Author Options:

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.



For around 20 bucks you get a dedicated laptop charger for 12V car use - why go through all these troubles? ;)

making use of some of the scavenged parts box means the wife tolerates it if something of use comes her way it looks like I've made once in a while

This seems strange to me. I mean, the existence of two DC voltage supply rails (three including ground) in your laptop charger seems strange.

I would be tempted to try charging the laptop with just two of those three wires, namely +19.5 volts, and 0 volts = ground, and see if that works.

I am hopeful your laptop can be happy charging itself from just a 19.5 V DC supply, because if it can do that, then this will simplify your plan to charge your laptop using a DC-to-DC converter.

Also if you have the ability to measure the currents in the wires of your laptop charger, these measurements might tell you something.

My basic hypothesis here is the laptop is only actually drawing current from one of those supply rails, and I am guessing it is the 19.5 V rail, and the 14.1 V rail supplies zero (or close to zero) current.

That the mains charger can supply more than one style of lap top is a possibility seeing if it will charge on just 19.5v must be worth a try and if it doesn't try just the 14.1v thanks