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Author Topic: Raising 0.95V to 12V?  (Read 769 times)
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Hello,

I'm sure this is the dumbest question ever: I have 0.95V on a wire and I need to raise it to 12V to turn on a light in my car. For some reason the wires don't supply 12V anymore but only 0.95V. How should I go about this?

Thanks,
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Maryland, USA
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I'd go try to figure out why the wire produces 0.95V.  Otherwise it sounds like you're playing with fire here...
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I must add the battery is fine and everything else in the car is working fine, other lights etc are getting a steady 12V.
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I'd go try to figure out why the wire produces 0.95V.  Otherwise it sounds like you're playing with fire here...

You reckon I could damage something?
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I'm kind've wondering if the wire's cut, or partly shorted somewhere, and you actually try to pull significant current through some circuit (like a DC-DC stepup converter) you could overheat the wire at wherever it's failed and cause fire/smoke that way.  Trying to compensate for damaged/broken wiring is just playing with fire IMO.
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I see. Could there be an easier way to check for damage than pulling the wire out? I have no idea where it's running and I don't want to take the car apart. Or maybe I should put one or two LEDs instead of the 12V lightbulb? They won't draw too many mA and they will be happy with 1V?
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A few questions...

Do you know where that "12V" wire is coming from?   For example, is it "power" from a switch or something, or is it some sort of low-current "control signal" ?   If you don't know what the wire is for, you shouldn't be connecting things to it...  or at least you shouldn't be connecting anything that draws more than a fraction of a milliamp.   

What's normally connected?    The light is something you're adding, right?

Do you get 12V without the light connected?   I mean, what makes you think there should be 12V there?  Ort, did you connect the light and blow something, so that there's no longer any voltage there?
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.95 volt could even be phantom voltage to some multimeters...  There may be no current at all there.
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A few questions...

Do you know where that "12V" wire is coming from?   For example, is it "power" from a switch or something, or is it some sort of low-current "control signal" ?   If you don't know what the wire is for, you shouldn't be connecting things to it...  or at least you shouldn't be connecting anything that draws more than a fraction of a milliamp.   

What's normally connected?    The light is something you're adding, right?

Do you get 12V without the light connected?   I mean, what makes you think there should be 12V there?  Ort, did you connect the light and blow something, so that there's no longer any voltage there?


I was a bit evasive in my OP, sorry about that.

The light in the boot of the car is not coming on anymore when the boot is open.
I checked if the lamp was fried but it's not.
Then I checked if the switch to detect whether the boot was opened or closed was working, and it is: when the switch is pressed (= boot closed), there is 0.00V flowing to the lamp. When the switch is not pressed (= boot open), there is 0.95V flowing through. But the lamp is not coming on because it is rated for 12V 5W.

So I thought I could just use a voltage regulator to take the voltage back up to 12V and have the light work normally. Or I could make a small PCB with a 3.3V regulator, a resistor, one or two 12k mcd white LEDs and a fuse (the original Toyota lamp looks like a fuse and I'm thinking it doubles up as a fuse) and place it there so I could get light with minimal voltage and current draw.

Alternatively, I could use a ATtiny to monitor the wire for voltage and turn on the above mentionned LEDs PCB powered by a battery or two that I would change every now and then.

My main problematic is that I have very basic electronics knowledge and I don't have the tools not the courage required to take the car apart to dig up the wire and check it all the way to the (brand new) battery.

All in all it's a minor annoyance, everything else in the car is working fine, but I thought it could be a useful and fun mini-project. Thoughts?
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just use a voltage regulator to take the voltage back up to 12V
The voltage regulator you are thinking of only reduces the voltage to 12v. It will not boost it to a higher output than the input.

I would tend to think that there is some problem with the wiring in the car, and "making do" with what you have isn't going to solve the root problem. Maybe the switch is bad? If the contacts are dirty then you might see a lower voltage than normal.
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Yeah, there's no way around it--you have to figure it out.  If the boot's switch is accessible, maybe you can remove/disconnect it and check the wiring from the battery and the wire going to the lamp, then check the switch itself and make sure it reads 0 ohms when closed.  If it's a bad wire from the switch to the lamp you might be able to splice a new wire in there.  If it's a bad switch then see if it's possible to disassemble & clean it or find a new one of similar type.
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Down load the service manual for your car.   It will take time to trace it out but that is FAR better than just leaving it.  That is a very odd voltage for a car application.  If it is shorted some place better to find it now than after an electrical fire.

Also look on line in car forums for similar failures in your make and model car.
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In most older car designs the boot switch is actually a grounding device.  ie the lamp is permanently supplied with a 12 volt line and the other side of the lamp then runs to the boot switch.  When the boot is opened the switch actually takes this line down to the chassis of the car, so completing the circuit to the lamp. ie a "grounding " switch

So the first question you need to answer is  " Does the switch have 1 or 2 terminals"   

If only 1 then you have a "grounding" switch. If so then I suggest that there are at least 5 possible options

a) the 12 volts line to the lamp is defective
b) the lamp holder connections are corroded
c) the lamp is dead
d) the wire from the lamp to the switch is defective
e) the switch is defective (possibly just corroded)

You have not advised the model or age of the car  --all information that might get you an informative answer
« Last Edit: April 18, 2012, 12:42:16 pm by jackrae » Logged

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What are you using as your ground when measuring the voltage coming from the switch? If a wire comes to the switch, another leaves the switch and goes to the bulb and the bulb is grounded then you should have 12V at the switch. If you don't have 12V at the switch then you need to check fuses and/or damaged wiring.

When you get an odd voltage reading on a device that was once working then you have something that failed and the best fix is to find out what failed and replace it. Not hook up some device you pulled out of your imagination to fix a problem you have not troubleshot.

The problem in this case is that for some reason you are not getting 12V to a device that should have 12V to work. The problem is not that you have .95V and you need it to be 12V.
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In most older car designs the boot switch is actually a grounding device.  ie the lamp is permanently supplied with a 12 volt line and the other side of the lamp then runs to the boot switch.  When the boot is opened the switch actually takes this line down to the chassis of the car, so completing the circuit to the lamp. ie a "grounding " switch

So the first question you need to answer is  " Does the switch have 1 or 2 terminals"   

If only 1 then you have a "grounding" switch. If so then I suggest that there are at least 5 possible options

a) the 12 volts line to the lamp is defective
b) the lamp holder connections are corroded
c) the lamp is dead
d) the wire from the lamp to the switch is defective
e) the switch is defective (possibly just corroded)

You have not advised the model or age of the car  --all information that might get you an informative answer

The car is a Toyota Avensis T22 from 2002. I cannot check the resistance or anything of the switch because it is not easily accessible. It is burried under the trunk latch...
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Please factor this in

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