confused about transformers
 

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.

In article ,
(listerofsmeg) writes:
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?

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

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.

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

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?

No, they output high frequency AC.

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?

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.

--
Andrew Gabriel

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

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

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?

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

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.

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

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.

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

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?

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

--
*I love cats...they taste just like chicken.

Dave Plowman London SW
To e-mail, change noise into sound.

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.

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.

listerofsmeg wrote:

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?

Try CPC order code PW0074966

http://custom1.farnell.com/cpc/produ...ct%5Fid=263743
or
http://tinyurl.com/6qvys

2.5A @ 12V DC for about 20 quid by the time you have delivery and vat on
it.


--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"listerofsmeg" wrote in message
om...
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.

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 www.cpc.co.uk part number
SR0407066 should do you!

Sparks...

On Fri, 08 Oct 2004 13:17:43 +0100, "Dave Plowman (News)" wrote:

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

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

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?

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

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.

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

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?

No. These put out AC. Your cameras want DC

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.

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

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?

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

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.

In article ,
"listerofsmeg" says...
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?

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

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.

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

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.

Pass.

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?

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.

listerofsmeg wrote:

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?


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

Thanks,
Lister.

Rob Morley wrote in message et...
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.

What kind of cable thickness are we talking about for 20m runs then? I
bought 0.5mm twincore for the power. I suspect this will be too thin
now...

Mike Harrison wrote...
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.

Excuse my ignorance but what are ground loop issues?

Mike Harrison wrote:
On Fri, 08 Oct 2004 13:17:43 +0100, "Dave Plowman (News)" wrote:

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

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

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

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.

'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.

"listerofsmeg" wrote in message
om...
Hi all,

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

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.

In article ,
jim. wrote:
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!)

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

--
*Don't worry; it only seems kinky the first time.*

Dave Plowman London SW
To e-mail, change noise into sound.

John Rumm wrote:

2.5A @ 12V DC for about 20 quid by the time you have delivery and vat on
it.

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 wrote:

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

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 ;-)

Stefek Zaba wrote:

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!

--
Cheers,

John.

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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Stefek Zaba wrote:

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.

Ian Stirling wrote:

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! ;-)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
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