I'm building my first arduino project, and I could use some help building up the relay portion.

My goal is to control a water pump that's running at 110V, 3/4hp, rated at 9A, and measured to run at 500 watts.

Initially I selected a 10A relay module from Sainsmart:

  • amzn.com/B0057OC6D8
  • www.sainsmart.com/arduino-pro-mini.html

However, after reading about motor loads, I'm worried that I undersized the relay. Now I'm looking at:

  • A 20A EM relay:

    • amzn.com/B00R7TTSMI
    • Is this enough of a safety factor for my motor size
    • Would the Arduino have enough current to drive the coil, would I need additional circuitry to drive it?
  • A 25A SS relay:

    • amzn.com/B0087ZTN08
    • Pro, comes with a heatsink
    • Would the 25A really be enough to support the startup spike of the pump motor?
  • A 40A SS relay:

    • www.sparkfun.com/products/13015
    • amzn.com/B00HV974KC
    • amzn.com/B00E1LC1VK
    • Does not include heatsink. Do I really need a heatsink for a constant 9A running through a SS relay?

How about a flywheel diode? I'm guessing that for an EM relay I should include a flywheel diode. I've seen reference that SS relays normally include snubber circuitry. Is this the case, and would either of the SS relays basically be drop-in with my application (5V arduino logic pin, directly to VDC side, 110VAC hot line through the VAC side).

I've read that EM relays are beefier and more reliable than SS. And I've also read that EM tends to fail open, whereas SS fails closed (potentially destroying my pump). However, the idea of a relay with no moving parts, zero-crossing switching, and an onboard snubber is really attractive.

What's your recomendation? Thanks for your help in choosing.


2 Answers 2


USA standards require motor horsepower ratings for relays used for motor control. If you look carefully at the amzn.com/B00R7TTSMI relay you will see Hp ratings on the top of the relay. Some online info for the SSRs mention heating control - not good for a motor.

Addes re Coil Etc.

If you look up the Songle SLA-05VDC-SL-C that you can see printed on the relay, you will find that the coil requires 185ma. If you look carefully at the pictures on Amazon, you will see a diode that must be across the relay coil. For an AC motor, you would not use a diode across the motor, that would be a short circuit on the power supply.

Solid state relays that are rated for motor use have Hp marked or stated in the spec. sheet.

Note that the above relay is marked with UL's cRUus mark (backwards R). That indicates that the relay is a UL listed component that is also listed per Canadian requirements for the ratings published.

The Amazon material mentions "amp transfer for heating ...." The amp ratings and transfer duty are only for resistive load. The Hp ratings are only ones applicable to motor duty (non reversing).

Before asking questions here, you are generally expected to find and attempt to interpret the spec sheets. Then ask about the parts that you don't understand and give a link to the sheets that you have questions about.


Anything you do with 120VAC must be Code legal. Fortunately, you can raid the parts bin of the heating/cooling industry, stuff that's safe, code legal and cheap because it's made by the millions.

Think the way a smart thermostat works: use the Arduino to switch 24VAC control power. Then use the 24VDC circuits to control large power contactors to switch the motor. This also keeps 120/240V far away from your Arduino.

Here's a common 24V transformer. Notice how it screws into a 1/2" knockout on a steel junction box. That satisfies Code requirements to keep low-voltage wiring separate from 120/240V.

Switch the 24V with the Arduino, then at a workable location along the 120V wiring, install a steel junction box to contain a suitable A/C contactor or fan-control relay. Run the 24V wire to it, that can be thermostat wire and you can legally run it the same way.


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