter in your application but for cars , they have to survive reverse jump cables and 2xVbat jump cables so reverse diodes are verbotten as they would blow. So, they use Snubber power resistors, often inbuilt.

enter image description here

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  • \$\begingroup\$ This is the type of automotive relay I'm using in my project: !(images-na.ssl-images-amazon.com/images/I/410SFrIB44L.jpg) !(images-na.ssl-images-amazon.com/images/I/410SFrIB44L.jpg) \$\endgroup\$ – geraddavis Oct 19 '16 at 14:41
  • \$\begingroup\$ as I suspected , so no reverse diodes on coil needed, but load may have arcs \$\endgroup\$ – Sunnyskyguy EE75 Oct 19 '16 at 16:13
  • \$\begingroup\$ Okay, so just the one diode on the trigger feed then? \$\endgroup\$ – geraddavis Oct 19 '16 at 16:17
  • \$\begingroup\$ Yes the one diode isolates the pair of relays for just the Fan. In logic , we call this Diode OR logic. The relay coil has a wide operating range, so the diode drop is neglible whether it is 0.3V Schottky or 0.7V Silicon, won't matter. I would choose Silicon for greater PIV with snubber R built in, I am not certain of value hence spike voltage but Silicon 1N4005 .. no sweat \$\endgroup\$ – Sunnyskyguy EE75 Oct 19 '16 at 17:19

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