I have a door lock that requires 12VDC 750mA to get it to unlock, then it drops to 350mA to keep it open. My door controller has a max load of 500mA, so it won't work connected directly to the door controller. I have a power supply nearby that has the available load to unlock the door, but I am unsure what kind of relay to use so that when the door controller outputs it's 12VDC, the relay closes and supplies the power from the nearby power supply to the door lock? I'm looking for something already mounted to a board with screw terminals for connecting it up.


  • \$\begingroup\$ So, what have you found so far (bearing in mind that questions seeking recommendations for purchases are off-limit)? \$\endgroup\$ – Andy aka May 17 '16 at 20:12
  • \$\begingroup\$ @Andyaka, I've been overwhelmed with the options and the even what specs would be required! I'm just looking for the actual specs of a relay, didn't mean to make it look like I wanted a product recommendation. \$\endgroup\$ – Drewsonian May 17 '16 at 20:18

I hear you, the selection can be overwheming.

Start with the coil voltage. It must match what your controller outputs within 10% or so, that's mandatory.

Next look at coil amperage. You cannot exceed your controller's output, and since relay coils are inductive loads, I'd aim for more margin still, e.g. 200ma or less.

Next look at the contact voltage rating of the relay contacts, though that won't be a factor at 12V. Your door solenoid is an inductive load, and those love to arc, so the more DC voltage rating, the better. (Not to be confused with AC voltage rating: because AC current often goes to zero, this tends to snuff arcs. DC doesn't have this advantage, so a common relay rating is 250VAC or 28VDC. Yes, a factor of 9.)

Next look at the contact current. I like to go by a 2-3x margin for a resistive load, and a lot more for an inductive load like this. For this 750ma load, 10x is 7.5A and that is not excessive especially because it won't cost you much more.

When I have a wide selection, I look at packaging. Does this relay solder onto a PCB? Does it use blade terminals? Screw terminals? Loose leads? A connector? How does it physically mount? 90% of electronics is physical.

Next I look at stocking. How many does the distributor have in stock? I like to see hundreds in stock - a relay they have 3 of with a 12-week backorder is a bad choice.

Last but not least, I look at price.

You'll find you can obtain relays with 20-30 amp contacts in the US$3 range, barely more than a 1A relay. While that seems like wild overkill, it's not with an inductive load.

  • \$\begingroup\$ Very nice write up \$\endgroup\$ – Passerby May 18 '16 at 0:34

You are looking for any common small relay, with a coil that works at 12V and under 500 mA, and supports 1 Amp or more switching (nice buffer for safety). You will find most hobby relays in this range will have 100 mA range coils fitting your needs.

  • \$\begingroup\$ Thanks, I believe I've found an appropriate relay based on this! :-) \$\endgroup\$ – Drewsonian May 17 '16 at 20:30

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