No voltage but device works fine.

How many things do you know of where a voltmeter shows no voltage, but the device works fine?
I bought an auto/air laptop power supply for a friend of mine, and it was a good brand (Targus) and it's hard to imagine too high a voltage coming from a car, but since it wasn't my computer, I thought I should check the voltage before plugging it into his new laptop.
So, out in the car, I measured the voltage and got zero. Even though the light was on on the brick.
It turns out iiuc that the device is so fancy that it waits until everything is connected to decide what voltage to deliver to the laptop, and before then, there is no output voltage present.
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mm wrote:

Same as a "switching" power supply in a PC: No voltage until a load is detected.
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mm wrote:

Heard about seroies voltage regulator? No load, no voltage!
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mm wrote:

If the adaptor has a built in series regulator, no load, no vvoltage. Parallel regulator has current sink. Without load it'll show voltage.
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wrote:

Good to know. Thanks all.
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I think that device needs to see a programming resistor to tell it what voltage to output..
No resistor, no output.
It has nothing to do with series vs parallel regulators.
Mark
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snipped-for-privacy@yahoo.com wrote:

May I ask what programming resistor is?
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if it is like this one
http://www.targus.com/us/accessories_power.asp
it needs a special "tip" for each laptop. THe tip has the right physical conector and also has some device (probably just a resistor but maybe a diode) of the correct value to tell the supply what voltage to put out. It's just a programmable power supply with the programming element in the tip. They tell you which model tip to use with each laptop.
If you are not skilled in electronics which appears to be the case (no offense) then the best thing for you to do is buy the correct tip that you need for the laptop that you want to use from Targus.
Mark
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On Dec 19, 10:07am, snipped-for-privacy@yahoo.com wrote:

I doubt if its a series regulator or a parallel regulator. I would have to be a switching regulator and these just shut down under no load conditions. This is to protect the internal circuitry of the regulator which would soon let all the smoke out if operated without a load.
Jimmie
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JIMMIE wrote:

There is no switching regulator. Switching power supply is follwed by a regulator circuit. Crow bar circuit is for protection.
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On Fri, 19 Dec 2008 07:07:11 -0800 (PST), snipped-for-privacy@yahoo.com wrote:

Yes, 3 pins. It's a Targus, like the ones in the url you posted in reply to Tony.
BTW, it turns out not all Targus tips will fit all Targus adapters. They have indents and things to keep that from happening. I bought this adapter for a friend and then his old Macintosh broke, and I thought I could use the adapter for myself, with a tip I had, maybe from an AC-only adapter. But the tip wouldn't fit in the other adapter. I forget the details, but my friend bought a later model Mac and the second tip fit it, so I gave it to him and everything is fine.**
If you buy one of these things in the store, and it's opened all ready, check for the tip you need before you leave the store. I had bought one early on that was missing one tip.
**Then I fixed his old Macintosh. He got an image of a floppy on the screen which meant the system was't on the harddrive or the harddrive had failed. I took it apart according to the directions in the manual, and I couldn't get the harddrive out to look at it, but while trying, I disconnected it a couple times, and when I reassembled the thing, it worked. It was all screwed together when this started. I don't know how the connection went bad.

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I know about this now. Wish I knew about 8 months ago when I installed a fancy pendant light. There was a short but I didn't know that and tested the power supply to find no voltage so I assumed the PS was bad until I learned a bit more. Until then, I never heard of such a thing and had to admit I was wrong. How embarrassing.

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