This is why you should take things apart.

[Pentium100]

I bought a DC-DC adapter so I can charge my Viliv N5 in my car (while the battery lasts quite long, I may need to use the GPS function when the battery is low). The UMPC needs 5V and 3A, so I just bought an adapter instead of making my own (I am terrible at making cases and a switching power supply would probably cost more in components, while a linear power supply would need a big heatsink). The adapter has selectable voltage and a plug suitable for my camera, so I may be able to also use it with the camera and a SLA battery).

In any case, I decided to take it apart to see how it was made. There is a SO8 chip with "SW" or "MS" written on it, depending on how you rotate it. And I saw one thing that I now have to remember when I'm using this adapter. It connects the positive wire straight to the +12V, while it regulates the voltage on the negative wire to make the difference be equal to the number on the selector. So, this means that if I connected my UMPC to this adapter and my tape deck (which I modded to add a line input in addition to the radio and tape) something would blow up, since "ground" for my Viliv would be 7V above "ground" for my tape deck. I would have to use transformers now.

My other, 5V 2A (USB) adapter does this properly, so I can charge my MD player while listening to the music over the car speakers.

[IanB]

There's no "properly" here to compare with.

There is nothing to say that connecting the 12 V negative wire straight through is any better than connecting the 12 V positive wire straight through. You are supposed to consider anything powered by such an adapter as floating and isolated. If you try to create any external ground reference on the device being powered you are asking for trouble.

[cybergibbons]

Maybe it's meant for a positive ground vehicle? It's not unheard of on older cars, and still is done on some bigger vehicles and planes.

[Pentium100]

The problem is that it is not isolated or floating. If I connect two devices to two AC adapters, they would be floating with respect to each other, so I could connect them together with no problems. The same is for two devices each powered by its own set of batteries. Here the PC is 7V above car-ground, not floating or isolated at all.

As for cars with positive ground - my car is 30 years old and has negative ground. Positive ground was probably used much earlier than 1982.

[NiHaoMike]

Many data centers still use 48V positive ground. It was a holdover from the telecom standards that used positive ground to avoid corrosion of the wiring. (Some newer data centers use 370V or so.)

[wkb]

Many data centers still use 48V positive ground. It was a holdover from the telecom standards that used positive ground to avoid corrosion of the wiring. (Some newer data centers use 370V or so.)

Hm?  48V DC was supplied by lead-acid batteries in the old days of POTS.

[rr100]

NEVER assume these complicated devices have common ground, especially when you connect them for audio-out. Sometimes the phone/PDA/UMPC will have a current sensing resistor on "-", sometimes the audio amplifier won't have the ground common with the "charging" ground. Even if your charger and your device are "well behaved" you still have thin wires and lousy connectors with 1-2-3 amps going through them. This will make your ground float considerably on the portable device, more than enough to disturb your audio (charging is usually pulse-modulated). So, use a transformer for audio input.

[Pentium100]

Well, until now, all the power supplies/devices I had were either isolated or common ground and, for example, connecting a laptop with its grounded power supply to a grounded PC did not produce any sparks. Sure I have some devices with positive ground but they are old enough to be powered by an isolated linear power supply (or are just battery powered). Not having common ground is an invitation for disaster.
Now, if the ground is floating because of the wire resistance etc, the only problem would be noise in the audio - not something blowing up.

There are some car tape decks with factory line-in (as opposed to my mod) so I don't think it is that unlikely that someone wants to plug in a MP3 player or some other audio device to the tape deck while at the same time plugging it in the cigarette lighter port (so as not to waste the battery). Oh, and manual does not say anything about not connecting the device that is powered by this power supply to any other device connected to the car (for example, a camera charged by this power supply to a portable TV that can work with 12V).

Sometimes the phone/PDA/UMPC will have a current sensing resistor on "-", sometimes the audio amplifier won't have the ground common with the "charging" ground.

Wouldn't that resistor be connected in series with the battery, not the external power supply? In that case the device could measure the current going to and from the battery (while measuring current coming from the external power supply is a bit pointless if the device can be charged and used at the same time).

[PStevenson]

forget all that, the reason you should take stuff apart is because it's fun!

but lucky save, usually people learn these things the hard way when something goes "bang"
it's easy to say "never assume" but unfortunately assumption is part of the human condition and sometimes it costs us dearly