DC Supply Question

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?

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

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In response to what John Miles posted in news: snipped-for-privacy@news-central.giganews.com:

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.

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

Break the positive side.

-- jm

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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.

posted

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

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.

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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. ;-)