A while back, I ran across an intriguing relay variant. It's a single pole double throw relay with two coils, where the contactor normally sits in the middle between the two contacts. In other words, it's normally open, and by triggering one coil or the other you can get it to contact one side or the other.

It's like a single-pole double-throw latching relay (the type with a separate set coil and reset coil), except that it doesn't actually latch - when power is released neither contact is contacted by the contactor.

Is there a common name for this?

  • \$\begingroup\$ So your saying its almost like a single pole with 2 double throws that share a common normally open contact? \$\endgroup\$ – crowie Dec 24 '16 at 5:57
  • \$\begingroup\$ I think you are describing is a differential relay. There are two main types....current and voltage. They are used to sense either current or voltage imbalance in voltage distribution networks and to protect multiple transformer installations. \$\endgroup\$ – Jack Creasey Dec 24 '16 at 6:37
  • \$\begingroup\$ @crowie - pretty much. \$\endgroup\$ – TLW Dec 25 '16 at 3:06

Probably that would be a SPDT-NO relay (form K). See Tyco documentation on relay forms taken from this datasheet:

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But I tried to find some, it seems unobtainium. Which could easily be explained by the fact it can simply be made using two standard, cheap, SPST-NO relays.

  • \$\begingroup\$ "it can simply be made using two standard, cheap, SPST relays." That loses the only major advantage of this type of relay, namely that the two outer contacts cannot be inadvertently connected. (Consider what happens when both coils are inadvertently (briefly) on at the same time...) \$\endgroup\$ – TLW Dec 25 '16 at 2:49
  • \$\begingroup\$ "it seems unobtainium" I know :( Which is a shame, as they are useful ("useful"?) for dual-rail logic. \$\endgroup\$ – TLW Dec 25 '16 at 4:05

You can do it if you use relays with an extra pole.

Interlock the relays with each other using the extra NC pole. Then put all of the terminals on the NO side of the relays.

Now it is impossible to inadvertently have both sides connected at the same time.

  • 2
    \$\begingroup\$ ... unless a contact welds - which can happen. Welcome to EE.SE. \$\endgroup\$ – Transistor Mar 17 '20 at 22:33

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