Power Inverters

[aquariaguy]

A quick question, I bought a Canadian Tire Motomaster Power Inverter. It's rated at 75W. However, I am not sure how to check the amperage/wattage for my laptop/pda. On the box of the inverter, it says "amperage x 115 = wattage". So i checked my power adapter for my laptop and it says: Input 1.7A Output 3.5A, 65W

Which amperage would I use to calculate the wattage? Or if there's another way or something? If i were to use either 1.7 or 3.5, they will both be over 75W obviously so....dunno!

Thanks.

[DragonZealot]

Input 1.7A Output 3.5A, 65W

Since input voltage is 115V and input amp is 1.7A so the wattage is 195W. This is how much power it would draw from the inverter when it ouputs 65W to the laptop. However the laptop does not draw 65W all the time. It depends on what you are running on the lappy.

My experience with the 75W inverter from CanTire is that it is marginal for my T42 laptop. If the laptop is low on battery and I connect this to my the inverter I have to keep my car running otherwise the inverter would cycle on and off.

[FastFokker]

If the inverter says 75W continuous and your laptop says 65W, then the inverter should be fine.

Not all inverters are created equal though.. so try it, if it gives you trouble just return it to crap tire for refund.

Your laptop would only take the maximum wattage if the battery was dead and you were booting up the system.

[fishcurry]

Since input voltage is 115V and input amp is 1.7A so the wattage is 195W. This is how much power it would draw from the inverter when it ouputs 65W to the laptop. However the laptop does not draw 65W all the time. It depends on what you are running on the lappy.

My experience with the 75W inverter from CanTire is that it is marginal for my T42 laptop. If the laptop is low on battery and I connect this to my the inverter I have to keep my car running otherwise the inverter would cycle on and off.

Err... To nitpick on your calculations... I don't think power is calculated like that since your 115V is an AC voltage. I think you have to do the root-mean-square thing that you learned in physics class. 65W is 65W, this should be the max power rating of the adaptor, and is the same (Power in = Power out, minus some watts for inefficiencies) for the input and output.

Regardless, a 75 W inverter should be ideal for a laptop adapter.

[DragonZealot]

Err... To nitpick on your calculations... I don't think power is calculated like that since your 115V is an AC voltage. I think you have to do the root-mean-square thing that you learned in physics class. 65W is 65W, this should be the max power rating of the adaptor, and is the same (Power in = Power out, minus some watts for inefficiencies) for the input and output.

Regardless, a 75 W inverter should be ideal for a laptop adapter.

You should go back to physics class. The 115V in our household AC system is indeed 115V RMS already. When we talk AC voltage, RMS voltage is implied and by default.

65W is the max output power of the laptop power adaptor. Power feeding the adapter (from 115V side) has to be higher than 65W because it is not 100% efficiency.

115V * 1.7A = 195W max assuming power factor is 1.0. But in fact the power factor is never 1.0 so this number should be less than 195W, probably significantly less.

If you don't know what power factor is go ask an electrical engineer.

[fishcurry]

my mistake, i forgot 120 AC is an rms voltage already. you'd probably be surprised to know what program i'm taking...

[aquariaguy]

my mistake, i forgot 120 AC is an rms voltage already. you'd probably be surprised to know what program i'm taking...

I know. Electrical engineering at UT. Hopefully not at UW!! ;)



Thanks for your help guys. One more question, if I charge my laptop AND pda, will there be enough juice? IF something were to go wrong, like i'm drawing too much power through my electronics, will my car fry? Or will the inverter just protect both car and devices, like i'd rather the inverter just melt instead of my car or laptop.

[fishcurry]

ya elec at UT, you are correct. um, ya usually there is a fuse on the cigarette lighter jack. if you open your fusebox there should be a description of which fuse that is, and you can check the fuse and on it should say 10 or 15 amps which is how much it can handle. or check your manual.