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Auto laptop 12V/mains switchover design | 26 comments (26 topical, editorial)

Re: Auto laptop 12V/mains switchover design (3.00 / 0) (#3)
by RP (russp located-at fidnet (dot) com) on Thu Jun 28th, 2007 at 01:02:05 PM MST
(User Info)

Most 12 volt relays will continue to "Hold" once on down to 9 volts or so. Even if it did turn off at 11.5v for instance, as soon as the load is removed, the voltage will rise and turn it on again.

I suspect you'll need a comparitor circuit with adjustable on and off points to prevent this.

Also, I think you'll only need 1 double pole single throw relay to do the switching. Use one pole to switch the positive side of the input to the DC-DC convertor. The negative can stay connected. The other pole can switch the "hot" side of the AC input to the computer charger. The 3rd relay you show at the output of the DC-DC convertor can be replaced by using 2 diodes (one from each charging source) to prevent backfeeding.

[ Parent ]

Re: Auto laptop 12V/mains switchover design (3.00 / 0) (#4)
by DamonHD (d@hd.org) on Thu Jun 28th, 2007 at 01:35:27 PM MST
(User Info) http://www.earth.org.uk/

Hi,

Even if I use Schottky diodes, I think that I'd be wasting ~1W (0.5V forward voltage times a couple of Amps) out of a total power budget of ~25W or less. I did think about diodes, but I'm trying to be very energy efficient here.

And you're probably right that I could avoid DP relays, but I get a warm feeling inside totally isolating the unwanted supply, and the DPDT relays didn't seem (more) expensive... B^>

But, yes, thanks for the observations: they're spot on!

Rgds

Damon

[ Parent ]

Re: Auto laptop 12V/mains switchover design (3.00 / 0) (#5)
by DamonHD (d@hd.org) on Thu Jun 28th, 2007 at 01:38:29 PM MST
(User Info) http://www.earth.org.uk/

BTW, I may well see the oscillation problem that you suggest: I'm just going to have to find out. I believe that my controller has a delay on LVD shutdown, and I hope they've chosen the upper LVD switch-on limit so that shedding the load doesn't immediately cancel the LVD as you suggest that it might.

Here is the fun of DIY... B^>

Rgds

Damon

[ Parent ]

Re: Auto laptop 12V/mains switchover design (3.00 / 0) (#8)
by DamonHD (d@hd.org) on Fri Jun 29th, 2007 at 04:30:06 PM MST
(User Info) http://www.earth.org.uk/

I've knocked up a simple little comparator circuit diagram with hysteresis as you suggested... Off at 12V and not back on again until 13V. I'll post the scan tomorrow.

Thanks for the prompting!

Rgds

Damon

[ Parent ]

Auto laptop 12V/mains switchover design | 26 comments (26 topical, 0 editorial)

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