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Small Tablet DC converter horribly inefficent?

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vonsteimel

vonsteimel

Mechanical
Joined
Oct 19, 2010
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132
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Greetings,
I'm a mechanical guy so I wanted to make sure I'm understanding this right.

We are going to start using tablets as scanning stations to record job costing, inventory.etc (why tablets? I don't know, not my decision....)

Anyway, there will be quite a number of them which will never go to sleep. So I was having a look at the power consumption and was a little taken back by the inefficiency of what I found.

The DC converter/charger reads:
INPUT: 100-240V 0.3A
OUTPUT:5.35V 2A

So if the wall is 110V, then 110V X 0.3A = 33W "sucked from the wall" (the number used to figure the cost of powering the tablet)
When only 5.35V X 2A = 10.7W actually used by the tablet...? That's a 68% loss !?

Is this correct? I know the converter puts off heat but I didn't figure it was this much.

Thanks,

VS
 
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They are likely listing the peak line current. That converter is likely 80% efficient.
 
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There may also be a Power Factor effect too, where the peak current is not exactly in-phase with the peak voltage. Basically, the power is not quite as high as might be suggested by the current x voltage. Usually these are subtle adjustments, but may be higher for low power gadgets.

Probably several such explanations all combined, including generous rounding up on the "0.3A".

 
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I was under the impression that a DC converter like the charger I'm concerned with, are constantly drawing power even when the tablet is not plugged in. Is this just an old wives tale?

Even if they are peak values, the ratio is still over 3:1. (3 watts consumed by the converter for every 1 watt consumed by the tablet). I assume the consumption ratio is linear?

So even if it is consuming only half of the peak value, it is still 3:1 ?

BTW, this is an el-cheapo model.
 
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If this is in support of a major project, then perhaps you should buy one of those $25 "kill-a-watt" or equivalent AC power monitoring gadgets to display the actual current draw, and power consumption.

Other advice: at low power levels, you should abandon the ratios and percentages, and just focus on the watts. By way of example, if something was using a nanowatt, would you worry about it if it was only 1% efficient?

 
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Why all these theoretical questions when just measuring the device would answer them once and for all.

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
 
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A little off topic, but I have used these little wall warts on solar projects to power a small micro. That was at just 50V, half the rated input voltage. Of course they could only supply a tenth of the current. That was more than enough for my application. It wqs a handy way to get isolated power. If I remember, these only draw about a half watt with no load. A limiting factor for low input voltage is that some of the FETs in these chips have a 35 ohm on resistance and input resistors for EMI can be 10-40 ohms too. There is an interesting trick you can use with the TNY series chips, add a small pullup trsistor to the bypass cap and these will operate in the 12V region.
 
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There is a lot of variability on these 'wall wart' chargers. Once a model goes into production in China there will be many design tweaks that can affect efficiency, lifetime, and emissions. They figure if they can build and ship a million or so and save a few pennies each, who cares as they probably won't get caught? If you think I'm kidding, consider this - a colleague's retired father was hired (I believe he still is) by a major American store to disassemble the power supply for every new product they plan to offer; if the power supply has shoddy workmanship/design at that stage of the life cycle they won't sell the product.

Z
 
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" if the power supply has shoddy workmanship/design at that stage of the life cycle they won't sell the product. "

So, if it has shoddy workmanship later in life, they'll keep selling it? ;-)

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
 
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