This thread could also be entitled: “Fight my electrical ignorance”
I’m looking to buy a car power inverter for my laptop. I just need to know what power rating so that the damn thing will work. I thought this owuld be an easy task, but now I’m confused all to hell.
The AC adapter “brick” for the HP says
So it appears it wants to feed 65W to the notebook. So all I need is this el-cheapo 70W inverter. Great!
WAG is that the input is just the rated capacity, it never actually draws that much. It certainly doesn’t waste 140W, otherwise, it would be hotter than a light bulb.
If it’s a switched-mode power supply, then I’m guessing the 1.7 amps is the current when the input voltage is 100 VDC. And should we assume the 1.7 amps is an RMS current value? Perhaps it’s a peak current value?
Let’s assume the 1.7 amps is the peak current when the input voltage is 100 VDC. Now you might be saying, “O.K., I know how to compute the input power: it’s 100*(1.7/1.414) = 120.2 watts, right?” No. This is because the current waveform of a switched-mode power supply is usually not a nice, pretty sine wave, which mean you can’t simply divide the peak by √2. Furthermore, the phase angle between the voltage and current waveforms may not be zero degrees (unless it has PF correction circuitry built in to the power supply).
What I’m getting at is this: calculating real power isn’t easy when you have so little information. In your case, we do not know if HP is referring to peak or RMS current, and we do not know the phase angle between the voltage and current waveforms. Not knowing this information can give you wildly wrong answers when trying to calculate real power.
As Crafter_Man said, in AC circuits you can’t just multiply volts times amps and get watts. You have to multiply voltsampscosine of the phase angle between volts and amps in order to get power in watts.
Or you could just ignore the input, and figure total wattage based on the output power and the typical loss for such a device (I have here an estimate of 85% efficiency for a steel EI power tranny, but this is from an old reference).
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Yeah, but notebook car adapters = expensive. Low-wattage inverters = cheap.
I’m going to guess that the losses in the transformer are small enough that the input shouldn’t exceed the 70W rating by much if the output is 65W. As an engineer, I’m familiar in factors of safety, and put my faith in them often. That’s what fuses are for. What’s the worst that could happen?
In my admittedly limited experience, you’ll probably be just fine. Even though the AC adapter for your laptop says it outputs 65W, keep in mind that that’s when it’s called upon to feed the laptop at maximum power draw. My PowerBook will draw 45 watts at maximum, but during typical use it’ll probably only draw about 18 watts. So, as long as you’re not rendering a Pixar film with your laptop while running on the inverter, your fuses should remain unblown and happy.
Since the main questions have already been addressed, I’d just like to add a note about inverters. Most low-end inverters don’t output a true sinusoidal AC waveform, instead providing a stepped square wave, called pseudosine. This pseudosinusoidal AC has similar characteristics to a true sinewave, and most electronics will function properly with it. BUT (there’s always a “but”, isn’t there?) there are some things that don’t like the pseudosine output at all, and can even be damaged by it. If you’re using an inverter to power fairly expensive things, like a laptop computer, I recommend you look for one that outputs true sinusoidal AC.
Just my experence, I have 2 inverters, a cig lighter plug in rated at 300 watts (over 150 they tell you to cut the plug off and connect direct to the battery), and a 1000 W one. Both give the modified step wave and both I have run my laptop w/o problem. Also the 1000 W (w/ 3000 W surge start ability) inverter has run my main computer (and refriergator) as well.
A step wave is one that goes from full peak (+) voltage to full (-) peak voltage every 1/60th of a sec. A modified step wave goes from 0V to full + voltage to 0 volts to full - voltage then back to 0 again in that same time.
I’m not saying your laptop won’t fry, but just if it was me I would personally try it, YMMV.
Maybe I need further clarification from the electrical gurus out there, but how bad would a square wave really be for the laptop? After all, whatever kind of AC comes out of the inverter is just turned right back into DC on the other side of the brick. Maybe it causes ripples or spikes in the DC side, but shouldn’t most of the AC characteristics be “processed out” by the transformer? It may be cause for concern, and I’d like some insight into this behavior.
That being said, I’ve done some pretty unsavory things to all kinds of electrical and electronic devices, and have generally found them to be a lot more robust than most give them credit for. Of course, I’ve just guarantted myself a dashboard fire by saying that, but it has been my experience.
My guess is that most switched-mode power supplies wouldn’t have a problem with a yucky-looking sine wave or square wave on the input. This is because a switched mode power supply simply rectifies the input voltage, and stores the energy in high-voltage capacitors (where it is later modulated by a transistor and sent through a transformer). A linear power supply, on the other hand, may be more problematic, since the high-frequency components in the square wave might overheat the transformer.
I have a pseudosine power inverter, and in my experience laptops worked fine, so long as I wasn’t charging a battery. Once the battery fully charged, the laptop’s power brick started to overheat. Sometimes the inverter shut down, and an HP laptop shut down its charging circuitry. If I removed the battery everything was fine, so far as I could tell.