Laptop shutting off when LCD connected via VGA

hari-bhari · 03-12-2007, 09:26 AM

I've been having a really strange problem as I work through my install. On the bench, connected to AC power, all this worked fine. However, in the car, this is what I noticed...

-My car has two power outlets (cig lighter plus a second one) and when I connect the dc-dc laptop supply (providing 15V @ 3.75A) and the lcd into each, they work just fine.

-If I leave the laptop off and connect the lcd's vga cable to the laptop, and then power on the lcd, everything also seems in order (lcd has no signal obviously, but both power supplies' LEDs are lit)

-HOWEVER, if I connect the vga cable for the lcd to the laptop, and then turn the laptop on, the fuse on the dc-dc supply blows and the laptop shuts off. The monitor seems just fine and its supply does NOT blow.

-Another interesting thing that I noticed is that if the laptop is on when I try and connect the vga cable, I see small little sparks when the metal contacts touch, so I immediately quit. The sparks do NOT seem to affect the laptop or the lcd.

My questions are:

1) My laptop power brick is rated at 15V @ 4A, max. Could it be that the .25A difference is causing this behavior?

2)I have never had any problems connecting or disconnecting the vga cable to the monitor while it was being powering on my bench (no sparks or abnormal behavior, even if laptop is on and lcd has power). Could it be that the lcd is getting too much current from the cig lighter? It's a xenarc 700tsv, and there's no voltage regulating 'egg' on the car adapter line, however all this testing has been done with the car on ACC and without being cranked.

3) I am using a 15ft extension cable with ferrites, could it have something to do with this (although I find it odd, because it worked fine on my bench).

4) This is a bit of an off topic question, but my CNX-P1290 provides 75W at 12V, and the DC-DC regulator states that its input is 8A max from 11-15V, even though its output is only 3.75A/60W max. Does this mean it will be drawing more power than I can supply (8Ax12V=96W). Is it safe to just connect it and test it?

Any help would be appreciated, as this is a very very strange issue.

I'm really perplexed by this behavior

bbalazs · 03-12-2007, 09:36 AM

Try to remove the laptop battery as a beginning.

And yes, that 250mA could be important in certain cases.

Can you measure the current by a multimeter at 10A range?

hari-bhari · 03-12-2007, 11:22 AM

This is all done with the laptop battery removed. Thats the first thing I've thought of. The weird thing is that when I've taken trips with just hte laptop plugged into the cig lighter and GPS connected (current draw 300mA), everythign works fine. Does VGA out really need that much juice??

Diesel300s · 03-12-2007, 11:32 AM

Im not positive about this however I thought that the outer metal casing of the VGA cable that houses the pins is used by the computer as a ground. If for some reason you have the positive and negative reversed on the monitor then it would be expecting a positive signal from the VGA cable but getting a negative one from the laptop and shorting out. Definitely not positive about this but it’s a thought.

hari-bhari · 03-12-2007, 11:40 AM

hmm thats an interesting take. i have no idea how +/- could be reversed though, because im using the included cig adapter that came with my xenarc and it powers on just fine and displays av1 before powering off....