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I have this 4 pole single throw relay which needs to switch the 3 phase mains to the output.

My question is, I am providing the control signal (to open and close this relay) from a 3.3V MCU GPIO.

Whether this GPIO 3.3V control signal be able to open and close the relay? Or should any other circuit need to be added?

Freshman
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  • If the relay coil is rated for 12V/330mA then you need a driving transistor to switch the coil. – Michal Podmanický Jul 18 '23 at 07:17
  • It is considered good practice to include the relevant data from the datasheet in your question as text. At least do copy & paste. When the manufacturer eventually will change the structure of its catalogue the link will be broken and the question along with all answers rendered useless for future readers. – Ariser Jul 18 '23 at 07:30
  • Here's the datasheet from Digikey. The relay is the 12 V @ 333 mA version, not the 24 V, if the question's part # is correct. – periblepsis Jul 18 '23 at 07:32
  • They note "Release time will lengthen if a diode, etc., is connected in parallel to the coil. Be sure to verify operation under actual conditions." Given that this is normally open (I believe), a diode probably should be replaced with a zener+diode for faster release reducing contact wear. (Were it normally closed, a diode alone may be better.) – periblepsis Jul 18 '23 at 07:43
  • @periblepsis, thank you for the input. So, when you mention that the relay is 12V @ 333mA, whats do is actually imply? I need to provide a 12V gate signal, is it? And what is the 333mA? It would be helpful, if you could write an answer with an example sample relay driving circuit – Freshman Jul 18 '23 at 08:09
  • @Freshman Relay drivers abound. This site is replete with them. But yes, you will need a source of voltage that is higher than 3.3 V. You need to find it somehow. It will need to share the ground with your existing 3.3 V supply. And you will need a transistor of some kind, plus some basic parts that will include protection for the transistor. – periblepsis Jul 18 '23 at 08:45
  • @Freshman When you ask questions like 'what is the 333 mA' then this tells me that you really need someone to just "give you the construction details" and not waste time on discussion beyond that. There's a decent answer here that includes some options for you to consider and the zener+diode option I'd mentioned. Look at it for a moment and see if it helps any. – periblepsis Jul 18 '23 at 09:06
  • @periblepsis, how to identify whether the above relay is Normally open or Normally closed? – Freshman Jul 18 '23 at 13:24
  • @Freshman Usually, I find it clearly stated in the datasheet. But not in this case. So i have to assume. – periblepsis Jul 18 '23 at 13:28
  • @periblepsis, yes. Could you help to arrive at what your assumption would mean? – Freshman Jul 18 '23 at 13:30
  • @Freshman For AC mains relays, which this appears to be, i would assume it is NO. But that's my assumption. I've no justification other than limited experience. – periblepsis Jul 18 '23 at 13:34

1 Answers1

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The relay you have is rated for much higher voltage than 3.3V and generally MCU GPIO pins are incapable of currents required by the relay coil.

The GPIO can't control the relay directly. You need a relay driving circuit.

Justme
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  • Thank you for the answer. Can you suggest a driving circuit for an example? – Freshman Jul 18 '23 at 08:07
  • No, because I don't know the requirements you have for driving the relay. If I suggest something without knowing what you are using the relay for and how, it will be dangerous as a three-phase mains switch. Besides the web page you link to has application notes how to use relays. – Justme Jul 18 '23 at 08:18
  • Ok, I just want to connect a 3 phase mains AC input to the output. The Relay is only to help the make/break that connection. – Freshman Jul 18 '23 at 08:19
  • Would this BC817-40,215 transistor help to drive the relay? – Freshman Jul 18 '23 at 08:26
  • That is impossible to say as I don't know how you are going to use it. Your orignal question is now answered. Draw a schematic diagram, link to datasheet of each exact component and ask a new question about it. – Justme Jul 18 '23 at 08:33
  • Ok thank you. Could you tell me how to identify whether the above relay is Normally open or Normally closed? – Freshman Jul 18 '23 at 13:24
  • The datasheet you linked says one version of the relay has four Form A contacts, the other has three Form A and one Form B. A Form A contact is Normally Open, Form B is Normally Closed. – Peter Bennett Jul 18 '23 at 15:48
  • @Freshman You need to know which exact relay you intend to buy as the data sheet is for a range of similar relays in the same series. When you know the exact part number, you can look from the data sheet e.g. which coil voltage and contact type it has. – Justme Jul 18 '23 at 16:48