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Author Topic: Voltage reading on a disconnected wire ?  (Read 631 times)
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I know this sounds really strange, but here's the setup :

12V supply with a 3A diode on the positive line, used for other components and then to an 8V voltage reg for the Mega board.

The 8V voltage reg is currently off ( physical switch ), so can't (?) have any effect on this problem.

Common Gnd to 'com' pin of relay 1 on a driveway gate motor ( about 30 meters away ).
Same relay, pin 'N/O' returns to the board ( but is not currently plugged onto the board - just loose ).

These wires are in a cable containing other wires that do carry 12V to other items.

Application is that when the drive gate is opened, the relay triggers and feeds Gnd back to the board, which will then activate an optocoupler and feed a 'clean' signal to the Mega input pin.

But if I measure between the supply ground, and the disconnected 'N/O' pin wire, I get a reading of  0.07 V.

If I turn off the 12V supply, the reading slowly reduces to  - 0.008V.

The 12V+ supply is also connected to a number of other items, one of which is a siren ( not in the same above mentioned cable, and not even anywhere close to it ).

The 12V+ for the siren is fed from the supply.
The Ground for the siren is supplied via a relay ( Supply Gnd to 'com' and N/O to the siren ). Relay is currently off, therefore the siren is not 'on'.

If I disconnect the sirens 12V+ wire from the supply, the reading drops to  - 0.006V

Any ideas ?

« Last Edit: April 25, 2013, 12:23:22 pm by DaveO » Logged

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A disconnected wire is floating - the only thing defining its voltage is leakage through the voltmeter and
leakage in the wiring.  30m is a long cable run, plenty of leakage possible.

You probably need a 1k pull-up resistor on the N/O relay contact to provide stable output when its open?
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A disconnected wire is floating - the only thing defining its voltage is leakage through the voltmeter and
leakage in the wiring.  30m is a long cable run, plenty of leakage possible.

You probably need a 1k pull-up resistor on the N/O relay contact to provide stable output when its open?

Thanks for the reply MarkT.

Would it be preferable to have the 1K pull-up at the central board ( ie .. closer to the Arduino where the power supply is ) or at the relay ?

I do not have a positive line at the gate relay, so would have to run a new wire if that is where it would be required.

Also, would you recommend the pull-up in the 'loose' wire be connected to the supply 12V or the regulated 8V ( and I also have a regulated 5V available after that ) ?

I do also have 10K and 100K resistors - would using either of these not reduce current required when the relay does activate and the Ground signal is connected from 'com' to 'NO' ?

Regards
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Pull up to 5V at the Arduino end if you are connecting to an Arduino pin.   I'd recommend a pretty strong pull-up because its
a long cable run - especially if the cable isn't shielded.
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30 meters of "mutual inductance". The floating wire is acting just like a transformer. If you switch your multimeter to Amps and crank it down to milli/microamps, it'll read about 0.    BTW, shielded cable encourages the phenonomon as it makes it more tightly coupled. If you cranked up the frequency on the wire with a signal on it, that voltage on the disconnected wire would increase.
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