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I'm new to electronics and am trying to construct a hardware level safety check for a pair of relays so that only one can be triggered at once. The relays will be driven by two separate DIO's on Arduino. I originally tried to do this using a pair of npn's as switches triggered by a single xor but I'm curious if there is a better way.

[Reply #1 on:]

I'm new to electronics and am trying to construct a hardware level safety check for a pair of relays so that only one can be triggered at once. The relays will be driven by two separate DIO's on Arduino. I originally tried to do this using a pair of npn's as switches triggered by a single xor but I'm curious if there is a better way.

Well if you use DPDT relay contacts one pole could be wired up such that it removes coil voltage of the other relay if the first is engaged and visa versa. Very basic but very safe.

Lefty

[Reply #2 on:]

You want an arrangement called XOR or exclusive-OR. It is common in relay logic or "Ladder Logic"
Here is one example:
http://de.wikibooks.org/wiki/Digitale_Schaltungstechnik/_XOR

[Reply #3 on:]

I've seen this mentioned in some of the pages i've searched through. However all the examples seem to only have one load attached to the last relay in the pair. I want to run separate loads off each relay but still maintain exclusivity between them. I haven't been able to find a schematic that demonstrates that yet so i'm having a hard to envisioning it. Is that were the DPDT comes in one set for the relay to relay and the other set for the load?

[Reply #4 on:]

I've seen this mentioned in some of the pages i've searched through. However all the examples seem to only have one load attached to the last relay in the pair. I want to run separate loads off each relay but still maintain exclusivity between them. I haven't been able to find a schematic that demonstrates that yet so i'm having a hard to envisioning it. Is that were the DPDT comes in one set for the relay to relay and the other set for the load?

Exactly, one set of contacts for each relay form an interlocking arrangement such that only one relay can be energized at a time. And of course the other set of contacts on each relay is free to perform whatever the purpose of the relays are.

 It's very simple actually. Say you are working with 12vdc relay coils, wire +12vdc to the common contact of both relays, then run a wire from relay #1 normally closed contact to one of the coil terminals of relay #2, and also a wire from normally closed contact of relay #2 to a coil terminal of relay #1. The other coil terminal of each relay wires it's driver transistor collector or drain which pull to ground to enable the relay coil.
 So draw that out and see if it's not both a simple but elegant solution of mutual exclusive operation of the two relays.

Lefty

[Reply #5 on:]

See a quick drawing attached.

As soon as you energise one of the relays it will switch that load on, but it also breaks the circuit for the other relay, meaning you can't energise it.

Whichever relay is switched first will switch it;s load and prevent the other from being used. When you release that relay you can then energise the other one.

There is no way you can energise both relays at once.

[Reply #6 on:]

I think i understand now that is most definitely a more elegant solution thanks for the help all!