Wallwart to replace car battery?

[Lobster]

One of my kids came home from Xmas shopping with a very tasteful(??) illuminated Xmas decoration for her bedroom, but when she got in she discovered that unfortunately(??) it had a cigar lighter plug and was intended for use in a car.

This "thing" includes ten 2.5V 0.5W bulbs, and the blurb says it wants a

12V/5W power supply (and must be used only with a car battery etc etc). I tested it with a spare wallwart from my Useful Box, rated at 9V / 200mA / 1.8VA and it illuminates the bulbs fine. But could someone just confirm it's OK to do this long-term, ie that I can't overheat the PSU or something daft?

Thanks David

[Andy Dingley]

Sounds like you're under-running the bulbs. I'm guessing that they're wired as two paralel strings of 5 bulbs in series, thus expecting a design voltage of 12.5V. Running this on 9V will make them glow, but more orange and at a fair bit less power. If your PSU was providing 12V then it would probably be significantly under-powered at that current rating. However at 9V if it's working OK so far, then I'd be happy to use it - the total power demand drops off a lot because you're under-running them.

Don't wrap the PSU in anything flammable and check that it isn't running too hot (by touching it). Apart from that, I'd happily use it.

[fred]

In article , Lobster writes

Although the lights will use less power on 9W, the supply you're using is waaay underrated for the job having only 40% of the required capacity and will almost certainly overheat. Get digging in the bin again & find one rated

9V 400mA or 12V 500mA :-).

- fred Plusnet - I hope you like vanilla

[Dave]

Watts are the same as VA for a load like this - your lights would need a

5W supply at 12V (probably 3-4 W at 9V) but yours is rated at less than 2W. These wallwarts usually have a fuseable link built-in to the transformer, I wouldn't be too surprised if it gave up after a while.

Dave

[Dave Plowman (News)]

A 12v 5 watt supply suggests at basics that the load is 29 ohms.

9 volts into 29 ohms will draw 0.31 amps, or 310 mA.

All approx, of course, but you're sailing very close to the wind.

A 12 volt 500mA wall wart shouldn't cost much. If they are incandescent lamps you could probably use an AC one.

[Christian McArdle]

It will probably burn out the transformer. If it was a resistive load, it would draw 2.8W (or 2.8VA) which is considerably larger than the rating. However, the bulbs are probably not ohms-law compliant and the situation could actually be worse than this.

Christian.

[Suz]

Well according to that lot your either fine or about to be toasted. I can't decide which. Lectrics are confuddling. Suzanne

[Bob Eager]

You can get 12V PSUs (try a caravan shop) with cigar lighter sockets fitted! Useful for priming those coolboxes, too.

[Lobster]

OK... I also have an old 14V / 500mA wallwart (that's the nearest alternative):

So, 14V into 29 ohms = 483mA! Sounds like that would that run OK without setting fire to my daughter, then?

Not a chance! This monstrosity she's bought only cost her a quid (it uses Xmas tree bulbs AFAICS) and I'm not going out to buy one specially... no further than a rake through the garage!

Thanks David

[Dave Plowman (News)]

Should be OK - if they really are designed for a car this is about the 'on charge' voltage.

[Grunff]

Yes, but you will shorten the bulb life considerably. They'll burn bright though!

Thinking about it, it's likely that this decoration will be thrown out after xmas, right? Go for it.

I must have half a dozen 12V 500mA and anther half dozen 12V 1A psus in my drawer. Unfortunately the time and hassle of packaging and posting is just a little too high :-(

[Ian Stirling]

2.5/0.5W = 200mA. So, 400ma for 2 strings.

In other words, yes, you are probably running the PSU at somewhere over double its ratings. Probably the cheapest PSU would be an old PC one...

[Dave]

It would have to be a very old one then. Most computer PSUs will not output 12 Volts until the mobo gives the thumbs up on the 5 Volt line.

Dave

[Lobster]

Hey, I'd forgotten all about that as an option!! I've got an ancient one here which says: max output 145W; then DC Voltage /Max current = +12V / 4.2A

Sounds like just the job, yes?!!

David

[fred]

By the time you've finished faffing around sorting out a dummy load for the

5V to make it start up properly you'd be as well using the 14V number and live with the fact that the tree bulbs will be a bit short lived. Sounds like you wouldn't shed a tear if it popped before the new year & it would be a useful education for your daughter in the what-do-you-expect-for-a-quid rules of life ;-)

The plugin would be safer too, no open slots.

[Ian Stirling]

Or until you jumper the correct 2 wires on the PSU. I forget which two - they are next to each other on the plug though.

And IME, most PSUs work just fine with no load on 5V. YPSUMV.

[Rob Morley]

As long as it's AT rather than ATX it should work fine. Otherwise some fiddling will be required.