Is it volts x watts =amps ?

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or what. I could not get my Laptop to work on a 100w cable type inverter, do I need a 300w cable inverter then. Oh the back of PC says, input 20v ~ 3.5A

"The Med has got me" (no not the Medway the other Med)
 
so therefore 20 volts (input to laptop) x 3.5 amps = 70 watts so I wonder why the 100 watt inverter didni work.

"The Med has got me" (no not the Medway the other Med)
 
Therte will be some inefficiency in the inverter but presumably you were using an adapter to take the 240v fromn the inverter down to the 20v level. what is the rating of the adapter?

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Its the adaptor which comes with the Dell says : input 100-240v ~ 1.5A 50/60HZ
output 20v ~ 3.5A Maybe the invertor was naff but it was new.

Now what wattage of invertor do you think I need.

"The Med has got me" (no not the Medway the other Med)
 
For DC ONLY P=VI

Power (watts) = Volts x Amps

So your laptop draws 70W steady state.
The most your inverter can produce at 20V is 5A.
The current req by inverter for 100W from 12V supply is 9.8A (85% eff.)

If it won't work off a 100W inverter then the problem is one of the following

1, The laptop PSU has a high "inrush" current of more than 5A when you switch it on.
2, The 12V supply dips to less than 10.5V where the battery saving mode for the inverter turns it off.

Check the connections carefully and make sure the inverter is wired in as close to the battery as possable with nice big 20A wire.

KevL






--
It may be the early bird that catches the worm but its the second mouse that gets the cheese.
 
Ah ha

OK they gota state the highest current the supply will need at which will be at the lowest supply voltage so (and I'm going to ignore power factor here as it'll be a switch mode power supply with that input range):

From P = VI we get 100V X 1.5A = 150W

You need a bigger inverter.

KevL

--
It may be the early bird that catches the worm but its the second mouse that gets the cheese.
 
Not sure about that. P=VI holds true for AC as well as DC. It doesn't need a steady state to hold true. OK, the absolute values might be varying in a sine wave, but P always equals VI. The numbers like the 100w for the inverter are usually quoted as RMS, but so too are the V and I numbers. I reckon the inverter is bust, why not try another appliance in it to see what happens?
 
Sorry jfm but for AC there is a little problem of leading or lagging power factor. It is only true to say that P=VI for purely resistive loads on AC but as no such thing exists for accurate calculation you need to know the power factor. In some devices it is small but in the majority of cases it is large causing P=VI to give an error of about 30%.

Do a search on power factor if you don't believe me.

KevL

--
It may be the early bird that catches the worm but its the second mouse that gets the cheese.
 
Re: Right on Kev

I agree, the info suggest a max current of 1.5A therefore at the lowest voltage = 100V
therefore it needs 150Watt input. It will obviously take a lower current at higher voltage but still needs 150W to get it going. As the charge in the laptop builds up the wattage MAY reduce.
By all means check the inverter eg 40-60 Watt light bulb because the charger might just be smart enough to work slowly at a minimum wattage but I doubt it.

Anthony
 
Power factors eh!

I do believe you! Not picking an argument, fully aware of power factor (engineer, till i gave it up) but I guessed the PF of a laptop power adaptor to be very high - 95% so not significant.

But if, as you are suggesting, the PF of a laptop power adaptor is significant then your own statment "110Vx 1.5A = 150W" is not correct. 100V x 1.5A is 150VA, not 150W.

But splitting hairs here. I agree, he needs a bigger inverter now that we've seen the input rating of his laptop adapter. And if the PF were 95% he'd still need a 142.5W inverter. Which ahem nobody sells so he'd better get 150 :-)
 
Re: Power factors eh!

Sorry jfm, I was only answering the "P=VI holds true for AC as well as DC" bit of your reply,

I did say I was ignoring PF coz it was a switch mode PSU but didn't say why.

And I knew someone would come back with this arguement. Anyway it's been over twenty years since I needed to know any more than the basics of this and that for AC it's P=VI (cos Ø).

Glad we agree and sorry to bore everyone else to death

KevL



--
It may be the early bird that catches the worm but its the second mouse that gets the cheese.
 
The problem is the efficiency and power requirements of the laptop power supply. An invertor rated at 150W with a maximum output of 300W will probably power it ok (as has previously been stated on the forum) I would look for one about 200W minimum just to be sure.

Iain
 
hi all this hi tec theory is great stuff but before
you go out spending more spondulix you should try hard wireing your inverter direct to the battery (inline fused ) cos if you are pluging your inverter into a ciggy socket you aint getting max current and your 12 volts dc will drop and you wont get 100 watts from your inverter ..the ciggy sockets dont like 6+ amps flow they tend to get a little warm and you will get a volt drop across the ciggy plug andsocket..if you buy a 300 watt inverter and plug that into a ciggy socket it will charge your mobile fone but will still proberbly not work your pc..output needs input..im just trying to keep it simple..Bill

Bill
www.aegeansailing.co.uk
www.iggedo.com
www.macsyachting.com
 
Re: Power factors eh!

20 years for me too (almost!). This thread brought back memories (sad). Praps we should have some threads on thermodynamics and entropy. Or praps not.

Stupid thing about modern electronics is they all work on different DC voltages, slightly. If only the manufs would standardise, then one black power adaptor would power your PC, mobile phone, PDA, etc. Would save us having to lug loads of them everywhere. And it ought to be possible to run them off 12vDC, then would not need to invert 12vDC to 220AC then back down to 15vDC or whatever. Such a waste of time imho. Rant over
 
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