Simple stuff I know, but I need more than 4 poles from 24VDC 4PDT miniature relay.

Do i parallel the coils of two miniature 4P relays, or do I use one of the poles of the 1st relay, to energise the coil of the second?

I don't like the idea of paralleling the coils, (big back EMF?)

Whats the preferred method?

 

Good question: I would consider the following:

Inductors in parallel divide rather than add:^). Current doubles but inductance is halved so the dI/dT is a push.

You introduce a timing factor as well as reduce reliability driving a relay with a relay. A 10 cent 1N400X diode will clamp the inductive spike which is not the same as back EMF which is the voltage developed building the field. A fast Schottky diode will do even better and is very cheap insurance in any DC relay circuit. Omitting them is common, but very poor practice. That's why many relay manufacturers build one in.

I hope these help you decide:^)

Regards
cww

 

Personally, I do insist it is only my personal feeling I don't know if there are "norms" for your answer, But I would put both relays in parallel, this is not the first time I see this type of configuration without any further trouble with it, on some sites you can have some "safety" relays which will cut out all field powered equipment in case fire is detected and this can imply much more that two relays in parallel, I have seen up to a minimum of 10 relays connected like this
Of course you will need to put reverse diode in reverse parallel of the coils (one diode per relay) to avoid reverse High voltage, this will cut the back EMF as you say.

Now, of course there is the reverse of the medal, in case ONE (at least) of the coil is shorted you will short the whole "line".
With your second possibility of relays in "series" (one relay commanding the second and so on if more are needed) this will not happen. But in case one is shorted all the remaining one’s AFTER this one will not work either!

You also need to consider (I don’t know if it is important for your application) the response time of the whole system.In parallel configuration all relays contacts will close at the "same time" (I know this is physically impossible but you get the idea) in series configuration you need the first relay to close before the second on is closing and so forth if more than two are considered, in some applications this can just be not acceptable!

So I would choose the “parallel” configuration and not the "series" one. Once again this is from my experience I don't know if some technical papers are covering this type of request

 

I don't know that there IS a preferred method. I've seen both. While two or more relay coils exactly in parallel is not extremely common, look at a typical relay logic diagram and see how many coils may be downstream of a given set of contacts. Just because they're not exactly in parallel doesn't mean that set of contacts won't turn off all the coils downstream of it. And contactors too! If a set of relay contacts can handle the coil in a contactor switching several hundred amps, I doubt two or twenty-two ice cube relay coils are going to cause it any grief. Snub the coils if you're worried about it.

 

Mark... if you are interested in knowing which connection is the most reliable, I suggest doing a Reliability Analysis!

If you are interested, the formulas can be found in my SIS Paper, "Probabilistic Risk Assessment of Equipment Safety Systems" More detail can be found at:

http://control.com/thread/1026175097

Regards, Phil Corso (cepsicon[at]aol[dot]com)

 

Thanks guys, Ive also seen both methods all over the place, and have listened to arguments for & against each method from colleagues.

After reading your responses, its paralleled coils with two reversed Schottky diodes, here we come.

(Im pretty sure the Schneider relays i'll be using will have internal diodes anyway...)