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I am testing the Songle SRD-05VDC-SL-C 5pin 5V 10A Relay without any module board.

It works with 5V as it is supposed to.

It also works with 3.3V pin and digital out pin of an Arduino Nano.

If I operate this relay with 3.3V, will it cause any problem in the future?

enter image description here

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  • \$\begingroup\$ The pull-in voltage is specified as 75%, so for a 5V coil that's 3.75V so there's no guarantee that it will close at 3.3V - or it may close but not properly, adding resistance to the contacts and generating heat if it's a high current load. I wouldn't advise it. \$\endgroup\$
    – Finbarr
    May 7, 2021 at 9:37

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Supplying the relay coil with 3.3V instead of the 5V it expects may make it less likely to close or may make it close slower. That could lead to arcing between the contacts. Other than that it shouldn't be an issue for the coil side of the relay.

However, you mention that you're driving it directly from the output pin of your Arduino. That will fry the arduino sooner or later, it is not built to supply the current needed by a relay coil.

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  • \$\begingroup\$ Ther rellay at 5V takes around 100mA, so If you are lucky and the rellay switches at 3V3, the current can be more than 200mA and that will definitely blow up your Arduino very fast. \$\endgroup\$
    – Steporkak
    May 7, 2021 at 10:19
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    \$\begingroup\$ @Steporkak Relay coils don't draw constant power, they have a constant resistance. It would be more like 66mA \$\endgroup\$
    – user253751
    May 7, 2021 at 10:22
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    \$\begingroup\$ The coil resistance is 50 ohms so it tries to draw 66mA from 3.3V. Obviously the AVR pin can't provide 66mA due to output impedance and besides it would be way past the absolute maximum rating of 40mA for any IO pin. So yes, it is harmful. \$\endgroup\$
    – Justme
    May 7, 2021 at 10:36
  • \$\begingroup\$ Oh, sorry, I was wrong. \$\endgroup\$
    – Steporkak
    May 7, 2021 at 10:43
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Best way of interfacing any microcontroler and a relay is with a transistor for driving and a diode across its coil. A DC relay coil can be considered as a normal resistor (after the transients of switching on & off have been disregarded) so if you connect it to a voltage source it will allow current to flow according to that resistance (I=U/R). Additionally relays in their datasheets have SWITCHING ON current which is a minimum current source should provide in order to switch the relay ON and there is also a HOLDING current which is minimum current at which the already switched ON relay will switch off. HOLDING current is noticeably lower than SWITCHING ON current. You could for a short time power the relay just to turn it ON and then reduce (PWM or otherwise) the voltage to get little bit above HOLDING current. In fact to conserve power on some of my projects that required the use of relays i have used this technique with great success.

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