DC Supply Question

W

Will

Guest
Hi All,

I am a newbie to this group so if there is a more appropriate place to post
these types of questions, please let me know.

Essentially, I would like to put a rocker switch inline with the 12VDC power
cable that energizes a basic surveillance camera (<1A) remotely high in the
corner of a given room. With its mounting location as high as it is, I need
to put a separate switch in there to allow the camera to be powered off as
needed (without unplugging the power cube). I have purchased a 25ft
extension power cable terminated with a 2.1mm jack on one end and a 2.1mm
plug on the other end. My goal is to simply take a small enclosure and put
a 5A rocker switch (SPST) inline on the negative DC lead of the extension
power cable.

Thus my question: I presume, given that DC flows from neg to pos, putting
the SPST switch on the neg power lead is most appropriate. I can't think
of any reason why I would need to use a DPST to switch out both the neg and
pos leads.

Any concerns with this approach?

Thanks in Advance,

--Will
 
"Will" <a7b98208@telus.net> wrote in message
news:lu4te.63488$HI.13214@edtnps84...
Essentially, I would like to put a rocker switch inline with the 12VDC
power
cable that energizes a basic surveillance camera [...]
Thus my question: I presume, given that DC flows from neg to pos,
putting
the SPST switch on the neg power lead is most appropriate. I can't think
of any reason why I would need to use a DPST to switch out both the neg
and
pos leads.
Click to expand...
DC does not "flow from neg to pos" in any useful sense. A circuit is either
completed or not. If you break the circuit anywhere, current won't flow.

So, from that perspective, it does not matter where you put the switch; and
you are right that you don't need a DPST - you only need to break the
circuit in one place. (It would be slightly more common to break the
positive side than the negative, for reasons I won't get into because they
don't apply to your situation.)

Why not just plug the power cube into a power strip that has a switch on it,
though?
 
In article <lu4te.63488$HI.13214@edtnps84>, a7b98208@telus.net says...
Hi All,

...
Any concerns with this approach?

Thanks in Advance,

--Will
Click to expand...
You want to break the positive side, not the negative. Otherwise,
power-supply current may find another route through traditional
"ground" conductors, like the outer shield of your video cable, or the
camera's mounting bracket.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------
 
In response to what John Miles <jmiles@pop.removethistomailme.net> posted
in news:MPG.1d1e84e5f326f18d9896d0@news-central.giganews.com:

You want to break the positive side, not the negative. Otherwise,
power-supply current may find another route through traditional
"ground" conductors, like the outer shield of your video cable, or the
camera's mounting bracket.
Click to expand...
No it will not. Power supply outputs of this kind are floating - no
'earth' connection.

--
Joe Soap.
JUNK is stuff that you keep for 20 years,
then throw away a week before you need it.
 
In article <Xns967A5B3A068C4831041831041@130.133.1.4>, me@privacy.net
says...
In response to what John Miles <jmiles@pop.removethistomailme.net> posted
in news:MPG.1d1e84e5f326f18d9896d0@news-central.giganews.com:

You want to break the positive side, not the negative. Otherwise,
power-supply current may find another route through traditional
"ground" conductors, like the outer shield of your video cable, or the
camera's mounting bracket.

No it will not. Power supply outputs of this kind are floating - no
'earth' connection.
Click to expand...
I don't know that from the poster's description, though, and neither do
you.

Break the positive side.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------
 
Thank you all for the posts!

The camera is a 2.4GHz unit which is actually being used as a baby monitor
so no separate coax feedline. (I am trying to avoid using AC power
extension cables and power strips to make this installation less obtrusive).

The power cube is rated as 115VAC / 60Hz input and 1A @ 12VDC output with
positive tip / negative sleeve. So I guess since this cube has a step down
transformer, the output would be floating due to the magnetic coupling
effect of the transformer.

So what I am getting from the various posts, is that, it although there are
no other ways current can find another route to the camera b/c there is no
video feedline, it can't hurt to interrupt the + side nonetheless as good
practice should other factors be involved.



You know whats also interesting, when I decided to relocate the camera, I
took the existing stock power cube and spliced in additional cabling to
accommodate the added length requirement and measured the open circuit
voltage on this cube at the newly added barrel plug termination on this
extension and read 18VDC on both a portable DMM that has 10MOhm's input
resistance as well as my Fluke 8840/AF which has 10GOhm input resistance.
I then looked through my junk box and found another similarly rated power
cube and it too read 18VDC ? What's going on here ... I would expect the
regulation circuit in these cubes to present 12VDC at the output from open
circuit right through to a 1A load.



"John Miles" <jmiles@pop.removethistomailme.net> wrote in message
news:MPG.1d1f25d697df09059896d1@news-central.giganews.com...
In article <Xns967A5B3A068C4831041831041@130.133.1.4>, me@privacy.net
says...
In response to what John Miles <jmiles@pop.removethistomailme.net
posted
in news:MPG.1d1e84e5f326f18d9896d0@news-central.giganews.com:

You want to break the positive side, not the negative. Otherwise,
power-supply current may find another route through traditional
"ground" conductors, like the outer shield of your video cable, or the
camera's mounting bracket.

No it will not. Power supply outputs of this kind are floating - no
'earth' connection.



I don't know that from the poster's description, though, and neither do
you.

Break the positive side.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------
Click to expand...
 
In article <MPG.1d1f25d697df09059896d1@news-central.giganews.com>, John
Miles <jmiles@pop.removethistomailme.net> wrote:

In article <Xns967A5B3A068C4831041831041@130.133.1.4>, me@privacy.net
says...
No it will not. Power supply outputs of this kind are floating - no
'earth' connection.

I don't know that from the poster's description, though, and neither do
you.

Break the positive side.
Click to expand...
JM-

I vote for switching the AC side of the power cube as Walter suggested.
That way there is no question of alternate return paths resulting from
positive or negative grounding schemes, and the life of the power cube
would be extended.

Fred
 
"None" <none@none.net> wrote in message
news:Tvite.81033$tt5.80348@edtnps90...
You know whats also interesting, when I decided to relocate the camera, I
took the existing stock power cube and spliced in additional cabling to
accommodate the added length requirement and measured the open circuit
voltage on this cube at the newly added barrel plug termination on this
extension and read 18VDC on both a portable DMM that has 10MOhm's input
resistance as well as my Fluke 8840/AF which has 10GOhm input resistance.
I then looked through my junk box and found another similarly rated power
cube and it too read 18VDC ? What's going on here ... I would expect the
regulation circuit in these cubes to present 12VDC at the output from open
circuit right through to a 1A load.
Click to expand...
Many wall-wart supplies are not regulated. So, without a load, the voltage
rises.

You can kind of break wall-warts down into four groups:

1. AC/AC adapters. Just a transformer inside a box; no rectification or
regulation. Output is AC.

2. Unregulated AC/DC adapters. A transformer, bridge rectifier, and a
capacitor. No regulation. Output voltage rises when unloaded or lightly
loaded.

3. Linear regulated AC/DC adapters. Same as above, plus a regulator. This
kind is actually somewhat rare, because a linear regulator has to dissipate
excess energy as heat, and it's hard to safely dissipate much heat from a
wall wart. Note that some regulators (eg LM317) rely on current through the
output for their own power, so with loads that draw less than a few mA (eg a
digital voltmeter), the regulator may not be able hold regulation and
voltage will rise.

4. Switch-mode regulated AC/DC adapters. These are becoming increasing
common, and can be very small - nowadays, some are small enough that they're
built right into the plug and don't really need a "wart" at all. Still more
expensive than type 2, above. Like type 3, these may need a minimum load to
maintain regulation.
 
"Will" <a7b98208@telus.net> wrote in message
news:lu4te.63488$HI.13214@edtnps84...
Hi All,

I am a newbie to this group so if there is a more appropriate place to
post
these types of questions, please let me know.

Essentially, I would like to put a rocker switch inline with the 12VDC
power
cable that energizes a basic surveillance camera (<1A) remotely high
in the
corner of a given room. With its mounting location as high as it is,
I need
to put a separate switch in there to allow the camera to be powered
off as
needed (without unplugging the power cube). I have purchased a 25ft
extension power cable terminated with a 2.1mm jack on one end and a
2.1mm
plug on the other end. My goal is to simply take a small enclosure
and put
a 5A rocker switch (SPST) inline on the negative DC lead of the
extension
power cable.

Thus my question: I presume, given that DC flows from neg to pos,
putting
the SPST switch on the neg power lead is most appropriate. I can't
think
of any reason why I would need to use a DPST to switch out both the
neg and
pos leads.

Any concerns with this approach?
Click to expand...
It has nothing to do with the polarity, but everything to do with which
is common or ground. If you put the SPST switch in the hot lead, then
you should never have a problem. But if you put it in the common lead,
sometimes very strange things can happen. For instance, if the common
or ground lead in the wall wart was connected to the ground pin of a
3-pin power outlet. The SPST switch might not turn off the power
because the DC return path is thru the ground to the equipment, and then
thru the coax shield, back to the camera.

I'm not saying that your setup, or any setup for that matter, will have
this problem. But some not-so-savvy switch installers have tried to put
a switch in the ground lead of a car radio, with a result that it didn't
work. Gee, I wonder why. ;-)

Thanks in Advance,

--Will
Click to expand...
 
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