Wireless power is already used in some types of parking transponders. These are the things that people have in their cars that open the gates when they drive up. Although most parking transponders still use a small battery (watch battery), batteryless transponders are becomming more common.
I'm an EE student and the parking permits at the university that I go to use this technology. Basically it operates on RF energy. The transponder has an antenna that picks up RF energy from a transmitter at the gate. A capacitor charges (quickly) and then the transponder sends out a signal which the receiver decodes and (depending on authorization) opens the gate.
I think there are a few points that everyone is forgetting (or don't know about). I'll try to clear up as many misconceptions as possible that people are getting hung up on.
1) Power over ethernet is dangerous to people that touch the bare wire
This is not true. PoE uses voltages that are considered (are in the class) 'very low voltage'. If a person comes in contact with a power wire at this voltage, it is harmless because the resistance of skin doesn't allow enough current to flow (remember V=IR, therefore I=V/R and since R~1kohm, I is relatively small for small voltages). This is not to say that someone wouldn't feel a shock, but feeling a shock doesn't make it dangerous (think of when you get a shock after taking off a sweater - you don't get electricuted, just a discomfort)
2) It has been mentioned that electricans might have to install it because 'it carries power'. And that it has to be 'properly installed/grounded'. This doesn't really apply, since currently, anyone can run speaker wire (ie for surround sound) that carries power (up and beyond 100W). We're talking about up to 39W here. Secondly, regarding the proper installation/grounding. Well, it is good practice to install anything proper, and as for grounding, it doesn't really apply here.
2b) an extention of 2, although the electrical code doesn't have much to say about low voltage, there are some things it does. For example, it states that a low voltage junction box cannot contain high voltage lines as well. If the two types of outlets are to be placed next to each other, there must be a barrier between the two sections. TIA/EIA (Telecommunication Industry Association) takes care of most low voltage code stuff (although inspections are not required for low voltage work).
3) P=VI (for DC)
High power devices cannot be run from PoE. Since UTP is 22 guage (maybe it's 24), it cannot handle large currents, and since the voltage has to be kept low for safely reasons, you cannot power high wattage devices from PoE. PoE was never designed to run a coffee maker - it was designed for such things as wi-fi access points.
4) PoE will damage non-PoE aware equipment
While this could be true for some equipment, it is not the general case. If PoE runs on +12V and the ethernet card is expecting +5V (that's what ehternet uses for it's communication), the card may have sufficient protection to limit the +12V and no damage will result. Although this would be a vaild concern because NIC manufacturers try to cut every corner.
5) Normal ethernet does not carry power
This is technically false. Back to ohms law and power equations:
P=V^2/R
In order for power transmitted to be 0W, either V=0 or R=infinity. Since V=+5V at some time when connected, and the impedance of the input to the ethernet card cannot be infinity, there has to be some power being transfered (although very little).
6) It was posted earlier that an AC shock is less dangerous than a DC shock. This is the exact oposite! When you are being shocked with an alternating current, you are 'stuck'. This is because your muscles operate by electric current, and since your muscles are sensing the alternating current, they oscillate and you cannot control them. When you are shocked by a direct current voltage, you have control over your muscles (and depending on the polarity of the voltage, you might even be 'kicked' away from the conductor that is shocking you).
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Wireless power is already used in some types of parking transponders. These are the things that people have in their cars that open the gates when they drive up. Although most parking transponders still use a small battery (watch battery), batteryless transponders are becomming more common. I'm an EE student and the parking permits at the university that I go to use this technology. Basically it operates on RF energy. The transponder has an antenna that picks up RF energy from a transmitter at the gate. A capacitor charges (quickly) and then the transponder sends out a signal which the receiver decodes and (depending on authorization) opens the gate.
I think there are a few points that everyone is forgetting (or don't know about). I'll try to clear up as many misconceptions as possible that people are getting hung up on.
1) Power over ethernet is dangerous to people that touch the bare wire
This is not true. PoE uses voltages that are considered (are in the class) 'very low voltage'. If a person comes in contact with a power wire at this voltage, it is harmless because the resistance of skin doesn't allow enough current to flow (remember V=IR, therefore I=V/R and since R~1kohm, I is relatively small for small voltages). This is not to say that someone wouldn't feel a shock, but feeling a shock doesn't make it dangerous (think of when you get a shock after taking off a sweater - you don't get electricuted, just a discomfort)
2) It has been mentioned that electricans might have to install it because 'it carries power'. And that it has to be 'properly installed/grounded'. This doesn't really apply, since currently, anyone can run speaker wire (ie for surround sound) that carries power (up and beyond 100W). We're talking about up to 39W here. Secondly, regarding the proper installation/grounding. Well, it is good practice to install anything proper, and as for grounding, it doesn't really apply here.
2b) an extention of 2, although the electrical code doesn't have much to say about low voltage, there are some things it does. For example, it states that a low voltage junction box cannot contain high voltage lines as well. If the two types of outlets are to be placed next to each other, there must be a barrier between the two sections. TIA/EIA (Telecommunication Industry Association) takes care of most low voltage code stuff (although inspections are not required for low voltage work).
3) P=VI (for DC)
High power devices cannot be run from PoE. Since UTP is 22 guage (maybe it's 24), it cannot handle large currents, and since the voltage has to be kept low for safely reasons, you cannot power high wattage devices from PoE. PoE was never designed to run a coffee maker - it was designed for such things as wi-fi access points.
4) PoE will damage non-PoE aware equipment
While this could be true for some equipment, it is not the general case. If PoE runs on +12V and the ethernet card is expecting +5V (that's what ehternet uses for it's communication), the card may have sufficient protection to limit the +12V and no damage will result. Although this would be a vaild concern because NIC manufacturers try to cut every corner.
5) Normal ethernet does not carry power
This is technically false. Back to ohms law and power equations:
P=V^2/R
In order for power transmitted to be 0W, either V=0 or R=infinity. Since V=+5V at some time when connected, and the impedance of the input to the ethernet card cannot be infinity, there has to be some power being transfered (although very little).
6) It was posted earlier that an AC shock is less dangerous than a DC shock. This is the exact oposite! When you are being shocked with an alternating current, you are 'stuck'. This is because your muscles operate by electric current, and since your muscles are sensing the alternating current, they oscillate and you cannot control them. When you are shocked by a direct current voltage, you have control over your muscles (and depending on the polarity of the voltage, you might even be 'kicked' away from the conductor that is shocking you).