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Current, not voltage Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 5 Jan 2009 18:07:10
In Reply to: Re: Won't work, Arabiflora  [Profile/Gallery]
, Sun, 4 Jan 2009 20:30:26
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The issue is current, not voltage. The stalk switch contacts have carbon buildup, which is a resistance. Current flow times resistance is a voltage (drop) across the switch. Lots of current, big voltage drop across the switch. Low current, less voltage drop.
Checking with a meter is a good idea. Disconnect the power lead from the pump and stick a voltmeter lead in, with the other meter lead on ground. With the key ON, hit the squirter. Pull the wipers off the windshield first so they don't scratch. You'll either see 12 volts or 0. 0 means the contacts are dead. 12 volts means the contacts are still there. The meter pulls almost no current, so it doesn't matter what the resistance is.
A pump motor will pull about an amp. More than that, it'll pull closer to 3 or 4 amps to start spinning; that 1 amp is once it is running. That's a lot of current. A typical automotive relay will pull about 0.1 amp; some less. So that's 30 to 40 times less current when you first hit the switch. A typical automotive relay will pull in with a little over 8 volts, so you can live with a voltage drop on the switch.
Is it possible the stalk switch contacts will show 12 volts, but won't pass enough current to pull in a relay? Yes. But about the only way to tell is to put a load on the switch, and unless you've got some spare resistors around, just buy the relay. Given a choice, get a smaller current rating - 5 to 10 amps is plenty for a WW pump. The lower the current rating, the less current the coil will take. And it'll cost less, too.
The relay has two leads that are the coil, and from two to six leads for the contacts. Two leads are an on-off switch; 3 would be a Single Pole Double Throw (SPDT), where the common ties to one lead with the relay de-energized, and the common tied to the other lead with the relay energized. Six leads is DPDT - a double set of contacts.
You want to take the wire that goes from the switch to the pump (12 volts) and tie it to the + lead on the coil. Ground the other side of the coil. Find a set of contacts that are open when the relay is off, closed when the relay is on. Run 12 volts to one contact, and connect the other contact to the + of the pump. Make sure the pump still has a ground contact.
As Justin notes, there are relays with a fuse built in so you can connect it directly to the battery - never run a wire form the battery without a fuse. I'd mount the relay as close to the battery as possible, to keep the unfused wire as short as possible.
posted by 192.249....
, Sun, 4 Jan 2009 17:05:16, Mon, 5 Jan 2009 17:39:35, Sun, 4 Jan 2009 18:13:28
, Sun, 4 Jan 2009 19:06:43, Sun, 4 Jan 2009 20:30:26, Mon, 5 Jan 2009 18:07:10 <-- Viewing This Message, Sun, 4 Jan 2009 23:16:04
, Mon, 5 Jan 2009 09:48:44
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