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I'm trying to understand the limitations of connecting multiple relays, and how it relates to current.

Consider a relay with the following specs (rounded for easier math):

  • Rated current: 100mA
  • Rated voltage: 12V DC
  • Rated carry current: 3A
  • Maximum contact current: 3A

Let's also consider a power supply of 12V DC and 10A.

Is the following correct?

  1. I could wire 100 (10A / 100mA) of these relays in parallel and be at the theoretical limit of my current.
  2. I could wire 30 (3A / 100mA) of these relays in series and be at the theoretical limit of the relay themselves. At that point, the contacts would get too hot or the device would fail in some other way.
  3. The power supply wouldn't really care how the relays are connected, as long as the total current draw is under its maximum. That is, "Rated current" * "Num Relays" <= "Power Supply Current"
  4. I could mix and match parallel and series connections as long as stay under the limits in #1 and #2.

I'm new to hardware and I'm trying to not kill relays or burn down my house.

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    \$\begingroup\$ Consider what happens during the time that relays are opening and closing, especially if they don't all switch simultaneously. \$\endgroup\$ – supercat Mar 30 '17 at 22:59
  • \$\begingroup\$ @supercat, makes sense that since these are physical devices, there will be some delay when opening closing. \$\endgroup\$ – jakekarnes42 Mar 30 '17 at 23:20
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You seem to be mixing up the ratings on the coils and on the switching contacts. Notice the ratings you quoted come from two different tables in the datasheet: enter image description here

If your power supply is being used to drive the coils, then you need to consider the required coil current to switch the relay. As your item #1 says, a 10 A supply could potentially supply 100 100-mA coils simultaneously.

If your power supply is being used to drive the load (switched by the relay) then you need to worry about what current your load draws when powered by 12 V. If it does indeed draw 3A, then you could only power 3 such loads with a 10 A supply.

If one relay is being used to switch current to the coils of a bunch of other relays, then you get the limit of your item #2. The 3 A allowed through the contact of the first relay is enough to power the coils of 30 100-mA coils that are its load.

Also, if you want your system to be reliable for a long time, you'd probably want to de-rate all of these specs rather than operate the power supply or the relays at their maximum current limits. (The coils will operate at roughly the spec'ed currents due to the coil resistance and Ohm's Law)

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  • \$\begingroup\$ Thank you so much for the clear and concise answer! This is exactly the information I was looking for. I definitely agree that running everything at maximum current limits is a bad move, I was just trying to simplify. \$\endgroup\$ – jakekarnes42 Mar 30 '17 at 23:28

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