Extending a cable with 'positive polarity'

[Lee Fowler]

I have basic knowledge from my IT background.

We were burgled so finally pulled my finger out to fit a camera...
we got one 5v 0.4a plug (1 metre)

you can prob guess where im going with this...

ok so purchased 5mtr '2 core flat flex' cable to extend it...
cut the original cable/plug in half so i could extend it...
Made the external cabling water tight etc...
Put a 3amp fuse break - safety etc...
Went into the bedroom where i put the cable through the wall...
the five year old pulled off my marker i originally tagged the cable with so i knew where to connect them back up...ooops

how to i know which way round to connect these cables again... I know i got a 50/50 but dont fancy blowing a £139.99 outdoor camera.

Now i have a plug 0.5meter cable with the original adapter/plug on the end with a positive polarity. As well as the new flex cable that is coming into the room.

I have a multimeter with a continuity tester etc etc. I just watch to know the simplest way as to remove my waterproofing on the 'outside join' would be a nightmare.

Any help would be appriciated.

 

[telectrix]

set your meter to d.c. volts. range whatever is a bit higher than the voltage on the cable. with the ends separated, connect to the dc supply and stick your meter probes on the 2 ends. red probe should be +. if it's wrong way round there will be a - sign in front of the value.strange to find 5v on cctv. it's usually 12v.

 

[xylophone]

The original flex might be "polarised" with a thin ridge down one of the conductors?

 

[Flanders]

Does the original cable for the camera have to colours on it ? If so as long as you connect the same colour cable at each end to the cable you using to extend it then you won't need to know which one is positive and which one is negative .

for example White cable from camera to brown flex
and then brown flex to white cable on plug

 

[Lee Fowler]

flanders & xy the original cable houses two black cables inside the outer layer...
i took a dc reading before i chopped it and it read 5.16v (the last one i had was 12v telextrix but that one came with a 10 metre power cable so never had this prob)

Flanders - that would have been my normal working method, but manufacturers cutting costs and using one cable = headache for me lol.

typical chinese budget cr*p

also earlier i stated it was 0.5amps which is wrong it is 2000mAmps

If i connect it back to front... will it damage it?

PS
Thank you all for your time here

 

[Flanders]

I have connected cameras up before the wrong way and not damaged them but you never no if you are going to guess make sure you have the video feed connect first that way you will know straight away if you have got it the wrong way around ,sorry I don't know any way of checking the polarity at the camera end May be best to break open you waterproof joint to double check

 

[Lucien Nunes]

You can often identify the polarity of an electronic device like a video camera that has a video or audio output connection (for connecting to the monitor) because the ground or screen of the signal output is usually internally connected to the DC negative. Using the meter on 200Ω range, put the black probe on the outer contact of the video output plug and test the two wires of the camera power cable with the red probe. One wire will normally show a low resistance of a few ohms - this is almost certainly the negative. The other will show a much higher resistance that will normally rise over a second or two - this is almost certainly the positive. If there is no clear difference or both readings are many tens or hundreds of ohms, then it is not a suitable way of checking the polarity of that particular unit. Don't connect either probe to the inner pin of the video cable - that won't tell you anything useful. I take no responsibility for any blown up cameras but this method usually works for me!

Lucien

Note if you are using an old analogue multimeter such as an AVO, use the Ωx1 range and put the probes the opposite way round i.e. red to the video cable screen

 

[telectrix]

if using an analogue avo on ohms, it doesn't matter which way round the leads go. but i like your method.

 

[Lucien Nunes]

My reasoning for the choice of probe polarity is to ensure that the test voltage applied by the meter is of the same polarity as the expected device supply when applied from DC positive to signal ground. The 1.5V O/C test voltage of an AVO is enough to put an antiparallel or parasitic diode into conduction if tested with reverse polarity whereas it is not normally enough to operate a switching converter or anything downstream of a linear regulator. Hence with the preferred probe polarity (possibly even with a DMM) the positive terminal of the device under test is likely to give higher reading that is easy to distinguish from that of the negative. On a DMM the red probe is positive on resistance and diode tests, while on most analogue meters it was negative.

Lucien

 

[telectrix]

i bow to your superior knowledge of modern electronics. never had these problems with valves. LOL.

 

[Lucien Nunes]

never had these problems with valves

True enough - if your AC/DC tabletop valve radio lit up but didn't make any sound when you first plugged it in on DC mains, you swapped the leads in the mains plug or turned over the domino connector on the back so that the rectifier was the right way round. Thank G*d there weren't any AC/DC valve security cameras. They would have been lethal!

Lucien