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post #1 of 19 Old 03-12-2009, 11:02 AM Thread Starter
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Voltage drop off

Hey Everyone. First official post on this forum. Anyways, I was reading a prievious post where the OP was getting some expert advice. So I thought that this was the place I needed to be. I'm an AC electrician. Been in the business a long time. I have been doing electrical mods to my 07 2 door X when I ran across a situation that I am unfamiluar with. I did a "rock light / convienance light" mod where I picked up the dome light circuit to trigger on of my relays. Everything works as expected untill the voltage begins to fade down due to the "dimm to off" function of the dome light. Is there an easy way to get the relay to drop out as the voltage begins to fall off.
Thanks in advance
Randy

P.S.: I have an inline fuse at the connection point where I picked up the dome light circuit. And I have an in-line diode at the relay And using a standard 12v automitive relay.
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post #2 of 19 Old 03-12-2009, 11:30 AM
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There are a couple of ways you can go about this. The first, and most simple is to apply enough resistance to the relay coil input that you're getting _just enough_ voltage to trigger it at the 14V of a fully-charged battery (or an actual 12V if the circuit is only pushing 12V). Of course, if the coil its self is putting load on the line (not that likely), you might need to increase the hysteresis, or have it switches back and forth a few times.

You can also use a transistor -- since the base current modulates the collector-emitter current, you could design a circuit in such a way that a slight drop in the base voltage is enough to switch off the junction.

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post #3 of 19 Old 03-12-2009, 11:30 AM
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There are 2 ways they could be dimming the dome light, and I don't know which way they use.

The first way would be a transistor in the domelight driver module reducing output voltage. A relay should be able to deal with that OK but you could also build a voltage comparator circuit to shut off the relay at whatever voltage you wanted.

The second way for them to dim the light would be to use pulse width modulation. This could cause a relay to chatter or buzz when turning off. To work around this, you would need to build a PWM detector to turn off your relay.

One way you could fix it would be to wire a relay into the door switch circuit to turn on your lights. Remember, the door switch probably drives an input module that outputs to the CAN bus that lets the computer know your door is open. The days of simply grounding out a light when the door is opened are over.

Me, I'd just put a switch on the danged thing.

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Last edited by Alter Ego; 03-12-2009 at 11:40 AM.
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post #4 of 19 Old 03-12-2009, 11:33 AM
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Quote:
Originally Posted by Alter Ego View Post
The second way for them to dim the light would be to use pulse width modulation to dim the light. This could cause a relay to chatter or buzz when turning off. To work around this, you would need to build a PWM detector to turn off your relay.

Ahh yeah, forgot about all of the PWM they use in the jeeps now. That would render a simple resistor or transistor circuit useless.

Given the complexity of dealing with PWM, I'd just follow Alter Ego's advice and just wire off the switch that's triggering the PWM down.

!c
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post #5 of 19 Old 03-12-2009, 11:45 AM Thread Starter
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Quote:
Me, I'd just put a switch on the danged thing.
Ya, I knew that I was in the right place. I love this forum. Now, other than the switch because I have that also on an interposing relay, could one of you guys sudgest something in a little simpler terms. Thanks
Randy

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post #6 of 19 Old 03-12-2009, 12:48 PM
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It might help for us for you to tell us what you're intending to do. I see "rocklight mod" and I'm thinking you're adding LED rocklights as footwell lights? In that situation I see no need for a relay at all.

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post #7 of 19 Old 03-12-2009, 01:05 PM
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If you switch the interior lights to LEDs, you would be able to run LED courtesy lights directly off the dome light circuit and still not overload the dome light module. Just check the wattage of your existing incandescent bulbs and keep the total load with the LEDs under that number.

I assume since you're trying to run these lights off the dome light you only want these lights for getting in and out of the vehicle. Is this true? I can't see where you would want the dome light on when running rock lights to see the trail.

You never did say what kind of problem you were having as the lights were dimming or what kind of lights you're trying to use.

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post #8 of 19 Old 03-12-2009, 02:02 PM Thread Starter
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Sorry Guys. I'll try to be a little more specific. I'm using this system more or less like a curtisy lighting sys. I have two relays on this system. One is triggered by a simple switch inside the cab. The other is triggerde by the dome light circuit that I picked up inside the cab via the harness @ the passanger side floor board. I have connected to this system a couple of cheep 50w fogs mounted to the rocker pannels under each door. I would like to use these lights either with the switch or when a door is opened. The circuitry has worked perfectly with exception of the dome light dimming after door closure. As the voltage drops ( due to the dimming circuit ) the relay begins to chatter as the voltage approaches minamum clamping voltage of the relay. Not verry much chatter. Just enough to be annoying. And, I am not sure if this could cause premature failure of the relay. I realy like the curtisy effect of this system. I would just like the relay to completly drop out before minamum clamping voltage is reached. I do not have any electronics tranning. So I do not know how to make this happen. Thanks for any advice.
Randy

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post #9 of 19 Old 03-12-2009, 03:12 PM
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Driving it off the dome light will require an electronics tech to build you a special circuit to cleanly drive a relay. 30 years ago I could do that. Too gray now.

I think the best way to get the function you want is to switch to LED lights and not worry about relays. Unfortunately, they're not nearly as bright, but they will let you see where you're stepping. Use the big lights on a separate circuit controlled by a switch for trail crawling.

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post #10 of 19 Old 03-12-2009, 03:18 PM
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Just had another idea. Switch to a solid state relay and it will switch as fast as it needs to having the effect of dimming your rock lights along with your dome lights.

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post #11 of 19 Old 03-12-2009, 03:50 PM
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Depending on how much you want to build, this circuit contains what you need. Mainly the "low voltage cutoff" portion of the circuit.

http://www.electronics-lab.com/proje...042/index.html

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post #12 of 19 Old 03-12-2009, 08:55 PM
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Jk-ford, you can probably do something as simple as running a resistor inline from the dome light to the relay, reducing the current enough so that you start just above the minimum pull-in voltage for the relay. This doesn't mean it won't chatter at all, but it'll go off very quickly and should click for less time, as you'll spend less time around the window.

I would suggest getting a 100 ohm potentiometer, and wiring it up between your dome circuit, and the relay coil. Tun on your interior lights, and then adjust down the pot to where any more change will cause it switch off (you may need to give it a little more space than this). Then use your multimeter to measure the resistance of the pot., use a resistor near this value, and replace the pot. with the resistor.

Otherwise, you can go by the pull-in voltage and calculate the resistor value from there, but a potentiometer will allow for easier adjustment.

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post #13 of 19 Old 03-13-2009, 06:32 AM
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You could probably put a capacitor in parallel with the relay coil, this would(sized properly) hold the relay on until the PWM has finished and then the relay would kick out shortly after the PWM stops.
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post #14 of 19 Old 03-13-2009, 07:25 AM Thread Starter
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Thanks Guys. I am impressed by the level of experties that has been shown here. I like the idea of the solid state relay. Getting the circuit do dimm simular to the dome light would be an un-expected bonus. I'll try building the "cut-off circuit" also so I can test it out and use it for a backup plan. I had already thought about the in-line resistor ideal. My thoughts are that the drop-out span would be the same. Even if you start at a point just above the drop-out curve. So, I do'nt think that this would work.

I do have one other thought that I would like to run by U-guys. Does anyone know how the door switch works and where it's at. Other than the harness, I do not see any signs of a trigger device. I have assumed that , since this device is'nt obvious, that it's part of the buss system. Therefor usless to me. One of the undesirable effects of using the dome light as a trigger is that the lights outside are always on when the inside light is on. So when you want inside light. You also get outside light.
I have thought about adding an indapendent door switch. But then you run into the doors-off issue. And, I do not like the idea of a kill switch. Can U-guys think of anything else. Once again thank you for all of the excelent advice. I hope that I can be a contributer to this board.
Thanks
Randy

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post #15 of 19 Old 03-13-2009, 07:35 AM
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The door switch is a simple switched signal that goes to ground when the door is open. It switch itself is in the door latch mechanism. That way the lights stay off when you remove the doors. I do not have the wiring diagrams here at work but I can look it up tonight and give you the wire colors in the door cable.

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post #16 of 19 Old 03-13-2009, 08:01 AM Thread Starter
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Quote:
Originally Posted by jdrogers View Post
The door switch is a simple switched signal that goes to ground when the door is open. It switch itself is in the door latch mechanism. That way the lights stay off when you remove the doors. I do not have the wiring diagrams here at work but I can look it up tonight and give you the wire colors in the door cable.
So I would probably have identify the type of signal and try to find a CT device for relay contacts. Sounds doable but complicated.

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post #17 of 19 Old 03-13-2009, 12:10 PM
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Quote:
Originally Posted by DED HED View Post
You could probably put a capacitor in parallel with the relay coil, this would(sized properly) hold the relay on until the PWM has finished and then the relay would kick out shortly after the PWM stops.
Dang, DED HED, you beat me to it. This is the easiest way to stop the chatter. 100uF 15v elec. cap. This is what I used to keep the chatter out of my headlamp relays.

/jl

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post #18 of 19 Old 03-13-2009, 06:37 PM
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Add a small diode (1N4001 or similar) in series with the lead from the domelight, banded side towards domelight wiring. Next add a capacitor (2200uF or so, 16 volt or better) across the coil of the relay, negative lead to the same terminal that has the domelight lead. Dhe diode will allow the voltage to operate the relay and also charge the capacitor. When the domelight fades away, the diode will only allow the capacitor to discharge via the relay, it should release with a simple click (no buzz).

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post #19 of 19 Old 03-16-2009, 12:39 PM Thread Starter
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Quote:
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Add a small diode (1N4001 or similar) in series with the lead from the domelight, banded side towards domelight wiring. Next add a capacitor (2200uF or so, 16 volt or better) across the coil of the relay, negative lead to the same terminal that has the domelight lead. Dhe diode will allow the voltage to operate the relay and also charge the capacitor. When the domelight fades away, the diode will only allow the capacitor to discharge via the relay, it should release with a simple click (no buzz).
Thanks again guys. I was wondering if I could use a small cap to help hold the coil in. I didn't know if the cap would discharge any faster. I already have an in-line diode as part of my circuit protection.
Thanks
Randy

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