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Old 01-08-2011, 08:06 AM   #1
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The ubiqutious "power brick"


(A question for UK residents; 240v,13amp; British regs.):

I've never been able to fully conceptualize the difference between voltage and current. Usually, that has not been a problem, as carefully observe the normal 5 or 13amp fuse criteria, just like everybody else.

The "power brick" (step-down transformer), however, confuses me completely. I mean, everybody knows not to plug an electric kettle, and iron, an electric heater and a whole bunch of other power-hungry appliances into one extension lead or multi-socketóbut what about all the smaller appliances out there, (mobile phones, laptops, etc) which have low power demand but which all require the ubiquitous power brick? I mean, there may be only a few volts going out, but there is still 240v going in, right? And these things generate a fair amount of heat.

My question (when, finally, I get to it) is based upon the growing horde of power-brick type appliances I have around my house. These little snake-light blighters seem to be everywhere, so much that I can hardly sit in a chair these days without having to shift one. (And can never find the right one when I need it!) Thus, I am thinking of figuring out a way to consolidate them all in one, central, "charging station"óbut I see there, an obvious problem, hence my question:

Assuming they are all on at once, how many such appliances, with power-bricks, can I safely plug into a single 13Amp fused extension lead? In other words, how do I calculate the limit? I'm thinking, I add up all the output amperages and make sure the total does not exceed (or even come near) 13 ampsóbut am I right in that?

Thanks

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Old 01-08-2011, 09:43 AM   #2
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The ubiqutious "power brick"


Quote:
I add up all the output amperages and make sure the total does not exceed (or even come near) 13 amps—but am I right in that?
Ayuh,... I assume you're calling battery chargers, Power Bricks,..??...

If so,...
Add up the amp Inputs, rather than the outputs....

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Old 01-08-2011, 11:00 AM   #3
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The ubiqutious "power brick"


Hello Bondo, and thanks for replying.

Yes. Battery chargers all have step-down transfomers, if not typically half way down the cable, then as part of the plug. I'm sure you know the type and which reduce the current down to the fewer volts (DC) and amps required by the laptop, phone, etc.

Take, as two confusing examples, the specs (on two [13amp] transformer -plugs)

"Input: 100-240VAc 56-60Hz 49W
Output: 12V DC 3.3amp 40 VA Max"

and another:

"Input 220-240 V 50-60Hz 32Ma
Output 6.5V 350Ma"

(I also find it strange how one of the above includes a wattage input and the other does not.)

Exactly what should I be adding up here?

Thanks again.
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Old 01-08-2011, 02:05 PM   #4
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The ubiqutious "power brick"


Power = Volts x Amps (not quite that simple for Alternating Current, but close enough)

13 amps @ 240 volts = 3120 watts (or 3.12 million mAmps)

Your examples:

#1 Input watts given as 49 watts


#2 Input watts = 240 volts x 32 mAmps = 7.7 watts

These 2 powers transformers are using 56.7 watts from your 3120 watt supply. You could plug 63 of your example #1's in the same circuit and not overload your circuit. I doubt you have that many..
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Old 01-08-2011, 03:08 PM   #5
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The ubiqutious "power brick"


Good info above. Just to add one piece of info for you
1mA = .001 amps
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Old 01-08-2011, 03:13 PM   #6
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So, the bottom line is, I could easily have six of these (typical) things charging off the same 13 amp multi-lead, with no worries of overload?
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Old 01-08-2011, 03:27 PM   #7
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The ubiqutious "power brick"


Certainly no problem with 6. BTW, in the USA we call those things "wall warts".

In your first post you said "I've never been able to fully conceptualize the difference between voltage and current."

Think of electricity as water from a faucet. Volts is the pressure of the water coming from the faucet, amps is the amount of water coming out per second. If the faucet is off, the pressure (volts) is still there but no water (0 amps) is coming from the faucet. Turn the faucet (switch) on, and the water (amps) flows out at a certain rater per second.

Power is the voltage times the amperage. If an appliance uses 2 amps at 240 volts, it is using 480 watts. If that appliance is used for 1 hour it uses 480 watt hours (or 0.48 KWH (kilo watt hours)).
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Old 01-08-2011, 05:36 PM   #8
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Thanks rjniles. (I've copied and pasted your neat summary to Wordpad, and will pin the printout up on my wall!)

Uh,... while I'm at it, and you are obviously so good at conceptualization, perhaps you can explain to me why the wires in a 240volt system can be (and so, are) much finer than in a 110v system. That has always seemed to me counter-intuitive. Because electrons travel at the constant speed of light, one would think more pressure would mean a larger diameter pipe, but no. Here in the UK, the 240v wires seem to contain about 1/4 the wire found in ordinary domestic wires in the US.

But why?
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Old 01-08-2011, 07:30 PM   #9
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The ubiqutious "power brick"


The wire are sized for the current they carry. Not sure what the UK version are but in North America #14 wire is good for 15 amps. It doesn't matter what voltage you are using #14 is only good for 15 amps.
However 15 amps at 120 volts is only 1800 watts. But 15 amps at 240 volts is 3600 watts. So you can get more power(heat, HP, BTUs etc) from a 240 volts 15 amp circuit than you can from a 120 volt 15 amp circuit.
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Old 01-09-2011, 03:08 AM   #10
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However 15 amps at 120 volts is only 1800 watts. But 15 amps at 240 volts is 3600 watts.
Thanks for that. I always was more imaginative than mathematical, so even though I understand the basic arithmetic involved, it still doesn't sink in, how a smaller wire can cope with more wattage.

I've got my answers. Thanks to everyone,

Phrixos.
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Old 01-09-2011, 08:22 AM   #11
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The ubiqutious "power brick"


Another water analogy:

I have a 1/2" diameter water pipe with a vale on the end. The pressure in the pipe (voltage) is 12 PSI (pounds per square inch). I open the valve and 1 gallon of water (amps) flows out per second. If I let that run for a minute, I get 60 gallons

Now I double the pressure to 24 PSI, when I open the valve 2 gallons of water per second flows. In a minute I get 120 gallons

Same size pipe, twice as much water.

or

to get the same 1 gallon per second (amps)with 24 PSI(volts), I would have to reduce the size of the pipe (wire).

In my water analogy, instead of a 1/2" diameter pipe to get 1 gallon at 12 PSI, the pipe would have to be reduced to about 0.35" to get the same 1 gallon per second. ( I went back to some high school math for that one).

With the same size pipe I get more water with higher pressure. Or the same size wire I get more watts with higher voltage.
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Old 01-09-2011, 09:36 AM   #12
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The water analogy works even better (at least for me) regarding capacitance and inductance.
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Old 01-09-2011, 09:40 AM   #13
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The water analogy works even better (at least for me) regarding capacitance and inductance.

Explain that one to me as I do not see it.
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Old 01-10-2011, 06:43 AM   #14
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The ubiqutious "power brick"


Capacitance is analogous (in my mind) to something like my pressure tank. Even if I shut off my supply, I still have pressure until the tank is discharged. Also, I picture alternating water pressure at a source and how, depending on the size of hose (or pipe) and the propogation of pressure wave through fluids would tend to be degraded over distance.

Regarding induction, I am thinking in terms of how max flow rate can lag max pressure or how a rapid change in in flow can cause a pressure spike (water hammer effect).

I am always interested in a correction and improvement to my thought process should any care to offer.

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