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Topic: Wireless power transmission (Read 1 time) previous topic - next topic

tjones9163

Hello,

I am simply trying to understand the difference between wireless power transmission( in which a source device could wirelessly power another device) and something like RFID or WIFI or Bluetooth where signals at various frequencies are being sent and demodulated as useful information?

And also, what frequency is wireless power transmission being used on?, depends or application, range, power?

raschemmel

#1
Jul 08, 2019, 01:32 am Last Edit: Jul 08, 2019, 03:59 am by raschemmel
Power cannot be transmitted wirelessly (yet).
Energy can be transmitted inductively (inductive-couplingt charger, inductive isolated charge pump power supply , etc, or via laser beam (generating heat at the target which can be transferred hydrodynamically) , but
other than than, power , in general cannot (as yet) be transmitted wirelessly.

Normally, it is considered mandatory to define your terms before posing your question.
Obviously, the word "wirelessly" is all but meaningless without range specifications.
You might as well say "telepathically" , for all its worth.

The tags contain electronically stored information. Passive tags collect energy from a nearby RFID reader's interrogating radio waves. Active tags have a local power source (such as a battery) and may operate hundreds of meters from the RFID reader. Unlike a barcode, the tag need not be within the line of sight of the reader, so it may be embedded in the tracked object. RFID is one method of automatic identification and data capture (AIDC).[1]

In plain english, what the above means is that an RFID tag is an inductive device that responds to a nearby transmitted signal by collecting energy and using it to power a transponder which emits a stored code:
RFID TAGS

You could call this wireless power transmission but since the load device is restricted to a very small
device it is not generally discussed in the same sentence with the term "wireless power transmission"
as it would be misleading.
As far as wireless power transmission is concerned, the only notable project was NASA's in 1975:
Goldstone Project
But when you do the math, using a 500,000W transmitter to transmit 34000 watts over a range less than 2 miles doesn't seem very useful or economical.  The efficiency was >80% but you have to ask yourself, "If that was 44 years ago, why does it still stand as a world record ? (and why isn't that technology in use today ?)

Quote
Bluetooth:
Developed by Bluetooth Special Interest Group
Industry Personal area networks
Compatible hardware Personal computers
Smartphones
Gaming consoles
Audio devices
Physical range Typically less than 10 m (33 ft), up to 100 m (330 ft)
Bluetooth 5.0: 40-400 m (100-1,000 ft)[1][2]
Wifi:
Quote
an access point (or hotspot) has a range of about 20 meters (66 feet) indoors and a greater range outdoors.
difference-between-bluetooth-and-wifi

BTW, why do you ask ?

Southpark

#2
Jul 08, 2019, 03:34 am Last Edit: Jul 08, 2019, 03:45 am by Southpark
I am simply trying to understand the difference between wireless power transmission( in which a source device could wirelessly power another device) and something like RFID or WIFI or Bluetooth where signals at various frequencies are being sent and demodulated as useful information?

And also, what frequency is wireless power transmission being used on?, depends or application, range, power?
Passive RFID tag operation is based on 'wireless power transmission'. The wirelessy transmitted energy reaches the RFID tag's antenna. Current (and voltage) --- hence power is developed at the loaded antenna - for which the RFID tag can use the received power to operate (once it has accumulated/stored enough energy that is).

There are at least some documents that discuss the amount of power reaching passive RFID tags --- taking into account transmit power from the source, and radiation patterns, distance --- things like that, such as http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2175-91462018000100319

But, sometimes, there can be details within the articles that don't make sense, because the authors don't check for mistakes -- which is probably a life-story of lots of articles. Such as in equation (5), where it appears that the label 'P_tag' on the left-hand side of (5) should be P_ti.

I think in general, the amounts of power are relatively small at the tag ...... the article mentions values like 2 dBm at the tag ..... which is approx 1.6 milliwatt, and this is when their tag is a bit less than 1 metre from the reader hehehe

Paul__B

I suspect the OP is referring to the "wireless" charging of phones and Braun toothbrushes.  :smiley-lol:

raschemmel

Then why ask about Wifi  or bluetooth ?

MarkT

#5
Jul 09, 2019, 06:08 pm Last Edit: Jul 09, 2019, 06:09 pm by MarkT
Hello,

I am simply trying to understand the difference between wireless power transmission( in which a source device could wirelessly power another device) and something like RFID or WIFI or Bluetooth where signals at various frequencies are being sent and demodulated as useful information?

And also, what frequency is wireless power transmission being used on?, depends or application, range, power?
Wireless power transmission and RFID use the magnetic "near-field".  This means they operate rather like a transformer, coupling two circuits via a changing magnetic field shared between them.  For wireless power you need efficient coupling, which means really good proximity - basically its an air-cored transformer.  Typical frequencies are 10's of kHz to 100's of kHz (induction hobs work just the same, but generate heat in the secondary, which is the metal pan base)

RFID only has to transmit information, efficiency is less important, but you still need enough coupling to detect the signal against background noise, which means the range is only a few times the size of the transmit coil usually.  Typically the RFID tag modulates the magnetic field seen by the transmitter a tiny tiny fraction, and the transmitter filters out only the expected frequency of modulation.  Frequencies are higher to make the RFIDs smaller.  150kHz, 10MHz, that sort of range, there are various RFID standards.
(Though some RFIDs are actually far-field, and work by back-scattering a signal I think).


WiFi and BT are both radio transmission, ie electromagnetic radiation, propagating in the "far-field", electromagnetically.  The antennas used are tuned to the appropriate wavelength and a lot of amplification is
needed to boost the tiny signals (microvolt levels are common) till they can be detected/demodulated.

Far field propagation involves both an electric field and a magnetic field component, filtering either one out will
swallow the signal.  Radio antennas always generate both electric and magnetic fields.  Radio waves can propagate long distances as the power falls of with distance squared (near fields fall off with distance cubed).

Near field can be done just using magnetic field, or just using electric field.  Usually magnetic field is used and the electric field is screened out to reduce radio transmission.  Electric field coupling is seldom used as for any useful power levels the voltages are dangerous.  High voltage electricity pylon lines couple to fluorescent light bulbs rather well.  See this link (but note their explanation is completely wrong!) https://www.larkinweb.co.uk/miscellany/fluorescent_tubes_under_power_lines.html
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

raschemmel

@MarkT,
You should get a karma click for that explanation.

Southpark

#7
Jul 10, 2019, 02:46 am Last Edit: Jul 10, 2019, 02:46 am by Southpark
High voltage electricity pylon lines couple to fluorescent light bulbs rather well.  See this link (but note their explanation is completely wrong!) https://www.larkinweb.co.uk/miscellany/fluorescent_tubes_under_power_lines.html
Nice one!! Light sabres for everybody hehe. Is the mechanism of the glow related to electric field energising the gas particles? And when the gas particles lose energy, some photons are emitted?

Paul__B

Is the mechanism of the glow related to electric field energising the gas particles? And when the gas particles lose energy, some photons are emitted?
It is the (AC) electric field.  That is why the explanation given is so badly wrong as it refers to the magnetic field which is extremely weak as it essentially only exists between the wires themselves.

What you describe there is exactly how the tubes operate in their normal configuration; in this case the actual current - and so the brightness - is very small (and painless) but the voltage is so high that the gas can be ionised without the cathodes hot.

raschemmel

Quote
What you describe there is exactly how the tubes operate in their normal configuration; in this case the actual current - and so the brightness - is very small (and painless) but the voltage is so high that the gas can be ionised ionized without the cathodes hot.
 
or ,
Quote
another way to get the idea is to accidentally touch the leads of a 5000V neon transformer...

tjones9163

Then why ask about Wifi or Bluetooth?
To be more clear of what I was trying to ask is,
What is the difference between-
Option 1/ wireless power transmission (like wirelessly charging phones, RFID, etc).........and
Option 2/ Wireless signals like (WIFI, BLUETOOTH, etc).
I was thinking at first, that both options are both EM waves propagating through space, which after reading the post, I believe that only option 2 is EM waves that travel through space at a certain frequency, like 2.4ghz and are transmitted and the demodulated for useful information.
And option 1 is just inductive coupling like an air core transformer which just uses magnetism and magnetic lines of flux and these magnetic lines of flux "cut" through the target object's magnetic lines of flux, which will induce a small current.
Is the way I explained correctly?
Thank you very much for your responses.

Southpark

It is the (AC) electric field.  That is why the explanation given is so badly wrong as it refers to the magnetic field which is extremely weak as it essentially only exists between the wires themselves.

What you describe there is exactly how the tubes operate in their normal configuration; in this case the actual current - and so the brightness - is very small (and painless) but the voltage is so high that the gas can be ionised without the cathodes hot.
Thanks for mentioning all that Paul. Definitely nice that you and Mark mentioned those things. Not great when the wrong info gets passed along like that (on that website).

mauried

Theres no differance at all between 1 and 2 other than efficiency and how much power you need to transmit and how far you want to transmit it .
You can theoretically transmit 50 hz over very long distances if you want, but the difficulty is making an efficient 50 hz antenna. ( a 1/4 wave 50 hz antenna is 1500 kms high )
If you stand near a large power transformer with an audio amplifier and a loop antenna you will easily hear the
50 hz hum being radiated.
Hi frequencies like 2.4 Ghz and 433 Mhz are easy to transmit long distances simply because its easy to make efficient antennas at these frequencies.


Southpark

but the difficulty is making an efficient 50 hz antenna. ( a 1/4 wave 50 hz antenna is 1500 kms high )
Totally agree with you. The practicality of a 1.5 million metre length antenna hahaha.

raschemmel

#14
Jul 11, 2019, 08:14 am Last Edit: Jul 17, 2019, 06:19 pm by raschemmel
There is no Connection so you can't really call them Option-1 and Option-2 because WiFi and Bluetooth are not charging technologies.
They are DATA TRANSMISSION technologies and proximity charging is NOT a data transmission technology. In short , they should NEVER share the the same paragraph. Talk about one and then wait a day before bringing up the other one.

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