AC-DC Transformer Advice

Hi,

I have been trying to buy a small AC to DC transformer to power a 12V Fan off of a 240V light bulb lamp circuit. I have looked high and low and only seem to be able to get one from China (eBay).

Am I missing something? I thought they would be common items and I could get them from lots of places including Maplins etc.

Are there any good online electronic shops which might be worth me trying? Am i trying to do something that's not fairly normal?

Thanks in advance for any help,

Chris

I would have thought maplins do one.
Though you need to know wwhat amp output you need

or RS

I seem to recall maplins do one with variable DC output that includes 12 volt 1 amp, though it may be a plug in job

To be pedantic a transformer is AC to AC. To get DC out what you require is a power supply unit, which may be of a transformer based design or a switching design.

Searching Ebay for 12v dc power supply brings up more options: http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=12v+dc+power+supply&_sacat=0

Be wary of cheaper items originating in the East, some have fraudulent CE markings and are not fit for purpose

Thanks very much for your replies.

I only need a 0.16A output (one computer fan)

Thanks for the clarification, that actually helps a lot as it helps narrow down my search. I think I have found the one which will suit my requirements...

http://www.ebay.co.uk/itm/LED-Drive...146?pt=LH_DefaultDomain_3&hash=item43ba04d6d2

It looks right (to me), as long as the fact that it is designed for LED isn't a problem? As far as I understand, LED power supply are designed to give a constant voltage output to account for drop?

Thanks again for your advice, its greatly appreciated.

That device may, like most LED drivers have a minimum load that has to be taken to ensure ( possibly ) that the output is stable at 12 volts.. The CE mark looks bogus. The temperatures quoted in the write up do not match the temperatures marked on the device in the photo.

Looks dubious,

This more expensive but much more reliable.

http://uk.farnell.com/tracopower/tiw-12-112/psu-ac-to-dc-12v-12w/dp/1869150?categoryId=700000004428

You would need to check if it has a minimum load.

LED drivers come in two flavours, constant voltage or constant current.
The one you linked to appears to be a constant voltage version and likely to be ok. It is however rated to supply over 5 times the current you need and there is a chance of it not starting up with such a small load connected to it.

EDIT: Too late!

For a fan a cheap power supply is likely good enough the one shown is not regulated and I would start at 6 volt setting and increase one click at a time until correct voltage as 6V on switch could be 9V with very low load.

The PC fan is special it uses electronics to make it run it's not a simple DC motor so not 100% sure what a rough DC will do to electronics in the fan. But can't really see a problem.

As we get to regulated supplies there are two very different methods of regulation. One turns the excess into heat it is really an electronic variable resistor which keeps the voltage static what ever the load. The other switches power on and off rapidly and the on time (mark) and off time (space) ratio is changed to regulate the voltage. These produce less heat.

By also first changing the supply to DC and then back to high frequency AC the transformer can be much smaller and often the mark/space ratio is the supply to the transformer rather than done on the low voltage side although it monitors output. As a result the cost of the electronics is balanced by the reduced cost of the transformer.

However there are some down sides as well. They often have a minimum as well as maximum output and can't cope with sudden demand as well as the old type. Although now used to supply radio transmitters for a long time we had to use the old type of supply as they could not keep a steady voltage when the transmit button was pressed.

Also because the switch mode power supply has capacitors in it the life is limited where with the old type 20 years on they still work.

The problem is really to know what to ask for AC/DC adaptor, power supply and driver will all bring up different results. Technically a driver controls current not voltage, but for some unknown reason the lighting industry still calls a DC voltage regulated power supply a driver.

bernardgreen said:

That device may, like most LED drivers have a minimum load that has to be taken to ensure ( possibly ) that the output is stable at 12 volts.

You would need to check if it has a minimum load.

Click to expand...

DC power supplies of this type don't have minimum load problems. It is only the high frequency AC supplies used for halogen lights that have this problem.

winston1 said:

DC power supplies of this type don't have minimum load problems. It is only the high frequency AC supplies used for halogen lights that have this problem.

Click to expand...

Are a lot of them not 'high frequency', to keep the physical size down? Whatever, as bernard said, some of the cheap ones do have regulation problems at low loads - they usually do still 'work', even with minimal load, but the output voltage can sometimes rise appreciably under such circumstances.

Kind Regards, John

So many great replies, thanks very much for all of your advice. It has certainly made my understanding a lot clearer (mostly lol).

I will have another search now I kind of understand what I am looking for more clearly, otherwise the tracopower option looks good (thanks).

And there was me thinking finding the right device wouldn't be too complicated.

Thanks again for all your help and guidance, very much appreciated.

Hopefully my little project works soon!

winston1 said:

minimum load problems. It is only the high frequency AC supplies used for halogen lights that have this problem.

Click to expand...

Are you sure about that ?

Although high frequency and switch mode don't have to go together they often do. So as far as for this post goes I would say the switch mode power supply will be high frequency step down.

He says PC fan but not what size likely only a few mW. Likely it will not mind having slightly under voltage but the load will be reasonably static.

So a 200 mW fan with a 250 mW power supply will run for years without problem. The old power supply I showed was sold in near every corner shop. The switch mode power supply is now so cheap they have all but gone. But the one linked to is just £5 so will not break the bank.

There is really no need to buy a 5A supply to get milliwatts and as long as the supply is not a silly size then not real problem.

You missed a bit eric

giveitabashDIY said:

I only need a 0.16A output (one computer fan)

Click to expand...

bernardgreen said:

winston1 said:

minimum load problems. It is only the high frequency AC supplies used for halogen lights that have this problem.

Click to expand...

Are you sure about that ?

Click to expand...

Yes.

Winston. There are linear power supplies that require a minimum load to operate correctly.

Without that minimum load some types will shut down while other types become unstable.

To be fair to you these are not common and with your limited experience it is quite likely you have yet to come across one that is powered but without it's minimum load.

There are power supplies designed to shut down if the load current drops below a pre-set level as this would suggest a fault in the equipment being supplied and that requires the equipment to be shut down.