Electronics working out wattage of an AC-DC power adaptor.

Discussion in 'Modding' started by treble_x, 23 Aug 2004.

  1. treble_x

    treble_x What's a Dremel?

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    I want to get the correct wattage power invertor to go in my car but want to make sure that I get the right one based on the consumption of various pieces of equipment that I would connect.

    Here is an example:

    my camera charger is a 240Vac to 4.2V,1.5A output. Knowing the volts*amps = watts I have done 4.2v*1.5a to give 6.3W. Can you do it this way, i.e. working out the wattage based on the output of the adaptor? I would assume that the true consumption would be more than 6.3W from the input source but I'm not really sure and want some further advice.

    Thanks all.
     
    treble_x, 23 Aug 2004
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  2. Lynx

    Lynx What's a Dremel?

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    What you should do is work out a ball park figure (say 250-300W for a pretty decent PC e.t.c) then add 20-30% just to make sure
     
    Lynx, 23 Aug 2004
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  3. treble_x

    treble_x What's a Dremel?

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    Well I dont really want to power and PC related stuff off it, mainly chargers e.t.c.

    The invertors I am looking at come in 150W, 300W and 600W - now if the 150W will be able to cope with my chargers such as my description above then that would be ideal because it uses a cigarette lighter fitting rather than clips for the battery.
     
    treble_x, 23 Aug 2004
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  4. Lynx

    Lynx What's a Dremel?

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    OK then I would look at the adapter and on each there should be a VA or Wattage rating (in AC Volts x Amps =! Watts) add those up and then add a 20% safety margin
     
    Lynx, 23 Aug 2004
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  5. treble_x

    treble_x What's a Dremel?

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    That was my initial idea but when I looked at the specifications of the adaptor it didn't have a input amp/watt rating.

    I am however at work so haven't been able to look directly at the unit, only at the specs off the internet so I might have more luck when I get home. Will check later on.

    Thanks.
     
    treble_x, 23 Aug 2004
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  6. cpemma

    cpemma Ecky thump

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    Low voltage DC supplies are very inefficient, especially linear ones. Apart from transformer losses, a bridge rectifier will drop over 1.5V, a high percentage when the final ouput is 4.2V. If the adaptor is regulated there'll likely be another 2V dropped across the regulator. Doubling the rated output wouldn't be OTT.
     
    cpemma, 23 Aug 2004
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  7. Mark R

    Mark R What's a Dremel?

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    With cheap supplies, the input can be hugely more than their rated output.

    I've got a 6W output 5V supply here. I've tested it with a watt meter - at idle (no load) input is 8 W. At max load (6W output) the input is 18W.

    If you are running it off an inverter, you may need to triple the rating if the supplies are small ones.
     
    Mark R, 23 Aug 2004
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