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I'm planning to change a 5V relay module containing optocoupler and other components with a 3.3V relay. The 5V relay is from Songle, and I'm planning to replace it with relay from Bestep.

Would doing so require me to make any other changes in the module, which contains some resistors and diodes?

enter image description here

The schematic is for 5V relay module

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  • \$\begingroup\$ what are the other specs of the relay? \$\endgroup\$
    – ratchet freak
    Commented Jun 22, 2018 at 11:13
  • \$\begingroup\$ @ratchetfreak I am really sorry but I simply purchased them from China, but they are exactly like these : electrodragon.com/product/… \$\endgroup\$
    – Ouroboros
    Commented Jun 22, 2018 at 11:37
  • \$\begingroup\$ 3V relay like these: aliexpress.com/item/… \$\endgroup\$
    – Ouroboros
    Commented Jun 22, 2018 at 11:37

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The coil current will likely be about 50% higher for the 3.3V relay compared to the 5V, and the supply voltage is less, so it would be prudent to look at reducing the value of the resistor from the optocoupler output to the base of the drive transistor. The optocoupler LED series resistor maybe should also be reduced.

You will need to supply a schematic with part numbers and values for us to make more specific recommendations.

You've added a schematic, but the part values are not readable, also I suspect maybe some parts are not installed as there appears to be two options (with and without isolation).

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  • \$\begingroup\$ Thanks Spehro. Can you tell me what as per you would happen if there's no change in the resistors and I went ahead and changed the relay? Would that screw things up? What anomalies can happen, and under which scenarios? \$\endgroup\$
    – Ouroboros
    Commented Jun 22, 2018 at 11:46
  • \$\begingroup\$ Transistor might get hot, relay might not pull in properly or at all under some conditions of temperature, voltage, perhaps intermittently. Insufficient margin for aging may cause it to work okay today but fail later. Relay not pulling in smartly may cause premature contact failure. These things are not necessarily mutually exclusive. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 22, 2018 at 11:49
  • \$\begingroup\$ I've put the schematic. Can you explain the changes I'd need to do now please? \$\endgroup\$
    – Ouroboros
    Commented Jun 22, 2018 at 18:55
  • \$\begingroup\$ You've added a schematic, but the part values are not readable, which doesn't help much, also I suspect maybe some parts are not installed as there appears to be two options (with and without isolation). Please indicate if all parts are installed, and if not, which ones are or are not installed (may as well confirm values while you are at it). \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 22, 2018 at 19:40

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