Current consumption

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Oct 6, 2006 | 12:47 AM

fmc400

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MSEE

Joined: Apr 2004

Posts: 10,386

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From: Austin, TX

Current consumption

I'm building a car alarm based off 555 timer IC's. Long story short, the horn and headlights are going to be run off 12V relays, and I need to know what contact ratings to get. This is a '77 LTD. How much total would the left and right low-beams, and all parking lights (turn signals, tail lights) draw? That is to say, if every outside light in the vehicle were on, except the high beams and the brakes (when you pull the headlight switch all the way out), how much total current would I be pulling?

Also, does anyone have any idea how much current the horn draws?

Thanks

Oct 6, 2006 | 07:28 AM

subford

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Joined: Aug 2005

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From: Easton,Ks

You say 77 LTD, how many bulbs (tail, front and side) and their number are you talking about?

The horn is already on a relay and horn relay coil does not draw much. Just ground the relay ground wire like the horn switch does.

Oct 6, 2006 | 07:55 AM

fmc400

Thread Starter

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

I've got 2 bulbs in each tail light; and one bulb in each front turn signal. No parking lights on the fenders. I think the license plate light and the dash lights are on that circuit as well. As for the horn, I don't believe I have a relay. I'll disconnect the horn and listen for a click, but I'm almost sure there isn't one. I know that some LTD's had some and some didn't; I think the ones without cruise control didn't have them (mine doesn't have cruise). My wiring diagram shows both setups....relay and no relay. I'll double check, but if I still don't find a relay, would you happen to know how much it draws?

Thanks for the help so far.

Last edited by fmc400; Oct 6, 2006 at 07:57 AM.

Oct 6, 2006 | 09:24 AM

ness0012

Senior User

Joined: Mar 2006

Posts: 130

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From: Ottawa

real easy way for you to find out. Take a blown fuse and trim the plastic off the top of it so you can solder some small wires to it. Then grab a muti-meter set to Amps DC and hook it up to the wires in series. Next turn everything on and take a reading.

******************Warning****************
Just cause your meter says it can read upto say 20 Amps dosn't mean that you can run even 10 amps for more than 30 seconds without melting your leads. Don't belive me I'll send you a picture of my leads.

Oct 6, 2006 | 04:54 PM

Franklin2

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Joined: Jan 2001

Posts: 56,950

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From: Virginia

You need to worry about the current draw for good reason. But in a way, you don't. The reason is the most inexpensive common relay is usually rated for 30 amps. This will be way more than the total current draw of all the lights.

Thinking about what you are doing, I am assuming you are going to need a relay for the running lights(one circuit), the lowbeams(2nd circuit) and several relays for the turnsignals(there are 4 different circuits here).

So at 30 amps each, you have a LOT of headroom left as far as amperage of the contacts.

Oct 6, 2006 | 05:12 PM

Bob Ayers

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Joined: Jan 2003

Posts: 4,417

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From: Durham, NC

One thing to remember, if you are using an open collector driver to pick the relay coils, just make sure you have diodes across the relay coils to dampen the induced voltage when the relays are de-energized. The cathode of the diodes go to +12V on the relay coil, the anode goes to the collector side.