12v DC wiring question - advice appreciated

[Bert Coules]

From a local boot fair I bought a 12v air compressor, designed for use with a car's lighter socket. Unfortunately, some helpful previous owner had cut off the plug, leaving just bare wires. From eBay I bought the necessary plug, complete with leads, but I'm puzzled about connecting it the correct way round.

One of the plug leads is printed with text: "FT1 80C 300v 221WGX2C KELONG" and so on. The other is marked with intermittent grey bands.

One of the leads from the compressor has text: "TAISHAN DIANLIN" and more. The other is completely unmarked.

Is there a convention that text is always printed on either the positive or negative leads? That would be handy. Failing that, am I likely to do any damage if I just connect things up experimentally and see if the compressor works or not?

Many thanks.

[Bert Coules]

It would have made more sense if I'd said that the leads were all black, rather than being helpfully colour coded.

It was easy enough to determine that the text-printed lead from the plug is the positive, but I'm still in the dark about the compressor.

Bert

[tabbypurr]

no

no, dc motors work on opposite polarity.

NT

[Bert Coules]

Does that mean that if I get the connection wrong, the compressor's motor will run in reverse and it will suck rather than blow?

Bert

[Dave Plowman (News)]

If it's a permanent magnet type, yes. Which I'd guess it is. Things like older starter motors and windscreen wiper motors were sometimes series wound and not polarity sensitive.

As to whether it would damage anything by reversing it, not sure. Might be worth opening it up to see if there are any clues as to polarity inside.

FWIW, most (but not all) electronics components tend to have the negative marked, rather than the positive. Twin black cable often has a grey stripe to indicate negative.

[Bert Coules]

Dave,

Thanks for the reply. I just tried the compressor (text-print lead to text-print lead) and the immediate result was that the fuse in the car-socket plug blew. I didn't get as far as switching on the motor: the compressor has a built-in lamp with a separate switch, and it was that I tried.

Whether that affords a clue to the connection, I don't know. And the fuse is a non-standard size (longer than a conventional mains plug fuse) so until and unless I can get a replacement I'm going to stay in the dark.

Bert

[tabbypurr]

may mean it was connected wrong way round. I'd not expect a low cost compressor to have electronics in, but who knows.

NT

[Brian Reay]

The marked lead is generally the positive one, even if the markings are 'dashed' implying negative signs perversely. I've just had this on a lead I've salvaged to use for something in my motorhome.

I don't recall ever seeing it the other way around but of course that doesn't mean it doesn't happen.

I would expect the motor to run in reverse but I doubt if the compressor would 'suck' due to the arrangement of the ports in the chamber etc. However, it is possible is suppose.

The fuses in those plugs do vary from plug to plug. I often modify them to fit resetable fuses or fit an external inline fuse. It saves keeping a multitude of different fuse types around.

[Bert Coules]

Thanks for the new replies. Before I replace the fuse and trust to luck again, I might open up the compressor and see if there are any clues as to the polarity of the leads.

Bert

[Dave Plowman (News)]

If the switching is single pole, it's generally in the positive side.

Is the fuse a 1.25" long type? They can be a bit of a pain to find these days. You might have to go to a spares supplier for older cars. I know that Rimmer Bros stock them in a variety of ratings.

[Bert Coules]

That's useful, thanks.

Just a tad under: it's actually 1 3/16" or perhaps 3cms. That's not good news about the size being fairly scarce. Mind you, as I recall I only paid something like £2 for the compressor so it won't be a disaster if I have to write it off. Annoying, though.

Bert

[charles]

They seem to be known nowadays as 32mm.

Try

they seem to have good selection of values.

[Brian-Gaff]

That depends if its controlled by some control circuit to limit current at the start of course. Brian

[Bert Coules]

Happily I've just found them on eBay, with the size given as 30mm and specifically listed as being for car 12v sockets.

Bert

[Bert Coules]

It seems to be a fairly cheap and cheerful sort of unit, so perhaps the probability is against it. Once I have some replacement fuses I'll try again and see what happens.

In the meantime, I wonder if it would work connected up to one of those wall-plug 12v DC PSU units designed for AV gear and the like?

Bert

[Dave Plowman (News)]

Not a lot, I'd guess. A compressor will take several amps running - more at start up. What was the fuse rating?

Most wall warts are less than 1 amp.

[Bob Minchin]

That will have nothing like the current capability required

[Bert Coules]

Thanks, Bob and Dave. I realised that the idea was impractical as soon as I posted the question. The car plug fuse was 5A.

Bert

[ARW]

I'll take the opposite view. I say is the positive.

[Bert Coules]

Thanks. When I get the replacement fuses (I ordered several) I'll just have to experiment the connections and see what happens.

Bert