Can I control a relay with a two signal switches?

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MikeAqua

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#172108 12-May-2015 09:05

I need to built a circuit that uses a 12VDC relay to control a 12VDC pump. A relay is being used because the pump is close to the power source but some distance from the control point, and the relay saves long heavy cables ($$$).

The pump is usually controlled by a level switch, but on occasion I need to bypass this and force it to run.

Is it possible to set-up the relay like this: -

The pump is connected to the Normally Open terminal
The control signal comes from a single pole double throw switch.
- On1: Connects directly to the signal input on the relay, switching on the pump
- CentreOff: The relay stays open and the pump doesn't run
- On2: Connects to the signal input on the relay via the level switch, switching on the pump only when the level switch is activated.

I would solder the two signal leads together and have a single lead connected to the signal input terminal.

Mike

Niel

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#1302970 12-May-2015 16:44

I do not understand your last sentence, but the rest looks fine. (I also do not usually look in this forum so might not see replies.)

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gregmcc

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#1303006 12-May-2015 17:27

basic AOM (auto/off/manual) switch, very easy to do.

In the manual position the relay is powered from the switch
In the auto position the relay is powered via the level switch

the type of switch you are looking for is a 3 position centre off

MikeAqua

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#1304551 13-May-2015 14:33

Thanks for your reply. I have the switch and the relay sorted.

I'm struggling with how to connect both signal leads to the one relay.

Can I simply join both signal leads to a single lead that is connected to the signal terminal on the relay?
(I guess you would call this a y-junction or t-junction).

gregmcc: basic AOM (auto/off/manual) switch, very easy to do.

In the manual position the relay is powered from the switch
In the auto position the relay is powered via the level switch

the type of switch you are looking for is a 3 position centre off

Mike

Clima

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#1304601 13-May-2015 15:25

Do you wish to over-ride the level control completely (possibly causing overflow) or just force the pump to run when below normal cut-off?

andrewNZ

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#1304698 13-May-2015 17:40

Here are your two options.

https://onedrive.live.com/redir?resid=bae8cbde9ef3b738!70509&authkey=!AIcR8csoRuprc-g&v=3&ithint=photo%2cjpg

Edit: sorry cant seem to manage to get OneDrive stuff to display as images.

k1w1k1d

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#1304700 13-May-2015 17:44

Switch you need is ON-OFF-ON.

Connect the relay signal wire to the centre common terminal.
Connect one of the normally open terminals to power.
Connect the other normally open terminal to the level switch.

Moving the switch from the off position will select either manually on or auto from the level switch.

Clima

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#1304717 13-May-2015 18:29

If you want to have over-riding control just connect your contact across the existing contact.

If you want the high-level cut-off to prevent overflow you will need to modify the level controller.

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SepticSceptic

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#1305103 14-May-2015 13:04

Looks like you just need to parallel the level switch connections ?

Leave your switch at On2 - the level switch, and use the paralleled switch to override the level switch. Though as others have mentioned, you will need to be alert for overflow ...

MikeAqua

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#1305331 14-May-2015 17:50

I'll post a circuit diagram tomorrow, but in words ...

The system includes: -
1 x sump,
1 x 200W submersible 12V DC pump,
1 x float switch, 1x on-off-on switch.

The objective is to keep the water level in the sump below the level of the float switch i.e. stop the sump from overflowing. I want the sump to be nearly full most of the time

At times I need to be able to run the pump to almost completely empty the sump, well below the level of the float switch.

So I need two system modes: -
- Auto (float switch activates pump to stop sump overflowing)
- Manual (pump just runs continuously)

This is how most bilge pump circuits in boats work - which is where I got the idea.

What I'm doing differently is adding a relay to turn the pump on and using the switch as the source of the control signal for the relay. A relay saves me long expensive cable runs, because my power source is close to the pump, but the switch location isn't (can't be).

I want to run leads from the switch to this pin - one via the float switch, and one direct.

The relay has a single signal input terminal. I have separately tested connecting a switch to the input terminal directly and via the float switch. Both work.

What I don't know is whether I can: -
Join the control leads together at a point between the float switch and the relay; and connect this combined control lead to the relay's signal input terminal.

Mike

MikeAqua

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#1305665 15-May-2015 10:37

Thanks for all the replies (and thanks to Clima for helping me via PM). I think the top circuit in the picture posted by AndrewNZ is what I am trying to do.

Below is a sketch of the circuit I have in mind. The blue dotted shape encloses the party of the circuit I am unsure about. Basically the connection of

I'm now pretty sure it will work. Confident enough to risk the relay by testing.

One last question: Does the physical orientation of the relay matter e.g. should it always be mounted with the terminals facing down?

Mike

Technofreak

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#1305701 15-May-2015 11:17

Hi Mike,

It doesn't matter where you join those wires. You could use a chock block connector with two wires into one hole. You could solder two wires onto the switch terminal or two wires onto the relay terminal or two wires onto the float switch terminal. Electrically it doesn't matter.

I would make my decision based on what is easiest or the most practical. In other words where is it easiest to join the two wires together. Use the terminal that is easiest to solder two wires onto or the terminal that requires the least amount of wire.

So far as the mounting position of the relay, that doesn't matter too much either. Do what makes sense and gives easy access to the terminals for fault finding.

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MikeAqua

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#1305726 15-May-2015 11:46

Thanks for your help. I should have posted the circuit diagram to start with.

I usually solder, liquid tape and heat shrink connections. The connections along with the control switch and relay will be in an IP66 enclosure.

Mike

Niel

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#1306072 15-May-2015 18:54

Ideally you should have a diode across the relay coil in the reverse polarity. When you switch the relay off there is a magnet field in the coil which collapses resulting in a large negative voltage generated (similar principal to a car ignition coil, and BTW also an electric fence energizer) so a diode is often placed parallel to the coil so that the negative spike is shorted out instead of burning the switch contacts (both the manual and float switches).

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Technofreak

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#1306082 15-May-2015 19:14

Niel makes a very good point.

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MikeAqua

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#1307110 18-May-2015 12:24

The relay I am using has inbuilt diode protection.

Mike