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laptops need a supply and it is a DC voltage isn't it? so why my lab power supply can't turn my laptop on. it is tune on the desired voltage and polarity, when i press the on button, the laptop starts, leds on, tries to boot and suddenly dead with a beep!!

the power supply can provide 3A and it shows on its front Ammeter that the laptop is consuming 1.5A.

what technologies makes my laptop adapter different?

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    \$\begingroup\$ My small Fujitsu Lifebook 810 (a TINY laptop - 7" diagonal screen) has a 3.75 amp @16V supply. I suspect the laptop is trying to boot, hitting a need for more power and exceeding the supply limits. Unless the supply's ammeter has a 'max hold' mode on it, you're seeing what it's drawing NOW, not what it was drawing for that instant that it hit the wall and ran out of power. \$\endgroup\$ – Michael Kohne Jan 20 '13 at 14:16
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This depends on the type of laptop and power supply, some have additional logic signals, etc.

In all likelihood, your lab power supply is not able to provide enough current to power your laptop. Laptops use a good amount of power, if you have the original power supply it came with, you would see a current rating in mA or A. This is the total amperage the power supply can provide, and my guess is that it is at the least, 2 Amps or more. You may be able to charge the battery with your lab supply though.

There are other possibilities too, but without knowing the laptop model and the specs on your lab power supply, I couldn't say. Your lab supply may very well be able to power the laptop, however when the laptop is first turned on, your power supply may not be able to quickly adapt to the load.


Edit

the power supply is able to provide 3 Amps and the Current measurement shows that consume doesn't exceed 1.5 Amps

There are a lot of possibilities here, perhaps your lab supply isn't accurate or there were power fluctuations. Maybe your adapter plug was not the correct size, so you had a poor or intermittent connection. The laptop may have had an unrelated issue, that caused this problem, I've seen some that turn off as soon as you plug in external power. Perhaps the battery was bad, problem with the motherboard, or maybe a fan wasn't working. There are just a ton of possibilities.

the laptop is an old lenovo... "the beep and dead" seemed to be a power security act... HOW can the laptop distinguish between the its adapter

Usually a beep like that is a bios problem or warning. Also, according to this, it seems that Lenovo has different power supplies and depending on what one is used, it will throttle it's performance. A power supply for a laptop doesn't just have to provide power, sometimes there is some data communication/feed back, perhaps even just a way to verify that the power supply is a real brand name one.

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    \$\begingroup\$ Above answer is almost certainly correct on the current draw. Although anecdotal a while back I was trying to boot an embeddded PC that stated a maximum 2A @ 12V requirement with a 3A supply and it failed at a similar time during the boot process. Worked fine when I moved to a 20A supply, which showed about 1.5A used so must have just been an inrush problem about that time while the HDD and other peripherals were fired up. \$\endgroup\$ – PeterJ Jan 20 '13 at 7:33
  • \$\begingroup\$ the power supply is able to provide 3 Amps and the Current measurement shows that consume doesn't exceed 1.5 Amps, the laptop is an old lenovo. i don't know the exact model because it wasn't mine. (i tested it for one of the colleagues who had forgotten to take his adapter) "the beep and dead" seemed to be a power security act, i had a feeling that the laptop could start but it'd rather not. i really don't understand HOW can the laptop distinguish between the its adapter \$\endgroup\$ – shampoo Jan 20 '13 at 7:37
  • \$\begingroup\$ Peter: but i think the original laptops adapters doesn't provide 20 amps, does they? \$\endgroup\$ – shampoo Jan 20 '13 at 7:41
  • \$\begingroup\$ You should probably update the question with that extra info, but just saying it sounds remarkably similar to my case. How are you measuring the current? Remember that some intelligent power supplies may make the shutdown decision in a millisecond or less if they detect a dip, not the sort of thing you'll see with a standard meter. Edit - no just saying in my case I had a 20A supply on hand and that worked fine. \$\endgroup\$ – PeterJ Jan 20 '13 at 7:41
  • \$\begingroup\$ thank you peter for sharing your experience, i know that it can be a valuable start point. i am curious about the technology in the adapter, how can the laptop distinguish between the adapter and the power supply, i think that it is not a power supply shutdown because of the strange laptop beep, it seems that somehow laptop understands that it is dangerous to use another power supply \$\endgroup\$ – shampoo Jan 20 '13 at 7:56
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Many laptops have a hidden connection that digitally communicates with the power supply to make sure it's an "approved" one before charging enables. Quite likely the system sort of starts up but the communications check fails. This is often done with a 1-wire EEPROM.

If there isn't such protection with your Lenovo setup, most likely the power supply cannot handle the load imposed by the computer, or as others have suggested your laptop has suffered some sort of hardware failure.

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Some devices may have a protection circuit that checks if the supply is providing a limited current. This is particularly true for Nokia cellphones. Try setting your laboratory power supply to provide both a constant voltage and current into the laptop. You can get the required constant current limit by measuring the current when the laptop is on and charging the battery from its dedicated supply. If that cannot be done, try using the indicated output current of the dedicated supply.

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Some laptops such as Dell's have a third pin on the power supply that sends data (Dallas 1 wire) to validate that the correct supply is connec

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Does your power supply have a current limit feature? If it does then that might explain it. When the laptop tries to draw more current than is allowed the voltage is drawn down to accommodate the increased current. This also might happen if the laptop demands a large amount of current quickly - the power supply voltage will drop before the supply can react to increase the current and the laptop dies as a result.

Your current measurement is probably wrong - you need more power.

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