Re: Power Inverters

spamme9 wrote on Mon, 24 Nov 2008 23:48:35 GMT:
BillW50 wrote:
spamme9 wrote:
BillW50 wrote:
spamme9 wrote:
nospam@xxxxxxxxxxxxxx wrote:
We're looking for a power inverter to recharge our Compaq notebook
in the car. Is it better to buy one with a larger rating (400W) rather than
a smaller one? Black & Decker has a 100W model ($13) and it gets
good reviews:

http://tinyurl.com/5fstl9

But the notebook uses something like 90W, so we'll be very close to
this inverter's capacity. The alternative is something larger, e.g.

http://tinyurl.com/6nfbrp

This one's 400W, but we're wondering if the extra cost is worth it.

Thanks for any info.

The 100W one isn't big enough.

Here's the ugliness of all this.

Any inverter you can afford will be a Chinese knock-off
of a knock-off of another knock-off of a maybe-good original
design.
The "designer" copied the circuit (not to be confused with the design)
and used the cheapest available
parts and the cheapest labor. They had not a clue to the original
design tradeoffs. They probably spent more money on the design of
the attractive display box than the design of the inverter.

It's tested to run a light-bulb in very infrequent use for as long
as the warranty...assuming you can find the vendor.

But your laptop is NOT a light bulb. It's a diode and a capacitor.
When the fast rise of the modified sinewave slams into the cap, you
get MUCH higher peak currents than you would with a light bulb.
Just twice the current for half the time is still twice the heat in
the output bridge of the inverter. And that assumes the transistors
can reliably take twice or 4-times or more the peak current.

When using a square wave or modified sinewave (not so square wave)
converter to drive a switching power supply, you need much more
"rated watts" than you'd think.

Typing this got me thinking...
Newer power supplies have built-in power factor correction to make
life easier for the power companies. What happens when you try to
run one of those from a square wave???

Nice theory! Although when you run at 110VAC and the power supply is
rated at 110v to 240vac, the slightly higher voltage won't matter. Plus
computers use power switching supplies and they don't care if you feed
them AC, DC, square wave or a variation of therefore.


That wasn't part of the discussion.
I'm NOT talking about the ability
of the load to run from a square wave source.
I was discussing the original topic of the thread.
I'm talking about factors that affect the ability
of the inverter SUPPLY to DRIVE a switching supply load.

I re-read my post multiple times. The ONLY mention of "voltage"
was yours...

And I am saying your statement is hogwash. And when you talk about a square wave source, whether you know it or not, you *are* talking about voltage. And computers use power switching supplies. And they don't care if the input voltage is a bit higher, square wave, sinewave, or DC.

Wash your hog in whatever you want.
I NEVER mentioned voltage in the quoted text.
Voltage is IRRELEVANT to the ORIGINAL TOPIC being discussed here.

You either don't know what you are talking about or are playing us for fools. Okay I'll call you bluff. I'm an electronic engineer. Here we go...

spamme9 wrote on Sat, 22 Nov 2008 04:09:33 GMT:
> nospam@xxxxxxxxxxxxxx wrote:
>> We're looking for a power inverter to recharge our Compaq notebook
>> in the car. Is it better to buy one with a larger rating (400W)
>> rather than
>> a smaller one? Black & Decker has a 100W model ($13) and it gets
>> good reviews:
>>
>> http://tinyurl.com/5fstl9
>>
>> But the notebook uses something like 90W, so we'll be very close to
>> this inverter's capacity. The alternative is something larger, e.g.
>>
>> http://tinyurl.com/6nfbrp
>>
>> This one's 400W, but we're wondering if the extra cost is worth it.
>>
>> Thanks for any info.
>>
>>
>>
>>
> The 100W one isn't big enough.
>
> Here's the ugliness of all this.
>
> Any inverter you can afford will be a Chinese knock-off
> of a knock-off of another knock-off of a maybe-good original
> design.
> The "designer" copied the circuit (not to be confused with the design)
> and used the cheapest available
> parts and the cheapest labor. They had not a clue to the original
> design tradeoffs. They probably spent more money on the design of
> the attractive display box than the design of the inverter.

Not so! As others have testified. Please provide evidence of your claims.

> It's tested to run a light-bulb in very infrequent use for as long
> as the warranty...assuming you can find the vendor.

Also where are the facts? Others have tested telephones, TVs, stereos, answering machines, and yes light bulbs too. So were is your evidence that they only test them with light bulbs? I bet you made that up didn't you?

> But your laptop is NOT a light bulb. It's a diode and a capacitor.
> When the fast rise of the modified sinewave slams into the cap, you
> get MUCH higher peak currents than you would with a light bulb.
> Just twice the current for half the time is still twice the heat in
> the output bridge of the inverter. And that assumes the transistors
> can reliably take twice or 4-times or more the peak current.

First of all a laptop isn't a diode and a cap. That is pure BS! It is a switching power supply with FETs, triacs, caps, resistors, coils, transistors, chips, etc. that sees the UPS, inverter, or wall outlet as its input.

When you speak of current slamming into a cap, you are talking about an applied voltage to cause a current flow. You may not understand this, but you are. It is impossible to have current flow without voltage being applied or created (i.e. magnetic field).

And no, a modified sinewave doesn't cause anymore higher current to a cap then a regular sinewave does. Caps are rated in volts anyway and not in current. Current doesn't not hurt a cap at all. It is the voltage that will hurt a cap. You can actually plug a cap straight into your outlet. And no, it won't blow (assuming you don't exceed the voltage rating of the cap). And no you won't even blow a fuse either.

And no the transistors don't handle twice or more current either. As the current remains the same in any case. Why? Because it gets switched on and off as much as it needs like you turning the water on and off in your sink. You only turn the knob enough as much as you need. The switching power supply does the very same thing with voltage.

> When using a square wave or modified sinewave (not so square wave)
> converter to drive a switching power supply, you need much more
> "rated watts" than you'd think.

That makes no sense whatsoever. Switching power supplies don't care if it is sinewave, square wave, modified sinewave or DC. And when you talk about such, what do you think you are talking about? Yes the sinewave in the AC outlet is talking about the wave of the voltage. Get it? And when you talk about the output of an inverter as square wave, modified sinewave, or whatever, you are talking about the shape of the voltage once again.

And when you are talking about watts... you are talking about current times voltage. Or voltage times current. The result is the same. Look it up!

> Typing this got me thinking...
> Newer power supplies have built-in power factor correction to make
> life easier for the power companies. What happens when you try to
> run one of those from a square wave???

Not isn't true either. It has nothing to do with making anything easier for the power company. You sure make a lot up in your stories I must say. It has to do with switching power supplies can adjust to what is fed to them automatically. All they need is enough voltage 50 to 60 times a second (anything between 110 to 240 volts usually works just fine). If you don't believe anything I say, then look it up. I dare you! ;)

--
Bill
Asus EEE PC 8GB 1GB SoDIMM Adata 16GB
Windows XP SP2 and Xandros Linux
.

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