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I am trying to use a 9.5A (max) 12v DC motor in the circuit of the coil side of a 10A 12v DC relay and although its opening the NC contacts, its not allowing the motor to run. What i'm trying to do is cut off an irrigation solenoid whenever the 12v DC motor is actually running, so as to avoid a sudden loss of pressure. Am I missing something basic here? motor not running here..

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  • \$\begingroup\$ A schematic of your circuit and links to the datasheets for your relay would help us to give you a better answer to your question. \$\endgroup\$ – The Photon Oct 17 '16 at 4:55
  • \$\begingroup\$ If the motor is in series with the relay coil, the coil will not pass enough current to allow the motor to operate - with a 12 volt supply, you may find 11 volts across the relay coil, leaving only 1 volt for the motor (numbers are a wild guess - you'll probably have much less than a volt across the motor). Find a datasheet for the relay - you'll probably find that the coil requires much less than 1 amp at 12 volts. \$\endgroup\$ – Peter Bennett Oct 17 '16 at 4:56
  • \$\begingroup\$ The datasheet is here baileyproducts.com/files/87452402.pdf. Its the FRM3W-2B10. Im unsure of how to post a diagram here, but it is hooked up in series with the dc motor, on the coil side. \$\endgroup\$ – Daryl Oct 17 '16 at 5:06
  • \$\begingroup\$ @Daryl you must be careful in Relay selection for DC motors due to the surge currents greatly exceed the average max load by a factor of 5 to 10 x \$\endgroup\$ – Tony Stewart EE75 Oct 17 '16 at 5:08
  • \$\begingroup\$ Check the DC resistance of the coil for your relay PN. Consider how that relates to what Peter said. \$\endgroup\$ – The Photon Oct 17 '16 at 5:12
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The relay coil should be connected in parallel with the motor, and the relay contacts connected in series with the irrigation solenoid, like so:

schematic

simulate this circuit – Schematic created using CircuitLab

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Im fairly sure this should work now. I trialled it on a breadboard with similar components. Seems I should have connected the relay in parallel. enter image description here

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  • \$\begingroup\$ The relay coil goes across the motor. \$\endgroup\$ – Spehro Pefhany Oct 17 '16 at 9:39
  • \$\begingroup\$ motor should be always on in this case, except if your relay connections on the other side are shorting the PSU \$\endgroup\$ – Wesley Lee Oct 17 '16 at 18:23

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