confused about transformers

Yes, as long as the cable runs aren't too long with too thin cable.

You'll be wanting a DC supply, so ignore any with the output rated in VA.

No, they output high frequency AC.

Try Maplin, CPC. Cash Converters had a large bin of them in the shop for a pound each, and I've often been able to find the rating I want by rummaging through it.

Note the word 'supply'. But you've missed out DC.

No, you'll need a power supply. The transformer is only part of this.

Volts x Amps gives you Watts. And VA can be treated as the same as watts for this sort of use.

They're for low voltage lighting. Which is AC. Pretty well all electronic devices require DC.

Maplin etc do them in all flavours. However, 8 cameras at 200 mA each will take a total of 1.6 amps, which is rather too much for a wall wart - due to size restrictions.

They do a free standing 12 volt 2 amp SMPS type for 20 quid - N95AT

VA is similar to watts, provided the power factor is corrected. If you fail to correct the power factor, then the useful wattage goes down. All of this is irrelevent to you, though.

What you need is a 12V *regulated* *DC* supply capable of at least 2A, or

Christian.

Try CPC order code PW0074966

In my CCTV System I am using alarm auxiliary power supplies. The usually come in a white metal case with a hinged door They also have the added benefit of having provision for a 12v battery, in case of a power failure!

Obtainable from anywhere that sells alarm systems (Not DIY ones though!)

...They don't usually come with a battery!

If you just want a non battery backed supply, then

Sparks...

No. These put out AC. Your cameras want DC

On a bad day, a switchmode PSU and a sensitive camera could have interference issues

- SMPSs are inherently more noisy than linear (e.g. wall-wart type) PSUs. Whether this is enough to cause visible noise in the image will only be apparrent once you have set everything up... Having a central PSU and long DC cable runs may also introduce significant ground loop issues - again using a SMPS may make this worse. Bottom line, it will work, but it _might_ cause noise in the images.

As long as the transformer output is rectified, and preferably regulated too.

Watts = Volts x Amps VA is like Watts but for an inductive rather than resistive load.

Pass.

Power supply or DC mains adapter rather than transformer, as it includes a rectifier and possibly a regulator. You'll not easily find a wall wart that supplies 12V at 2A. An alternative cheap source might be an old PC

- the switch mode PSU in all but the oldest PCs will easily provide a couple of amps at 12V, so if you can hide it away somewhere that might be an option to consider. Bear in mind that an SMPS requires a _minimum_ load in order for it to work properly, so don't just hook up a couple of cameras, connect them all before powering up. And obviously get the PSU checked by a competent technician if you're in any doubt that it's working properly, before you blow all your cameras. If the cameras are any distance from the PSU you should also probably use a fairly substantial cable to connect them to reduce any voltage drop.

Get a toroidal low voltage lighting transformer of at least 50VA.

What k> Having a central PSU and long DC cable runs may also introduce significant

Excuse my ignorance but what are ground loop issues?

issues - SMPSs are

enough to cause

loop issues -

'All' (the 6 I've investigated) cameras internally regulate the 12V to

Ok, 200mA camera. Assume a 2V drop is OK, and 0.5mm dia cable is used. That's about .1 ohms per meter, or .2 for both wires. For 200mA, and 2V, you need 10 ohms, so that's 50m. If it was 0.5mm^2 instead, call it 100m. I suspect it'll still work out to 2-3 times this distance.

Ok the rest of the thread has it covered, even down to the power factor difference between AC Watts and VA. One thought that occured to me, is that the black 'bricks' that come with a lot of printers tend to throw out somewhere between 7 and 12V DC at the sort of rating you'd be looking for. Of course they should be smoothed enough for the job too . (I liked the CD PSU suggestion too, never fort of that!)

jim.

If you mean a wall wart that plugs in direct, I've not seen one that large. Many printers have in line types.

Yup. Or, where I am, 0.00p for any number of well-built, 12V 3A or 5A cable warts from no-longer-loved laptops. Or indeed the PSUs from old computers or externally-boxed disk drives, CDs, etc: certainly the ones from old PCs have hefty 12V rails, though you may find they need a

Stefek

Glad to see I'm not the only one who chucks unneeded wall warts in a box rather than in the skip.

(Apart from the faulty one that killed the remote control extender for my stereo. Grrrr.)

No such thing as an unneeded wall wart/bit of wood/car component/piece of carpet/unwanted dentist's chair; only a need which hasn't manifested yet ;-)

I would tend to agree, however the old 9V AC external modem ones are proving harder to reuse!

Two, and you've got a teeny (though still big enough to kill) isolation transformer.

True.... although I seem to find a need for one of those even less often than I do for a 9 VAC wall wart! ;-)