Laptop power supply voltage

[Wing Commander Fowler]

Hi chaps,

read on another forum that it was considered perfectly acceptable to run a lappy requiring a nominal voltage of 19V on a 12 V car battery - the only downside being that the laptop battery wouldn't charge......

Does this sound like a feasible proposition if battery charging was not a requirement or is there a possibility of damage to the unit in some way? My thoughts are that an excess of 7V is an awful lot if that was solely required to charge a battery and my conclusion that the system components therefore require a cuppla volts more than 12 in order to function without drawing excess amps.

Any thoughts peeps?

:ugh:

[Ausatco]

It wants 19 volts, give it 19 volts, that's what it's expecting at the power input socket. Buy a 12-19 volt adapter. In Oz they cost about AUD$50.00 and we're usually ripped off with electronics so you may well do better.

My lappy needs a 19 volt power brick, but the battery is 13.2 volts. 12 volts from a car battery is within 10% of the nominal laptop battery voltage and would probably be ok if applied to the right terminals, especially as it's on the low side, rather than the high. In theory I think you could identify the battery output terminals and apply 12 volts to the matching terminals on the lappy, but really, why bother when DC adapters are available and cheap?

AA

[Wing Commander Fowler]

Thanx Ausatco - yes am aware of the availability of these devices but the advantage of NOT using one is simply one of current drain on the donor 12V battery. It'll last for a greater duration supplying the 12V as opposed to ramping it up. I'm just not sure how the lappy will receive it tho'.

[Ausatco]

Wingco,

My guess is that applying 12v to the power socket that is expecting 19v will not work. But it may, 'cos it was just my guess.:O You could probably try it with the lappy battery removed. It's unlikely that too LOW a voltage will harm anything, the worst that can happen is it just won't work. Make sure the polarity is right, though.

If you're very careful, maybe you could get a circuit diagram or pinouts of your battery and identify the +ve and -ve terminals, then apply 12v to the appropriate connectors in the lappy's battery socket (with the battery removed.) I'd get advice on this first, trial and error is NOT the way to go.

One thing about the adapter, though. Even if it uses more juice it may provide some spike protection to your computer that a direct connection would not. Maybe not important if you're using an isolated battery, but if you're plugging into a car electrical system I think you should consider that. Car power systems are pretty spikey.

AA

[Wing Commander Fowler]

Hi again Ausatco - thanx again for your thoughts but.....

It's unlikely that too LOW a voltage will harm anything, the worst that can happen is it just won't work.

V=IxR Resistance remains the same, voltage drops, current escalates. It's generally the cause of many a component burnout in my humble experience.......

Point regarding spiking hoisted aboard though, thanx fer that!

Well, guess I'm not going to risk it now. Better to play safe methinks.

Tallyho! :ok:

[under_exposed]

If V = IxR then V/R = I, assume R is 38 then at 19 v you have 19/38 = .5 amp, drop to 12v and you have 12/38 = .316 amp

[Ausatco]

Wingco,

You have the right formula but a wrong twist on it, methinks, unless I have misinterpreted you. (Been known to happen after a claret or two.)

I"ll make up numbers for easy math, but the principle will be the same..

Say your lappy's designed to run on 15 volts, and say that when given 15 volts it draws 5 amps. Using V=IR, then R = 3 ohms. Ie, the effective resistance of your lappy is 3 ohms. This is an oversimplification, but it will do for now.

Now, that resistance is fixed. The lappy will always be 3 ohms.

If you now apply 6 volts, then using V=IR, 6=I x 3. That is I, the current with a lower voltage of 6, will be 2 amps.

A HIGHER voltage than the design 15 will push a higher current in the theoretical lappy, so don't go there.

A LOWER voltage is safe - it can only result in less current.

Cheers

AA

AA

[419]

I don't think you are looking at this the right way.

You can ignore the resistance side of it. Any electrical device is designed to work at a set power rating (watts, or Kilo-watts for example).
This figure is derived from the resultant of the supplied voltage, and the required amperage. (power = volts X Amps)
So if a laptop uses 240 watts to run, it will be supplied with 240 Volts, and will draw 1 amp. If you then drop the supply voltage to 120 V, the laptop will then have to draw 2A to run correctly. (this is why US mains power devices draw twice the current as the equivalent UK item)

Ausatco, There is a simple flaw in your logic. you stated that
"Say your lappy's designed to run on 15 volts, and say that when given 15 volts it draws 5 amps"

In this case, the total power would be 90 W.

"If you now apply 6 volts, then using V=IR, 6=I x 3. That is I, the current with a lower voltage of 6, will be 2 amps"

The total power used would now only be 12 W. ie. The laptop would now only be getting 1/6 of the power required to run it.

Most laptop power supplies are regulated, so the output current is limited at the top end.


voltage drops, current escalates. It's generally the cause of many a component burnout in my humble experience.......

That sums it up perfectly.

Hope this helps.

419

[under_exposed]

It will only draw the correct pwer if it has an autosensing power supply.

[Tonkatoy]

In my experience, the laptops I've had are not really that sensitive to voltage differences on the supply. Having said that, 7V might prove to be too much. The chance of you doing damage with an undervoltage supply is smaller than overvoltage one but you need to be careful. Laptops are expensive kit to have repaired, particularly if you managed to release the magic smoke.

If you're in the UK www.maplin.co.uk do a laptop adaptor for cars (part no. N59AC).

Having said all that, if the output of the battery (check the label) is less than 12v you might be in with a shout, if it's over 12v you'll certainly need an adaptor.

Hope this helps.

[Keef]

It ain't necessarily so that lower voltage will result in lower current. Probably would be in this case - but I wouldn't bet my laptop on it.

I have a device here that will pull about 2 amps from a 12v supply. Up the volts to 24, and it will pull just over 1 amp. Give it 240v and a red light comes on and it draws no amps at all.

If you leave the battery IN and connect 12v, it should be OK. Remove the battery, and I wouldn't be so sure.

Try Ebay for a 12v to 19v converter. I got one for my laptop for about £30.

[flyingwysiwyg]

Or you could buy an inverter and use that (might also come in handy for a few other odds and sods):-



http://cirrus.tlc-direct.co.uk/Images/Products/size_3/TLINV150A.JPG

Although a little more expensive, they do allow battery charging whilst you're using your laptop.

I haven't done any maths on this, and I may be mis-interpreting your requirments, just a suggestion anyway.

F - Wyg

[Ausatco]

I went down this route for the same reason as Wingco - I wanted to run my lappy off a car system.

I considered an inverter because, as flyingwysiwyg suggests, I thought it would be a more flexible solution, able to be used by other 240v appliances but rejected it for two reasons:

[list=1] The better inverters produce a full sine-wave output. For a given capacity they are considerably more expensive than the alternative.

The cheaper type produces a high square wave component which many AC appliances, including laptop power adapters, do not like. While the appliances may work initially, their life may be greatly reduced, particularly if there's a motor in them.

To be generally useful the inverter needs to have a reasonable capacity, say 500 watts or more. They get very expensive very quickly above about 150 watts or so, though that would be sufficient for a laptop PS. [/list=1]
If money's no object a full sine wave inverter of reasonable capacity is a good way to go. From memory I was quoted $400 odd for 500 watts.

Money was an object for me - I went for a $50 DC to DC adapter that accepts 12-16 volts DC input (allows for sloppy 12v car electrical systems) and has 9 switch-selectable output voltages at 120 watt capacity (suits most lappy DC requirements).

AA