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I plan to use a solid state relay to control a couple of small 12 VDC motors. One is 12V, 12Watt (1A) and the other is 12V 11amp peak. The SSR can handle 15A DC. Some of my inputs (different part of the device) also use the same 12VDC power supply. I am using a SSR and the output already has a diode. Should I still include back EMF protection? Should I use a diode? Zener diode? Varistor? I am concerned about back EMF but also a possible drop in voltage that might cause the inputs to false read. Here is the SSR diagram.

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  • \$\begingroup\$ Does the motor only ever turn in one direction? \$\endgroup\$ – Spehro Pefhany Nov 8 '16 at 22:05
  • \$\begingroup\$ Ooops, yes, both motors are pumps, and will only ever run in one direction. \$\endgroup\$ – JoeChiphead Nov 8 '16 at 22:08
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Yes, you should put a diode across the load, just as shown in the SSR diagram.

Otherwise you can damage the SSR from the inductive energy in the motor when the SSR tries to turn off.

The diode can be a conventional silicon rectifier- it will have to conduct the motor current briefly and needs to withstand the supply voltage plus some margin. For the 1A one, a 1N4004 or 1N5404 will work fine.

For the 11A one, a diode rated at 10A or so should be fine, such as a 10A04-T (or you could use that type of diode for both).

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  • \$\begingroup\$ How do I choose the diode for this purpose? What about the possible voltage dip when on of the motors kicks on? \$\endgroup\$ – JoeChiphead Nov 8 '16 at 22:31
  • \$\begingroup\$ Added comments on suitable types. The diode won't have an effect on a voltage dip- won't help and won't hurt. \$\endgroup\$ – Spehro Pefhany Nov 8 '16 at 22:33
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    \$\begingroup\$ @MichaelKarcher There are two pumps involved. The 1A one is find with the 1A or 3A diodes. The 11A one might be but I'm recommending a 10A diode. The 3A diode is actually capable of very large current for short durations. Worst case is actually not the maximum running current but if the SSR turned off during the start-up surge. I would not recommend 20V Schottky diodes without knowing a lot more about the power source- they are prone to damage from reverse breakdown (and they can leak an awful lot of current when hot). \$\endgroup\$ – Spehro Pefhany Nov 8 '16 at 23:37
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    \$\begingroup\$ @SperoPefhany I completely agree to your comments about the suitability of the diodes. My comment was meant as response to a (now deleted) comment asking about the viability of 1N4001 or 1N5401. Having more than 20V reverse voltage handling capability will definitely add a good safety margin, but I fail to see how a 12V supply should be able to exceed 12V at the diode. Leakage might indeed be an issue, if the leakage current is big enough to keep the diode hot. \$\endgroup\$ – Michael Karcher Nov 8 '16 at 23:47
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    \$\begingroup\$ What @MichaelKarcher is saying about protecting logic is possible- what you could do is isolate the supply with a series diode and provide an electrolytic capacitor so that even if the supply craps out momentarily whatever it is that is driving the SSR won't lose power. Preferably, use a completely different supply for all such loads and maintain isolation. But that's an answer for another question I think. \$\endgroup\$ – Spehro Pefhany Nov 9 '16 at 0:17

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