Need to buy/build a momentary power interrupt switch 12v DC

Joined Apr 20, 2010
6
Hi All... Electrically challenged person here, so use layman terms if you will please...
Trailer project.....

On my trailer, I have a 12v pump (draws 6-8 amps) which I want to operate from the auxilary power pin in the trailer plug (7 wire) on my truck, OR from a stand alone dedicated battery on the trailer. I want the pump to shut off whenever I hit the brakes. There is another power lead in the 7 wire plug for electric brakes I can use, or if it draws little power, possibly use one of the turn signal leads as they are on the whole time the brakes are applied. My initial plan was to add a 'normal open' 12v solenoid valve.. Was speaking to someone about it and he came up with the idea of using a momentary power off switch for the pump intstead. Hoping it to be a lot simpler and less expensive. Is there an off the shelf product I could use, been digging, but no luck so far.

I 'subscribed' to this thread, so I will probably respond immediately if anyone has a question..

Thanks for the help

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BMorse

Joined Sep 26, 2009
2,675
An automotive relay properly rated for the load should do what you need,

here is an example:

The way this will work, as long as power is supplied to the pump, it will run, when you press on the brakes, the relay will actuate and open the connection to the pump power, thus shutting it off...

P.S.
the label "PUMP_+_IN" is where the power coming in would connect (from battery or harness), and the label "PUMP_+_OUT" connects to the pumps + lead.

B. Morse

Joined Apr 20, 2010
6
Thanks BMorse.. Will look for automotive relays... Any of the numbers on your sketch there to give me a hint of how to find it?

First one I found does the opposite (like an old time horn relay)

This 'sounds' like it may accomplish what I am looking for, do you agree?

Last edited:

BMorse

Joined Sep 26, 2009
2,675
Thanks BMorse.. Will look for automotive relays... Any of the numbers on your sketch there to give me a hint of how to find it?

First one I found does the opposite (like an old time horn relay)

This 'sounds' like it may accomplish what I am looking for, do you agree?
This is exactly what I was talking about....
any heavy duty automotive relay should work (you should be able to get these at your local auto parts store, but they can be very expensive!), most DPST relays will have a (NO) Normally open contact (when the relay is powered up the contacts will close) and a (NC) normally closed (when relay is powered up the contacts will open).

Some Automotive relays will also have the diode built in (as in the picture you posted) if it doesn't have one, you need to put one in to prevent and back EMF from causing any problems with the rest of the electronics.

B. Morse

BMorse

Joined Sep 26, 2009
2,675
I keep searching and can't differentiate between NO and NC... Most say both (at different amps)...
Here is one; http://www.allelectronics.com/make-a-store/item/RLY-351/12V-SPDT-30-AMP-AUTOMOTIVE-RELAY/-/1.html

What do you mean by expensive? The soenoid valve would be about $50.00... All of the relays I have found are under$10

The one you have linked to would work, that will have a NO and NC contacts, when I said expensive, I meant if you purchased it from your local automotive store, they usually charge around \$45.00 for a Bosch type relay..... on the web is a different story, you can get them cheap, but then you get what you pay for....

If the normally closed contacts on a relay you choose is rated to handle atleast twice the current draw of the pump, then it should work,

You could also use a Normally closed SPST relay (these only have 1 contact).

B. Morse