Hi,
The relay power switchover would not now be fast enough now to prevent the laptop from going down I think, so I'm doing more-or-less what some of you suggested a year ago, which is to essentially tie the mains-adaptor and 12V-adaptor outputs together and assume that neither will fry when its output is being held up by the other supply.
Actually, I'm not prepared to risk frying the laptop adaptor for a number of reasons, but I am prepared to replace the 12V car laptop adaptor (eg I have a spare in hand), and also I want to make sure that power is not drawn from the mains if off-grid power is available.
I measured the Voc of the two adaptors (nominally both 20V). The mains adaptor Voc is 20.85V and the car adaptor Voc is 20.47V.
So I've dropped the mains adaptor +ve output via one nice new 74p/$1.50 6A silicon rectifier with a forward drop of ~0.7V and tied that directly to the car-adaptor +ve output and on to the laptop.
Seems to work: mains adaptor reverts to ~2W 'standby' consumption when off-grid power is available, 20W--30W otherwise!
Because when NOT off-grid the laptop draws the minimum power it can, <20W, the maximum wasted power in the rectifier should be <1W. I'll have to be sure that I'm actually off-grid more than ~5% of the time year-round to be definitely ahead in energy terms, but I think it's probably that way even in the depths of December.
An alternative is to get rid of the diode and raise the converter output to 22V nominal (22.49Voc measured). If that doesn't blow anything up or waste power some other way then that might save the odd Joule. But I don't want to risk frying the laptop.
I'll have to keep my eye on it, but I think that it's an all round improvement.
Rgds
Damon
