# What size inverter to charge a 48w tablet



## ian100448 (Apr 10, 2012)

Hi,

May occasionally need to charge my tablet when no hook up is available. My power supply/charger is 220v 48W. What size inverter would I need and would it be best to use the leisure battery 12v outlet or the cab 12v outlet. 

I may be stupid here but having said the power supply is 48W does that mean a 150w or 300w inverter would be adequate or do amps come into play ?


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## thegamwellsmythes (Jul 29, 2008)

Which tablet is it?

Others may already have found a solution.

We use 12V cigarette lighter chargers for all of our gadgets.

There are either from Ebay or Maplin.


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## PaulW2 (May 30, 2010)

Isn't it as simple as:

...edited for nonsense. Second try

P= VI
48W = 220V x 0.218A
48W = 12V x 4 A

So a small (50W +) inverter should do it and should draw about 4 amps?


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## Morphology (Jul 23, 2010)

Most tablets will charge off a USB connection, so any cigarette lighter to USB adapter (plus suitable cable) will do and will be more efficient than converting 12v DC to 240v AC only to convert it all the way back to low voltage DC again.

However, in answer to your question, when using an inverter to power a mains adapter for charging laptops etc., by far the biggest consideration is the quality of the 240v AC Sine Waye.

A lot of cheaper inverters generate pretty rubbish AC Waves, which are fine for, say, 240v light bulbs but are rubbish for powering various types of laptop power supply even though the power rating would indicate that they ought to be fine.

Morph.


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## ian100448 (Apr 10, 2012)

It's a Microsoft Surface pro which cannot be charged via USB. The only way to charge it is a dedicated 240v power supply which is rated at 48W.

Everything else we charge via the cigar lighter sockets with no problem so I didn't want to waste money buying an over the top inverter when this will be its only function. I already understand that it has to be a pure sine wave but it's the size of inverter that I'm unsure about


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## Morphology (Jul 23, 2010)

Then the quality of the Sine wave output from the inverter will definitely play a part.

Personally I think you would be lucky if a 150W inverter would power it. I would go for a 200W or greater pure sine wave inverter to be sure.

Pay attention to the Pure Sine Wave bit!!

I have a pretty cheap (Maplin?) 150W inverter and it struggles to power anything.


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## listerdiesel (Aug 3, 2012)

Why does it need a pure sine wave? 

Where did you get that information from?

What is written on the power supply you have, (input & output)

48W at the output of the charger/PSU would equate to something in the order of 55W input power, depending on efficiencies, so a 100W inverter would be fine.

As most small PSU's are switch-mode types these days, I'd be very surprised if that PSU needs a pure sine wave input, it would most likely be fine with a modified sine wave input.

Switch-mode supplies rectify their input and then convert it to high frequency AC in the first stage of the power supply. That is then transformed down through an isolation transformer which is ferrite-cored and very light due to the high frequency being used.

Thus they can take a wide range of voltages and frequencies as input, even DC can be fed into them without problem.

Peter


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## ian100448 (Apr 10, 2012)

Thanks for all the help here.

The power supply says..

Input 100-240v - 1.3amp
Output 12v - 3.6amp

I got the previous info from the Microsoft web site where they show 2 models and say that the power supply for the RT model is 24W and for the PRO (which I have) is 48W

Ian


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## MotorhomeWiFi (Apr 16, 2012)

Plenty of Surface Pro car chargers on eBay for about £10 delivered, not sure if this is powerful enough but one is sure to be along shortly:
http://www.ebay.co.uk/itm/111102341174


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## ian100448 (Apr 10, 2012)

Thanks, but the only ones I've found on Ebay are for the RT which draws 24W. As soon as I got the tablet from the USA I ordered one of the one's you mention and it lasted less than 5 minutes - I guess because my Pro tablet is drawing 48W which it wasn't up to.


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## listerdiesel (Aug 3, 2012)

Ian:

12V at 4A regulated is probably within reach of a simple DC-DC converter, and if the input is wider than just 12V, ie 11V-15V then you just need a lead from the m/home leisure battery.

See if there is any further data on the back of the tablet?

Edit: Surface Pro also takes a 5V supply:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

AC adapter rating: Input: 100V - 240V @ 1.3A, Output: 12V @ 3.6A + 5V @ 1A ==> 48.2 W total power draw

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Peter


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## Morphology (Jul 23, 2010)

listerdiesel said:


> Why does it need a pure sine wave?
> Where did you get that information from?


I've tried quite a few different laptops (I work in IT, so have access to lots of different laptops) on my Modified Sine Wave converter and, although the inverter would appear to be completely adequate to power each Laptop PSU, it has often failed to do so.

A poke around Google would seem to indicate that the problem is to do with my inverter being a Modified Sine Wave rather than a Pure Sine Wave.

Here are a selection of comments:

http://www.dslreports.com/forum/r25454998-True-sine-wave-vs-modified-sine-wave-inverters
http://www.waterwaysworld.com/questionspost.cgi?post=757
http://www.powerinverters.org/pages/Inverter-FAQs.html
http://www.techiehq.net/computing/laptop-inverter-31218.html

I do understand how Switched Mode PSUs work and I do agree that the shape of the incoming waveform should make no difference, but there does seem to be a concensus amongst the laptop-toting fraternity that if you want reliability, go for a pure sine wave inverter.

Just because I have found this to be the case with a variety of laptops, it doesn't prove that it would also be the case with the Surface Pro's PSU (I haven't tried one), but my advice was (and still is) to opt for a Pure Sine Wave inverter.

Obviously the OP is at liberty to do whatever he chooses... :wink:

Edit: Totally Agree: DC-DC is the way to go if at all possible.


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## ian100448 (Apr 10, 2012)

Thank you, Peter.

I think I would prefer to go the inverter route because it's simpler (for me !). Given that the max power draw by your calculation is 48.2W what size of inerter should I opt for ?

Ian


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## Stanner (Aug 17, 2006)

As morph says if you go the inverter route do not expect it to work - I have tried 2 cheap inverters for charging several different laptops and never really had much sucess the alarm usually goes off and nothing much happens.

In theory they should work, but in practice they often don't - take your Surface Pro with you, park outside Maplins or whatever and ask if they will let you see if it will work OK.


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## listerdiesel (Aug 3, 2012)

ian100448 said:


> Thank you, Peter.
> 
> I think I would prefer to go the inverter route because it's simpler (for me !). Given that the max power draw by your calculation is 48.2W what size of inerter should I opt for ?
> 
> Ian


As per my first post, 100W, but you'll probably find that the volume market is for 150W so that may well be cheaper.

Peter


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## lgbzone (Oct 8, 2008)

ian100448 said:


> Hi,
> 
> May occasionally need to charge my tablet when no hook up is available. My power supply/charger is 220v 48W. What size inverter would I need and would it be best to use the leisure battery 12v outlet or the cab 12v outlet.
> 
> I may be stupid here but having said the power supply is 48W does that mean a 150w or 300w inverter would be adequate or do amps come into play ?


Hi

Are you going to the northern show rally in Knutsford?

If so i have a 200w modified wave inverter in great condition that has charged/powered several different laptops, you're welcome to try it, if it works you can keep it (saves me throwing it in the bin).

Lee


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## ian100448 (Apr 10, 2012)

Hi Lee,

Yes, we are and are in the MHFs group so that would be great. We're arriving Thursday afternoon - Rollerteam, Autoroller WA12HPC can be recognised by the ugly Maxview dome on top.

Regards
Ian


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## lgbzone (Oct 8, 2008)

Hi Ian

Nice one, we won't be arriving until mid/late Friday afternoon, also in MHF group, I'll look out for your van, if you see us first come and say hello KC02 KWK.

cheers
Lee


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## VanFlair (Nov 21, 2009)

Ian

Looks like a result, if it does not work on Lee's modified sine wave you can try our pure wave BUT I am not giving it away sorry. We are on MHF at Knutsford also will have MHF sign and username in windscreen.

Martin


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## ian100448 (Apr 10, 2012)

Martin

See you there and thanks for the response

Ian


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## grizzlyj (Oct 14, 2008)

listerdiesel said:


> Output: 12V @ 3.6A + 5V @ 1A ==> 48.2 W
> 
> Peter


Peter or anyone who knows  what does this bit mean? Why the + ?


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## ian100448 (Apr 10, 2012)

Jason,

The power supply has a USB outlet which is the 5V x 1A

Ian


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