Relay Dropping Resistor Needed?

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=77945
Printed Date: May 21, 2024 at 9:19 AM

Topic: Relay Dropping Resistor Needed?

Posted By: darioism
Subject: Relay Dropping Resistor Needed?
Date Posted: May 17, 2006 at 8:30 PM

I will be installing a pair of running lights into my 1993 240sx which use 1156 bulbs. I will power them using a relay (fused of course) controlled by a switch installed in the dash. My question was whether the relay would need a dropping resistor to keep it from burning up, or can it stay closed for a long time? I would have the 85/86 side directly connected to my ignition from the fuse panel, with the switch in between.

With all the current that a car battery puts out, I feel that a little coil in the relay will be no match. When testing at home with a 12V source, the relay coil does heat up. If needed, would I run the resistor in parallel or series, and how big would the resistor need to be? Thanks.

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(The loop I am concerned about is hilighted in blue.)

Replies:

Posted By: ff-mike
Date Posted: May 19, 2006 at 9:07 AM

If this is a typical 'bosch' 12v relay, you can run it continuously without a resistor.

Posted By: darioism
Date Posted: May 19, 2006 at 4:31 PM

Thanks Mike. It's a "Potter & Brumfield" automotive relay, so I assume it's almost the same thing.

Posted By: hotwaterwizard
Date Posted: June 20, 2006 at 12:21 AM

First off the Diagram is wrong. You have all grounds and no + wires. The relay just shorts out the bulbs. Second the relay is going to get hot and a resistor will just make it get hotter because more resistance just makes more heat. And if you put it across the coil then it will drop the voltaqge too low to activate the relay. If you are concerned about the heat just add a small fan.

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: darioism
Date Posted: June 20, 2006 at 4:37 AM

Yeah, I had realized that the diagram was wrong, but I guess everyone got the idea. Turns out that those relays are made to be on constantly, like any relay used to power headlights, air conditioning, fuel pump, etc. I installed it all and it works great. Since I put the switch on the ignition circuit, I don't even have to worry about turning off the lights. Also, with my remote starter, it looks awesome to watch the car start itself from a distance.