Mains to 12 volt DC

[petercharlesfagg]

Friends, In my daily job as an a woodturner I am often deeply involved in turning commissions or pieces for the next show or gallery etc.

The general public and my family have access to the main door of the workshop and the only way to attract my attention is to knock on the door BUT the window is closer and often I leap into the air in surprise!!!

On my Christmas list was the request for a flashing Amber light and I have given one by my family.

It is the type that has a magnetic base for the roof of a vehicle and a plug which goes into the Lighter Socket.

The bulb is a 12 volt 21 Watt DC and there is a small motor inside the housing for rotating the lens.

I want to rig it up to the mains via a transformer but do not understand what output I require from the transformer to power the light.

Anyone any suggestions as to the output required and a supplier, I have been studying the search engine results but it gets more and more confusing.

Regards and a Happy New Year to all, Peter.

[Bob Minchin]

You need a combined transformer and rectifier ie a DC power supply.

The motor should not need much so something rated at 2amps/25 watt absolute minimum, 3amp 36-40watt unit would have a comfortable margin. You do not need one that states it has a regulated output as you will have a constant load. watts = volts x amps

HTH

Bob

[Gordon Henderson]

Using:

P = I * V

and knowing the power (21 watts) and the voltage (12), rearrange to:

I = P / V

which is 21 / 12, or 1.75, add a bit more for the motor and you're looking at 2 amps.

So you need a 12 volt, 2 amp supply.

And there seems to be a lack of them online - which strikes me as odd, however since it's designed to be used in a car:

the bill exactly (that's a 5 amp unit - the only other one I've found in Maplin in a 1 amp unit), if a little pricey...

Gordon

[Mike Dodd]

Most certainly a 12VDC supply, of 30W (or higher) (or 12VDC, 2.5A-or higher). You shouldn't need a regulated supply, although a lot of the higher-capacity supplies are likely to be.

This is a bit of an overkill, but might suit the lamp if it's fitted with a cigar-lighter adaptor...

keep a look out for a cheap car-battery charger - these are often sold cheap, and should provide adequate current/power.

[Bob Eager]

Well, it's a bit less than 2 amps, and most suitable things you can get will do more than that.

Go to a caravan accessories shop. Buy a box with a mains cable on one end, and a car lighter socket on the other. Lots of them around, there and in other places.

[The Natural Philosopher]

Simplest way out is to buy off ebay somethinmg designed to run a CB set off the mains.

e.g.

or 13,8v I forget which - and enough amps to do the job. At say 50W that's about 4A at 13V give or take.

[Graeme]

In message , petercharlesfagg writes

An old Hornby or similar toy train transformer/controller should be perfect, and cheap, as long as it is rated at 2 amps. Most are, except perhaps basic train set controllers.

[andrew]

What's wrong with using an old pc case and power supply?

AJH

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember petercharlesfagg saying something like:

CPC Techno-junk flyer, bottom of page 7...

[Dave]

It would have to be a very old PC case. Modern PSUs have a link back from the mobo that tells the PSU that the 5 Volt line is good, before it puts the rest of the Volts to the Mobo.

Dave

[Andy Burns]

No, you have that wrong ...

Pin16 (power_on), shorting that to GND will turn on the PSU.

Pin8 (power_good) is what I think is what you're referring to, but it is a signal from the PSU to the motherboard, rather than an input to the PSU, you could ignore it to use the PSU in a "dumb" mode.

You may need to connect Pin13 (3.3v_sense) to one of the 3.3v outputs, e.g. pin1.

The other issue, is that if you are only using the 12v supply, and ignoring the 5v, 3.3v, -12v supplies you might "unbalance" the different rails of the PSU.

[Bob Eager]

IIRC, just ground the purple wire...

[The Natural Philosopher]

Bit tricky to get working. Need a load on the 5v to work, and may not do

4A at 12v.

Some modern ones need a bit more to get going as well.

[Andy Burns]

green.

purple is the 5v standby.

[Harry Bloomfield]

Dave formulated on Thursday :

Basically, it would need a load to be presented to the 5v line, which can be done with a resistor.

[Frank Erskine]

Which of course is a waste of power!

[Harry Bloomfield]

Frank Erskine brought next idea :

Not in this case. It resister does not need to be a particularly low value and the PSU would only be powered for a short period.

[Mathew J. Newton]

Hi Peter,

As the others have said you need a 12V 2A DC power supply. This is a fairly common supply requirement for CCTV cameras so something like the following switched-mode 'wall warts' would be ideal:

They're small, have well regulated outputs and cost less than a tenner.

All the best,

Mathew

[Owain]

I'm not sure how this is going to stop people knocking on your window. Is it to alert them to the fact that you're working, or to alert you to the fact there are people outside?

Owain

[mick]

Using transformer 88-3725 (with secondaries in parallel) and rectifier

47-3218 from Rapid Electronics you should get about 12v at 3A. Souds about right for this job.

The transformer is 15+15v 50VA.