Implementing a 1-Pole, 3-Way Switch Via Relay

Here's a sketch, using 3 of 3PCO relays.

If any relay is ON, it disconnects the grounded side of the coil of the other two.

There are other ways ...

... doesn't quite fit your last requirement (all off if attempting to turn 2 on at the same time), but does ensure only one can be on at a a time (on a first-come-first served basis).

Bolton_Alan is right!

Re-arranging the wiring on my drawing,

meets all your requirements.

Why thank you kind sir

Al

Thanks guys! It does solve my problems. I guess I'll really need a 3-pole relay instead of a DPDT.

Thanks for the help!

You could use 3 imput to 8 line decoder and diode logic to run 3 relays. The computer selects a 3 bit word, the only valid ones of which are 1,0,0, 0,1,0 and 0,0,1 The diodes are connected to output lines 2, 4 and 8 thence to read relay coils. Any input word other than the 3 above will give a logic output on other lines. Thus only one relay is activated.

If you can relax your last statement about 2 switches closed then neither A, B, or C will turn on you can do it with 2 3 pole relays.

relay 1 NO and relay 2 NC - output A

relay 1 NC and relay 2 NO - output B

relay 1 NO and relay 2 NO - output C

It doesn't meet your initial statement but it might meet your initial intent which I believe is to prevent 2 outputs from being on simultaneously.

Shawn

The solution you have diagrammed will work just fine if the normally closed contacts you show are activated by a separate coil, each energized at the same time as the corresponding relay. Let me elaborate:

PC "A" would energize a relay we will call RA. At the same time, a relay we will call Ra would energize. This relay would be a DPDT such as a Square D model MJN2C. Each control input, A, B, and C would have a corresponding auxilliary relay that would energize the same way as described above for A. The two normally closed contacts of the auxilliary relays would be used to disable the other two lines when energized. That is, for instance, in line A, there would be a normally closed contact from Cb and Cc and so on. Therefore, when any one of the main contactor relays, A, B or C is energized, neither of the other two could operate because the corresponding auxilliary relay contacts would prevent it. I hope this is all not confusing you. It's really quite simple.

Hi Commoner: Thanks for the inputs (no pun intended)

I've tried to sketch this out, and I think I get it:

I use 3 pcs of DPDT's (3 Aux: Ra, Rb, Rc)

I use 3 pcs of SPDT's (3 Main: RA,RB,RC)?

The coils of the main and aux are essentially in parallel (when PC energizes coil RA, coil Ra is also energized).

This would be the same as using 3pcs of 3-pole relays right (using the 3 SPDT's as the third pole)?

You are correct. The only exception is the MAIN relays don't have to be double throw, they can be single throw SPST using a normally open contact. The normally closed on the SPDT would not be necessary.

One last thing and I'll get off the subject. I have made a rough sketch of a wiring diagram for the system I described toyou. Look at it and see if there is anything you don't understand about it. I am assuming the your PC simply closes a contact through which you can run your control voltage (I have used 115 volt since that is the most prevalent here). The coils AC, BC and DC are going to activate whatever you are controlling. These are coils that may close (or open) to do the job you require. They may also be small electric motors, lights, motor starter coils, etc. The coils RA, RB and RC are the auxilliary coils tha will activate the NC contacts that will interrupt the voltage going to your main coils (or other load). As you can see, this is a rather simple wiring schematic, but I believe it will accomplish your objective. Good luck!! TheCommoner

Solrac,

You actually have four logical states--off, speed-1, speed-2, and speed-3. Let me suggest a completely different approach.

Use one SPDT relay and one DPDT relay (or two DPDT relays if you want to simplify the parts list and don't mind ignoring one set of contacts). Wire them as follows:

Power in to CR1-com. CR1-NC to CR2-com1. CR1-NO to CR2-com2. CR2-NC1 is left without connection (your off condition when neither CR is energized). The other three terminals of CR2 can be connected, one terminal to each of the three fan speeds, according to how you want your logic to control the two relays.

You have four boolean logic states off/off, off/on, on/off, and on/on. The above connections give the off/off state the off-condition for your fan. The other three are your three speeds. No need for interlocks, etc. because it is mechanically impossible to energize both the NO and NC contacts simultaneously. Depending on the programming language your bits are equal to decimal values of 0, 1, 2, and 3 which can correspond directly to the fan's speeds....

John M.

Thanks Jmueller for the alternative solution. I limitation which I failed to mention is the need for 3 inputs, which cannot work in boolean fashion (000,001,010,011,100,101,110,111), because I am avoiding the situation that any two bits are turned on at the same time.

all relays in their simplest design.. single pole double throw or 2,3 pole double throw are three way switches... common to normally open and common to normally closed is a three way switch

Bollocks.

A double-throw relay is a two-way switch - for a double-throw relay (or switch), each input can be connected to one (and only one) of the two possible outputs.

A three-way switch has three outputs. Each input can be connected to one (and only one) of the three possible outputs.

The number of poles is irrelevant. The number of "throws" (normally only used for single- or double-throw) is equal to the number of "ways".