Is there any way to get a 12V supply out of my dell inspiron 9400 laptop? I have an external 2.4Ghz video receiver which I want to power from my laptop without using a separate battery.
Could I just take power out of the charging port?
Obviously if I wanted 5V I could use a USB port as a power source, but I need 12V.
It is highly unlikely that you would be able to draw power out of the charging port. Simplistic devices sometimes let you do that, but there's probably a charging circuit in the laptop which would be in the way.
There may be an internal supply of greater than 12 volts which you could tap into by modifying the laptop, but that's far from certain (not much today would need that voltage - the display backlight would need more, but that probably doesn't have capacity to spare).
The most realistic path would probably be to use one (or more?) USB connector(s) to feed a DC/DC converter, provided that the power your require divided by the converter efficiency is within the USB wattage spec (which thinking simplistically would be a little under 2.5W after negotiation for a normal port, maybe 50% more for some special ones intended for charging phones)
You could also examine the video receiver and see what it really requires - there may be a 3-terminal regulator on there which you could bypass (if you were very lucky it would be a 7805, ie requiring 5 volts downstream). Though RF stuff, at least if it transmits, sometimes does need the higher voltages.
The quality of your power supply - if it is contaminated with digital noise - may degrade your video signal (or even cause spurious emission outside of the intended frequency range).
What is the output of the Dell supply? What is the current requirement of the Video Receiver? Yes, if there is enough power you could possibly "Y" the Dell output barrel connector to go to the laptop, and the other leg to a regulator circuit. The regulator could contain an LM317 voltage regulator set for 12 volts.
I agree with Chris that it is almost certainlynot possible to use power from the charging port without hacking it quite a bit.
Stepping up the USB supply would probably be the way to go, it would be pretty simple using something like this DC to DC 2W 5V - 12V module.
You can get them in higher power ratings, but I picked 2W as it's near to the maximum power per USB port (according to USB 2.0 spec - it is usually possible to unofficially draw a bit more though)
If it's only a one off you don't have to worry about meeting spec though.
