Electrical Advice

I have a 2 amp/13.8v power supply like this one, for powering my Celestron telescope, rather than use batteries:

http://www.rothervalleyoptics.co.uk/rother-valley-optics-2amp-13-8v-mains-power-supply_d3220.html

The power supply is connected to my telescope via two cables.

One is a cable with two terminals on one end, and a female cigarette lighter connector, like the one shown on this other 5amp/12v power supply, for other types of telescope:

http://www.rothervalleyoptics.co.uk/rother-valley-optics-5a-12v-bench-regulated-psu_d4671.html

The other cable has a male cigarette lighter connecter on one end and a 2.1mm power connector on the other, which plugs into the telescope, similar to this one:

http://www.rothervalleyoptics.co.uk/detail.php?id=946

Now somehow I have damaged the plug which connects with the telescope and I want to replace it, rather than buy a new cable.

I think I can source the actual plugs on ebay, but I don't know how to ensure that the wires are connected to the correct terminals to ensure that the polarity
of the plug is correct.

Can anyone with knowledge of these things tell me what I need to test the polarity of the wires, before connecting the new plug?

Will something like this do the job:

http://www.ebay.co.uk/itm/3-IN-1-MU..._Measurement_Equipment_ET&hash=item483fc59077

Or this:

http://www.ebay.co.uk/itm/Voltage-D...862?pt=LH_DefaultDomain_3&hash=item2c54d5c176

I don't want to spend too much, otherwise I might as well buy a new cable.

Incidentally, although the blurb on the power supply I have says that a power cable is required it didn't mention that I would need two, i.e. the bit with the two terminals and the female connector, even though that is on the illustration for the 5 amp/12v power supply. As Rother Valley don't appear to supply those separately, I was told I could get one from Halfords, but as I couldn't find one, I got one for free in the end, but I don't understand why they are supplied with the 5 amp and not with the 2 amp or the equivalent 5amp/13.8 v power supply.

What I don't fully understand is why they don't just supply a cable which connects directly from the power supply to the telescope, without the need for a cigarette lighter connector at all, but that's another story.

Thanks for the advice Straw Walker,
I think I will buy the one you suggest as it may come in handy in the future.
Do you mean the lead under the black outer covering will be red?
The black outer lead on the cable connected to the female cigarette lighter connecter does have a white stripe but the other lead doesn't.
Of course, once I cut the lead I won't know what the polarity of the telescope plug is, and I can't find any instructions in the telescope manual,
so I was going to test the plug while it is still attached to the lead.

Thanks again,
It actually isn't obvious to me - I will have to read the instructions a couple of times before understanding the significance of what you say!

First of all you don't need the cigarette lighter bit unless you want to connect to something like that !

You shouldn't really need a meter (as outlined above)

You can get bits like these:

http://www.ebay.co.uk/itm/2-1mm-Jac...-/170617652360?pt=UK_CCTV&hash=item27b99ac488 which are screw connections (It's unclear if your connector is plug or socket - so here is both)

But a better solution - do a search on CCTV power leads : http://www.ebay.co.uk/sch/i.html?_n...tension&_osacat=0&_trksid=p3286.c0.m270.l1313 or similar :icon_razz

Thanks beteljuice,
I had already spotted the plugs of the type you suggest, so I haver ordered one already, to avoid the need to do any soldering.
Since I have already bought the leads with cigarette plugs, I am stuck with them for now.
I don't understand why you say I don't need the meter.
Otherwise how can I ensure that the polarity is correct on the plug?

The 'lighter plug ' is identified by RVO as ' fuse-protected extension ' and the fuse is housed within that plug . It is a wise protection to have in the hinterland between the dry house/shed and the damp/wet optical kit and drive outside. It stops a lot of jungle juice finding its way in to your 'go to' electronics.'Frying tonight' can be very expensive.
When I make up my own DC cables (and the current level fits the wire I can obtain) I like to use the 'white stripe' on black cable as my positive identifier.
I don't know that thats a convention but I picked it up when extending / replacing damaged cables with the stripe.
Its always worth having a multimeter and a basic idea of how to use it. The Babani paperbacks are good and cheap on the subject and should be at your Maplins. There used to be a good one at The Gateshead Shopping Centre. Nearly lost the car there , and the motorcaravan (just from the size of the place).
One thing worth having from Maplins is their 'Tiny Polarity Checker' http://www.maplin.co...y-checker-32161 . It's the size of a matchbox and will flag up centre + ive on three regular plug sizes for DC voltages between 3 & 15 V.
You have a great guy in your part of the world for Cannon DSLR modding and astro cable making here http://www.astronomi...o.uk/cables.htm

Thanks,
The centre pin on the cigarettle lighter connector might be positive but I don't really know which wire is connected to that pin.
The black outer wire on the lead which plugs into the telescope doesn't have a white stripe but I suppose the inner wires will be coloured and I can disassemble the cigarette lighter
connector and see which wire is attached to the center pin (presumably red) and take apart the old plug which goes into the telescope and see which terminal that is attached to.
I hadn't thought of that but I suppose it will work, but it might be better to double check with the multimeter to be on the safe side.

Again thanks.
I had missed the bit about the cable being fuse-protected, and I suppose makes sense to have that as protection, so I see now why they supply that cable, although it's a pity they don't list the other cable which attaches to the power supply.
As I have said, the black outer covering on the cable doesn't have a white stripe, but presumably the inner wires are coloured, I was probably making a problem when there wasn't one.
Thanks for the suggestion of the tiny polarity checker, but I have already ordered the multimeter, so hopefully that will do.
The astronomiser looks interesting and I will bear him in mind if I need any more cables or if I decide to get my Canon modified.

Thanks to all of your help, my telescope is now receiving power again.
At first I wasn't getting any reading from the multimeter, but it turned out that was because the test lead plugs weren't in far enough, and they really
had to be forced in. I don't know if this is normal with all multimeters or just because I am using an inexpensive one.
It turned out that there was no colour coding on the inside of the wires on the lead connecting to the telescope, but on closer
inspection, the outer black wire did have a groove along the positive side, which I assume is an indication of polarity,
but unless you are "in the know", you might assume it was just a moulding fault.
Anyway I double-checked the polarity of that and it was positive.
Thanks again for everyone's help.