power supply problem

[doubleM]

i got a dc power supply off ebay for my gateway laptop to run it from the cig lighter in my car. its a universal with selectable voltage. my laptop takes 19 volts so i set it to 19. i used the correct dc plug. but my laptop wont come on. i checked the output voltage of the the power supply and on 19v its putting out 19.6vdc. the amp rating is 3.5 amps and the laptop pulls 3.4 amps.
i plug the laptop up with my ac adaptor and it works fine.

im thinking the voltage variance is too high and the power management in my motherboard is not allowing the laptop to be switched on for safety. i know it varies but 19.6v is kind of out of the tolerance i would think. i guess this is waht i get for buying cheap universal crap.

any ideas why this is happening?

 

[crutschow]

Did you measure the output of the AC adapter? And the polarity?

 

[doubleM]

yes i measured it with my meter.

 

[ANTlabs]

Are you sure the batery is working?

 

[trevor261]

How many pins on the DC laptop plug? Did you measure the DC input voltage when the laptop was plugged into it?

 

[Frosty_47]

19.6V is very close (3% error) and should not cause you any problems. In any case you can always put a diode in series to drop the the extra 0.6V but make sure the diode will handle the current load. Attach a dummy load of 3.4Amps to your power supply and measure the voltage across it. You can use parallel resistor network for this. If the voltage is still around 19V across the dummy load, than you have a bad connection somewhere...

Dummy load resistance = 19.6V / 3.4A = 5.7Ω

P.S. make sure to add a cooling fan to that PSU if it doesn't have one. It will get hot as you push it to it's current limit. Doing so will extend it's life dramatically.

 

[Menticol]

Is strange, because the laptop battery should do the dirty job, the car just recharges it. So even with dirty AC input, the thing should at least turn up! (laptop battery remains as the main source)

Check if the laptop battery is working!
Check the polarity of your car gadget! Check the attachment to see what I mean

Turn on your laptop using home AC, under Power management Menu select the "Economy" mode, and turn the LCD backlight brightness to the minimum
So, the load over the laptop battery will be easier, so recharging it.

 

[bill l]

i'd suggest you turn the laptop on prior to plugging in the power supply.

 

[Mikebits]

i'd suggest you turn the laptop on prior to plugging in the power supply.

Bad idea, that exposes the PC to noise and surge garbage.

 

[bill l]

ah, but i was thinking that maybe this supply he had was something where surge and noise would be filtered in the power supply, but to me, i'm going to bet that the power supply can't provide the surge that the laptop is drawing trying to turn on.

 

[Mikebits]

ah, but i was thinking that maybe this supply he had was something where surge and noise would be filtered in the power supply, but to me, i'm going to bet that the power supply can't provide the surge that the laptop is drawing trying to turn on.

An assumption that one should not make without all the details. Based on limited details, it is best to err on side of caution, this is an assumption as well. what is one to do?

 

[audioguru]

The cheap Chinese power supply lies about its output current which is not enough for your laptop.