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Help interpreting schematic with relay

  1. Mar 23, 2015 #1
    Hi, thank you for your help.

    I am having fun experimenting with an arduino. I have a solid state relay (SSR) which I am learning to use for the first time. I have found things sort of confusing, but have found the following image at the Arduino playground useful: Image here

    Here is what I have so far.

    The problem is that it tells me connect relay power + and relay power - at certain points in my network. What exactly does this mean? I would like to power a servo motor through my relay for fun, using the relay to provide some degree of electrical isolation. Does 'relay power' refer to the power which will drive this motor, or something else entirely? Can I just connect these points back to the 5V header and GND on my arduino board?

    As well, my actual SSR is the CN024D05. If I can figure out how to wire this up properly, does the motor--side of the relay need anything? I've seen some documentation warning about inductive loads: would a servo motor be a problem? I have some fuses since I had a feeling it would be a good idea.
  2. jcsd
  3. Mar 24, 2015 #2


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    A resistor and transistor is used when the digital output cannot handle the current needed by the relay coil. A relay coil is inductive so it needs a flyback diode to catch the voltage spike that occurs when it turns off.

    But, you are using a semiconductor relay; CN024D05. Look at the “INPUT SPECIFICATIONS for CN024 (1)” on page 2 of the data sheet. It shows that the SSR will draw typically 10mA when driven with 5V. The SSR will turn on when a voltage of more than 3V is available. The SSR has an LED and current limiter on it's input so you need no resistor or flyback diode there.

    I believe the Arduino digital output can source or sink up to 40 mA, so no transistor is needed, it can drive the SSR directly.

    The SSR output can switch a current up to 3.5A at 24VDC. If you are driving an inductive load such as a motor, you will need to use a flyback diode to protect the SSR output.

    Schematic attached...

    Attached Files:

  4. Mar 24, 2015 #3
    Thanks I really appreciate you taking the time to do that!
  5. Mar 26, 2015 #4


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    did you reconfig your circuit to match that shown by Baluncore ?
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