I am new to Arduino, also, I am not fluent in english.

I wish to create a controller for several circuits. All of those circuits are "in parallel" with additional circuits.

My question is:

  • Do a Relay Shield protects the Arduino board from the current and voltage from the power supply?

Suppose the following:

Eletric wiring

The retangular boxes are "parallel switches" connected to the devices.

R = Power Supply Phase

S = Another Power Supply Phase

N = power Supply Neutral

Arduino, C2 and C3 are 127 Volts, (S+N)

C0 and C1 are 220 Volts (R+S)
  • Are the Shields safe from oscilations from the eletrical supply?

  • What / Where / How should I look on what to ground? Local regulations state that I cannot let R and/or S be grounded.

  • Which relay shield? Construction is very important for safety reasons. Oct 6, 2015 at 18:35
  • Arduino: Mega 2560 R3. Relay (all of them are Sanyou SRD-S-105D) Circuits are UP to 2 Ampere each
    – Bonatti
    Oct 6, 2015 at 19:16
  • Okay, but which relay shield? Safety is more than just the parts, as I already said. Oct 6, 2015 at 19:20
  • So far, there is no "shield" for the relays, they are going to be "screwed" in a wood connector, and the Arduino will have welded wires to the Relay controller pins.
    – Bonatti
    Oct 6, 2015 at 19:36
  • Those relays don't have any ability to be screwed into anything. Oct 6, 2015 at 19:47

1 Answer 1


Do a Relay Shield protects the Arduino board from the current and voltage from the power supply?

A relay provides galvanic isolation in the circuit. The mains power is electrically isolated from the rest of the circuit since the device that performs the actual switching (the contacts in the relay) have no physical connection to the rest of the circuit - they are merely acted upon by magnetism.

That doesn't mean that it's safe, however. There are a number of things you still need to deal with:

  • The Arduino cannot directly drive a relay - it can't provide enough current. You must use a drive circuit.
  • Back-EMF from the relay coil as the magnetic field collapses can damage sensitive semiconductors - good snubber circuit design is important here.
  • Mains electricity is dangerous and must be treated with respect. Proper circuit design is paramount to ensure that the mains is kept away from the rest of the circuit. Proper connectors must be used and exposed high voltage points must be rendered inaccessible, either by the project housing or by the addition of plastic insulating shielding.
  • Relays as they turn on and off cause sparks. Sparks cause EMI. EMI causes induced currents. Induced currents cause damage to electronics. Minimizing the spark generation not only prolongs the life of your relay but the life of everything else around. Ensure your relays can switch fast enough for the currents you are dealing with, or consider using solid state relays (SSRs) that don't suffer from this issue.
  • When you wrotte: Arduino cannot directly drive a relay. I am having difficulty understanding this. Do you mean that Arduino cant provide the main power? (I expected that, the Relay has 5 pins, 2 for control, a common, and a Normal Open/Normal Closed), they are all understood, OR is this something else?
    – Bonatti
    Oct 7, 2015 at 11:57
  • @Bonatti The Arduino IO pins have an absolute maximum current rating of 40mA and a normal operation maximum recommended limit of 25mA. This is not enough to energise a relay's coil properly. Google "Arduino Relay" and you will see some simple drive circuits.
    – Majenko
    Oct 7, 2015 at 12:05

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