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Using LED's to indicate voltage from different sources

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takotruckin

New Member
Hi everyone, I'm trying to figure out a circuit here and need some help.

I am using a 12v UPS battery to keep memory alive in vehicles while replacing batteries or terminals etc. I have the UPS battery wired to a connector that plugs into the vehicles diagnostic port. The problem is, the fuse to the diagnostic port is sometimes blown, keeping me from feeding the cars electrical system that way. I would like to add an LED to indicate that the diagnostic port has power/ground before I unhook the vehicles battery. But I cannot figure out how to keep the LED from lighting off of the 12v UPS battery. Any help would really be appreciated here. I hope I was clear enough in my description.
 

Sceadwian

Banned
Connect an LED between the +12 and GND lines of the ECU module with a resistor valued to limit the safe current to the LED. Nothing else should be connected to the system. This will only work if the ECU connector/system is designed to allow current out of the system not just to let it in. Protection circuits will prevent power from coming out, if it has those there is no way aside from physically verifying the fuse.
 
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Pommie

Well-Known Member
Most Helpful Member
Can you make a second connector with just the LED and plug it in. If the LED lights then the fuse is good.

Mike.
 

Sceadwian

Banned
Weird, that's what I said =)
 

Sceadwian

Banned
In that case all you need to do is add a single additional switch to the + voltage of the battery on your backup supply, so that you can turn it off to see if the LED still is lit, then turn it on to actually work.
 
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takotruckin

New Member
Yeah, I can do that.

It just bugs me because I know this capability exists, I just cannot find the circuit for it anywhere.
 

tcmtech

Banned
Most Helpful Member
12 volt battery and one properly wired cigarette lighter adapter cord works wonders as well! :)

Just back feed the power through the cigarette lighter socket. Typically they are a full time power source with around a ten amp capacity.
Thats more than enough to be able to keep everything alive. Even when you open the doors and the interior lights come on. ;)
 

tcmtech

Banned
Most Helpful Member
So how do they charge their portable stuff when the key is off? :confused:

Are you sure its the actual cigarette lighter socket and not the power port socket they turn off? A friends car turns off the power port socket when the key is turned off but his cigarette lighter socket stays live all the time.

But yes its likely and possible some models may be turning both off now since so many people leave stuff plugged in and run their battery's down so often.

I still have an issue with back feeding the OBD port being its only supposed to supply at most several tens of Ma's for the scanners to work on.
When the car doors are opened there are typically numerous interior lights that come on and they can draw several amps combined. Thats way over the rated capacity of the OBD circuits.
If an in line resistor is used it will create a large voltage drop on the vehicle side when anything draws any power and that will render the whole memory keep alive purpose rather pointless if the vehicles power drops below 6 -8 volts for more than a few seconds.
 

delambo

New Member
I really cant see any simple way of doing what you are asking for because all the time the UPS is giving a voltage to the vehicle it will keep the led lit even if the fuse is blown. The biggest flaw in your plan is if the fuse blows at the point that you disconnect the battery. If the OBD socket has a 5 amp fuse (as many do) you may find that the vehicle has something alive which is drawing more than 5 amps (courtesy lights for example). When you connect your ups to this you may find the fuse is fine because the battery is supplying the correct current through the correctly rated fuse. As soon as you disconnect the battery it will try to pull that current through the fuse for the diagnostic socket and blow it instantly. The damage is done and it is too late to look back.
 

MrDEB

Well-Known Member
I may be missing something?

looks and sounds like a simple 12v rechargable battery in a fancy case with an AC adapter plug for recharging.
I need to dosome research about the OBD socket but looking at pic and description thats my first thought.
 

MrDEB

Well-Known Member
according to this link not all the pins are used.
pins 4 and 16 are battery ground and 12v+
would think that connecting a rechargable battery pak to these pins would do the trick.
according to the link you posted the lights only draw a minimal amt of power like lights on for several hours?
 

shokjok

Member
Connect a pushbutton switch inline with the LED to verify a voltage, otherwise the LED will drain your battery/ies over time.
 
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