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I need to design a circuit to drive a relay and I have some doubts.

  1. Do I need a resistor in series with relay coil, I mean, between transistor and relay coil ?
  2. Why I always find relays with 12V, can I use a 5V source to energize the coil?

Unfortunately, I don't have current specification neither a relay model. My doubt is about functioning, I'm afraid to fry the relay or the transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Relays with 5 volt coils are fairly common - one distributor lists 485 types. \$\endgroup\$ – Peter Bennett Jun 2 '20 at 23:59
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You only need a resistor in series with your relay coil if your are applying a voltage that is different than what your relay coil is rated for. At the rated voltage, the coil resistance will be sufficient to limit the current to the levels necessary to energize the coil.

However, if you apply a larger voltage than what the coil is rated for, the coil resistance is insufficient to limit the current to safe levels. In that case you must add enough series resistance so that the rated voltage appears across the coil and the rated current runs through the coil, even though you are applying a larger voltage.

This is easy enough to calculate using V=IR along with your applied voltage, and two of the following: the coil's rated voltage, current, and resistance from the datasheet

If you are applying a voltage less than the relay's rated coil voltage, you do not need a resistor but the relay will also not switch since there is not enough current.

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    \$\begingroup\$ Hi, Regarding: "If you are applying a voltage less than the relay's rated coil voltage [...]the relay will also not switch [...]". In that case, IMHO we can't say that the relay "will not" switch for any lower voltage. As an extreme example, let's say for a 12 V relay, then applying a voltage slightly less than that (e.g. 11.9 V) will still cause it to switch :-) I think it would be more accurate to say something like: "If you are applying a voltage less than the relay manufacturer's specified pick-up voltage [...] then the relay is not guaranteed to switch [...]". What do you think? \$\endgroup\$ – SamGibson Jun 3 '20 at 1:03
  • \$\begingroup\$ @SamGibson I didnt realy want to muddle the waters by talking about a chattering or a relay held so weak it chaters when you smack it =/ \$\endgroup\$ – DKNguyen Jun 3 '20 at 1:04
  • \$\begingroup\$ "a relay held so weak it chaters when you smack it" I'm not talking about that either :-) But I think it's important to point out that there is tolerance on the rated coil voltage (specifically, the pick-up voltage for energising & the drop-out voltage for de-energising) so trying to simplify things by stating that "less than the relay's rated coil voltage" won't work, is misleading IMHO. (Panasonic has a little article on this e.g. here). But it's your answer, so I'll stop here. Sincere thanks for the reply. \$\endgroup\$ – SamGibson Jun 3 '20 at 1:14

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