Can I use the same ground line for 2 different DC voltage out puts? Answered
It seemed a bit pointless to me to use an inverter to get mains voltage just to use a transformer to step it back down again to charge a lap top. This in hind sight may be the cheaper option.
So I decided to build a power supply from commercially available parts this step up dc to dc device seemed to fit he bill http://www.technobotsonline.com/pololu-adjustabe-step-up-9-30v-5a-max-input-current-2.9v-to-vout-input.html
coupled with a replacement charging lead from www.life4laptop.lt.
My problem is this, the mains power supply from HP doesn't mention that the central pin is a different voltage to the inside of the plug, as the new cable had 3 wires a stuck a multi meter on the mains adapter to check the outer is indeed negative, and the inner contact 19.5v as shown on the power supply label the unmentioned central pin is at 14.1v . I don't have any worries about running 2 step up devices in parallel from the same 12v supply. I'm a little concerned about using the same ground line for 2 separate devices even though the mains unit does.
It has occurred to me that there is probably a simple way to drop the 19.5v to 14.1v for the central pin but I don't know if they run at the same current and if my understanding of electronics was up to that I probably wouldn't need to be asking what I suspect is a fairly basic question. I am pretty adept at assembling and checking out puts of things others have designed I just don't understand the design bit.