confused about transformers

[listerofsmeg]

Hi all,

I am setting up a CCTV system, which will have about 8 cameras. Each of these cameras requires a 12V 200mA supply.

After a bit of research, it appears I can have one transformer (say

12V 2A), and take LV power lines off for the cameras in parrallel. Does this seem right?

Ok, my problem appears to be finding a transformer! Firstly, they seem to have different power ratings. Some are in amps, some are in Watts, and some are in "VA"s whatever they are. Anyone know how to work out one from the other.

Secondly, everywhere I look seems to have these strange light, white thin transformers that are primarily used to low voltage lighting. Can I use one of these to power my cameras? After googling a bit there are reports that these throw off a lot of RF interference which is no good to me. Also I will have some pretty long cable runs (20 meters), and I have read that some have restrictions on cable length.

What I was really after was a good old "black lump" transformer that plugs into a mains socket, but I can't find any of these online at all! Are they called something different?

Thanks, Lister.

[Andrew Gabriel]

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.

[Dave Plowman (News)]

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

[Christian McArdle]

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

24W continuous. It is best to overspecify a bit with regulated supplies, to keep the temperature down, especially with a continuous duty cycle. Be careful, some power supplies are rated at a reduced duty cycle, such as 50%. The continuous rating will typically be a half or a third of the 50% rating.

Christian.

[John Rumm]

Try CPC order code PW0074966

@ 12V DC for about 20 quid by the time you have delivery and vat on it.

[Sparks]

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

part number SR0407066 should do you!

Sparks...

[Mike Harrison]

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.

[Rob Morley]

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.

[The Natural Philosopher]

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

[listerofsmeg]

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

Excuse my ignorance but what are ground loop issues?

[Ian Stirling]

issues - SMPSs are

enough to cause

loop issues -

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

5V. (this generates heat in the camera, most are much cooler run at 8V.) As long as the supply voltage is over 7V, then it's fine.

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.

[jim.]

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.

[Dave Plowman (News)]

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

[Stefek Zaba]

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

1A-or-so dummy load on the 5V rail to fire up...

Stefek

[Huge]

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.)

[Stefek Zaba]

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 ;-)

[John Rumm]

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

[Ian Stirling]

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

[John Rumm]

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