Contact ratings are the same, only difference is the coil I have is 3v.
I'm thinking of using a resistor to drop the coil voltage down so
I don't burn up the 3v coil. I don't know why we don't just use
relays with 3v coils, and just add/change the resistor...

This is for a project to add an auto-off trunk/boot light on my car
(sometimes I leave the trunk open during the day, and don't notice it,
then day turns to night, and next morning my battery is flat.)

 

. I don't know why we don't just use
relays with 3v coils, and just add/change the resistor...

Because it is wasteful and not necessary, why not make a small time out circuit, 555 based?
Max..

 

I suppose it would be energy wasteful...

I am making a time-out circuit, but the relay is for the lamps, and
I am adding two more, to illuminate the walk. Will my 3v relay
with dropping resistor work okay, or should I get out and get
a 12v relay before proceeding?

 

Personally I would use a 12 volt coil relay, any light duty automotive relay will work. That said, yes, you can place a resistor in series with your 3 volt relay coil. Figure the relay coil current at three volts, know the output voltage of your delay circuit then subtract 3 from that voltage and divide by the relay coil current. Be it a 3 volt or 12 volt relay coil I would add a fly back diode across the relay coil.

Ron

 

Contact ratings are the same, only difference is the coil I have is 3v.
I'm thinking of using a resistor to drop the coil voltage down so
I don't burn up the 3v coil. I don't know why we don't just use
relays with 3v coils, and just add/change the resistor...

This is for a project to add an auto-off trunk/boot light on my car
(sometimes I leave the trunk open during the day, and don't notice it,
then day turns to night, and next morning my battery is flat.)

Post a schematic of the timing circuit and we can make a better recommendation.

eT

 

Contact ratings are the same, only difference is the coil I have is 3v.
I'm thinking of using a resistor to drop the coil voltage down so
I don't burn up the 3v coil. I don't know why we don't just use
relays with 3v coils, and just add/change the resistor...

This is for a project to add an auto-off trunk/boot light on my car
(sometimes I leave the trunk open during the day, and don't notice it,
then day turns to night, and next morning my battery is flat.)

Similar contacts take similar amount of effort to pull them in. The 3V coil probably needs 4x as much current as the 12V coil. A buck regulator would solve that problem - hysteretic topology is the simplest.

 

Buck regulator! I'll use an lm317, thank you.

 

I would prefer a 12V relay because it uses less current than a 3V relay and doesn't need a resistor to keep it safe. One less part to go bad.
On the other hand, I have built stuff with what I had laying around. It gets kind of, "Rube Goldberg" but it's quick and economical to use up spares instead of driving to the parts store.

 

Buck regulator! I'll use an lm317, thank you.

Whatever.