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Am a very beginner with electronic circuits and its components.. Im trying to turn on my water pump using Arudino nano. Ive got 25A solid state relay for that purpose. So according to little info I collected from google, I found we have to connect a diode to relay coil to avoid return current from the coil which will spoils the circuitary. but my questions are

  • how can we decide the diode to connect in this case. I mean any specific values?
  • Also does the diode to connect to the positive terminal of the coil in series with the out from Arduino? Or is it parallel?

This is the specification of the relay

enter image description here

If its not solid state relay what could be the solution

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The purpose of adding a reverse biased diode to a mechanical relay is to prevent a reverse voltage potential appearing at the driver circuit when the magnetic field around the coil collapses, which happens when the relay coil is suddenly disconnected.

A solid state relay does not use magnetic fields. Instead it uses light:

enter image description here

So there is no collapsing magnetic field to generate a reverse voltage potential. It follows no reverse biased diode (or flyback diode) is necessary.

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  • \$\begingroup\$ It is great if you kindly share answers for my questions if its not solid state relay?? \$\endgroup\$ – Sandeep Thomas Jul 25 '17 at 14:26
  • \$\begingroup\$ If you use an ordinary mechanical relay which uses a coil to pull contacts together - simply follow the link (blue text) in my answer. That will take you to another stackexchange question where using a diode on a mechanical relay is discussed in detail. \$\endgroup\$ – st2000 Jul 25 '17 at 14:37
  • \$\begingroup\$ Might well want a flyback diode across the PUMP, though!! \$\endgroup\$ – Scott Seidman Jul 25 '17 at 15:21
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You do not need a diode to protect a circuit and components if you choose SSR. Diodes are used as fly back diode in mechanical relay circuits.

It would help you, if you understood a difference between mechanical and solid state relay.

Electro- Mechanical relay:

Electro- Mechanical relays are being used with fly back diode protection. This is because, when these relays are powered by power supply they create magnetic field around the coil which makes NO contact to NC.

The magnetic field is created by electrons flowing through the coil which puts their energy into the coil to create a magnetic field. When the power supply is stopped, magnetic field will be changed into emf which is called back emf that will pass through the circuit which causes magnetic field to collapse. This voltage will be high enough to damage the circuit.This process happens based on law of conservation of energy. To prevent damage diode is connected in parallel with the relay which by pass the path of back emf. For in depth detail: http://www.douglaskrantz.com/ElecFlybackDiode.html

Solid State Relay :

Solid state relays are different than Electro mechanical relays. Opto coupling is the principle behind sold state relays. Solid state relays can be separated as input circuitry and output circuitry.

In input circuit, IR LEDs or other light emitting diodes are present in series with the current limiting resistor and a photo sensitive diode or phototransistors. There will be a small gap between the LED and Photo sensitive diodes. Small input signal is needed to energize a LED which illuminate light that will be focused on photo diode across the gap which in turn turn ON output circuit.

Output circuit consists of Triac, SCR or Transistor depends on the output type AC or DC. When Output circuit turns on, then power to the load will continuously pass on. In this SSR, isolation is provided upto great extent and there is no possibility of inductance problem etc. So, technically diode is not needed for SSR.

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If its not solid state relay what could be the solution

The difference between a solid state and mechanical relay is the inductive coil. For a DC mechanical relay:

how can we decide the diode to connect in this case. I mean any specific values?

The diode D1 must have a reverse voltage rating greater than the power supply V1. Example: if V1 is +15V than I would choose a diode with a reverse voltage rating of 30V or so.

The other thing to keep in mind (but is usually not a problem) is the power dissipation of the diode. When the SW1 goes from closed to open the current through the coil of RLY will continue to flow as the magnetic field collapses (this is what will generate large voltage transients.) D1 provides a path for the current to "flyback" which clamps the voltage transient. The power dissipated in the diode is the forward voltage drop*coil current, but for an extremely short period of time. Because it is such a short period of time it is not something that is typically a problem. I mention it because it is possible to pick a diode which is so small it can't handle the current, or if you have a high switching rate the diode should be pick a bit more carefully.

enter image description here

Also does the diode to connect to the positive terminal of the coil in series with the out from Arduino? Or is it parallel?

The diode connects directly across the relay coil as shown.
Note: If you are trying to control a mechanical relay directly from an Arduino you will have some trouble. Arduino's do not have the required power output for most mechanical relays. You won't find a 25A relay with a 5V/20mA coil. I would use an external bjt and power supply to switch the coil.

enter image description here

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  • \$\begingroup\$ If you dont mind can you tell me about that bjt in that case? I mean any specific value or details.. am really sorry am a very beginner with electronics.. am actually from software developer, but interested in these things.. \$\endgroup\$ – Sandeep Thomas Jul 28 '17 at 12:43

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