Wireless Networking Calculating power consumption...

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Subject Author Date
Calculating power consumption... ericm0009 09-23-08
Posted by on September 23, 2008, 12:52 pm
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Hi,

I am thinking about extending my wireless coverage by adding a WAP
(and a wireless-ethernet bridge) that will be powered by batteries.

The wireless-ethernet bridge will be the client feeding the WAP, so
the WAP can be run as a complete WAP and not a repeater.

The power specs for the bridge and WAP are the same:

INPUT: 100 - 120VAC (0.5A)
OUTPUT: 5VDC (2.5A)

Stupid question: How do you determine how many watts are actually
used? Is this simply by multiplying the voltage with the amperage of
the output specs?

Am I adding this up correctly for a 12V battery?

5V x 2.5A = 12.5W

12W x 2 (devices) = 25W

25W / 12V = ~ 2.10 AaH (Amps per hour)

So, say, a 12V 100A (deep cycle) battery should last ~47 hours if
supplying 5VDC (2.5A) continuously? (Well, somewhat less, because of
inefficiency.)

Then, later on, I could double that amount of time by adding a second
battery and connecting them in parallel.

Am I adding up correctly here? I would, obviously, be using a DC-DC
converters in place of the power adapters, not a DC-AC inverter with
the power adapters.

Thanks!








Posted by DanS on September 23, 2008, 2:52 pm
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34g2000hsh.googlegroups.com:

> Hi,
>
> I am thinking about extending my wireless coverage by adding a WAP
> (and a wireless-ethernet bridge) that will be powered by batteries.
>
> The wireless-ethernet bridge will be the client feeding the WAP, so
> the WAP can be run as a complete WAP and not a repeater.
>
> The power specs for the bridge and WAP are the same:
>
> INPUT: 100 - 120VAC (0.5A)
> OUTPUT: 5VDC (2.5A)

These specs are the for the power adapter, not what the unit actually
uses. The only way to know exactly how much is used, is to use a meter
and measure it....and that will then only be an average power
consumption. The unit uses far less in TX mode than while in RX mode.

And yes...power (W) = voltage (V) * current (W)



>
> Stupid question: How do you determine how many watts are actually
> used? Is this simply by multiplying the voltage with the amperage of
> the output specs?
>
> Am I adding this up correctly for a 12V battery?
>
> 5V x 2.5A = 12.5W
>
> 12W x 2 (devices) = 25W
>
> 25W / 12V = ~ 2.10 AaH (Amps per hour)
>
> So, say, a 12V 100A (deep cycle) battery should last ~47 hours if
> supplying 5VDC (2.5A) continuously? (Well, somewhat less, because of
> inefficiency.)
>
> Then, later on, I could double that amount of time by adding a second
> battery and connecting them in parallel.
>
> Am I adding up correctly here? I would, obviously, be using a DC-DC
> converters in place of the power adapters, not a DC-AC inverter with
> the power adapters.
>
> Thanks!
>
>
>
>
>
>
>
>


Posted by danny burstein on September 23, 2008, 3:04 pm
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>These specs are the for the power adapter, not what the unit actually
>uses. The only way to know exactly how much is used, is to use a meter
>and measure it....and that will then only be an average power
>consumption. The unit uses far less in TX mode than while in RX mode.

Wouldn't it be the other way around?

(And does a typical access point "power down" when
just sitting there as opposed to actively transferring
big data streams? I really don't know...)


--
_____________________________________________________
Knowledge may be power, but communications is the key
                 dannyb@panix.com
[to foil spammers, my address has been double rot-13 encoded]

Posted by DanS on September 23, 2008, 5:25 pm
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>
>>These specs are the for the power adapter, not what the unit actually
>>uses. The only way to know exactly how much is used, is to use a meter
>>and measure it....and that will then only be an average power
>>consumption. The unit uses far less in TX mode than while in RX mode.
>
> Wouldn't it be the other way around?

Uh...yeah....it was a long.....long day. Sorry. Boy did I blow that one.
Trying to program rtr's (real rtr's, not consumer gear AP's) that you're
not familiar with really strains the brain.

That is what I meant...TX is much more DC power than RX....DOH !!!!!

You can never go by what the PS says. The PS should always be over-rated
for the device....by 50% of the actual current draw, IMO. We've got a
900mhz DSSS radio that comes with a 1.5A 12v PS and is rated @ 700mA
(which is ~100% over-rated) @ 12V for a full 1W output.

Well I measured it, and in diag keyed mode, as in constant TX, it only
uses ~ 400ma @ 12v.

Posted by on September 23, 2008, 5:47 pm
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wrote:
> 34g2000hsh.googlegroups.com:
>
> > Hi,
>
> > I am thinking about extending my wireless coverage by adding a WAP
> > (and a wireless-ethernet bridge) that will be powered by batteries.
>
> > The wireless-ethernet bridge will be the client feeding the WAP, so
> > the WAP can be run as a complete WAP and not a repeater.
>
> > The power specs for the bridge and WAP are the same:
>
> > INPUT: 100 - 120VAC =A0(0.5A)
> > OUTPUT: 5VDC (2.5A)
>
> These specs are the for the power adapter, not what the unit actually
> uses. The only way to know exactly how much is used, is to use a meter
> and measure it....and that will then only be an average power
> consumption. The unit uses far less in TX mode than while in RX mode.
>
> And yes...power (W) =3D voltage (V) * current (W)

Thanks. That makes sense. I'll go ahead and measure the actual power
being used.

Jeff straightened me out on the battery. I didn't realize that the
battery is only good for the first 25 percent. Ouch!

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