6 ms·

Wait, why is she using a volt meter in that video? That's not useful information without knowing how many amps the system can push. I could probably get a volt

by ykl 11y ago

Wait, why is she using a volt meter in that video? That's not useful information without knowing how many amps the system can push. I could probably get a volt meter to read 8 volts just from my hair after walking on carpet a bit, but that's certainly not going to charge my phone...

jacquesm 11y ago
A static charge from the carpet would be a lot more than just 8 volts.
deleted 11y ago
[deleted]
cnvogel 11y ago
More specifically, THIS voltmeter, which is a little bit "under-spec'ed", to say the least: Radio Shack (22-223) https://www.google.de/search?q=radio-shack+22-223&tbm=isch https://www.google.de/search?q=radio-shack+22-223&tbm=isch It's set to the 10V-AC range. I'd expect a VC backed company to invest in a little more elaborate measurement equipment than that. On the other hand, given a a good RMS meter operated in a frequency range where it's spec'ed to work and a known circuit impedance will give you a very good measurement of the power: P=Vrms²/R. And analog meters make for rather pretty demos, to be honest.
- jacquesm 11y ago
  Actually, that makes it more believable, not less. Let me explain why: A voltmeter can't just measure volts, it has to make a tiny current flow through a resistor. The value of that resistor will be lower for cheaper volt meters because the cheaper volt meters need more energy extracted from the circuit under test to function. A more expensive one has a very high internal resistance, and so loads the circuit under test far less. In this case being able to supply 8 volts into a meter with a low internal resistance is actually a plus ;) That said, the current is still tiny, even a crappy volt meter will pull no more than a few hundred micro-amps from the circuit that it is measuring. So let's be generous and say that it pulls a whopping 10 mA (very likely on the high side (edit: it turned out to be 37 uA)), in that case the power generated would be 0.08 W. Not enough to charge your phone by a long shot, but enough for a pretty demo with a moving needle. Finally, these cheap meters tend to output complete nonsense values when presented with high frequency AC, one would expect that they would at a minimum measure power transfered using a dummy load and some kind of dedicated power meter. All in all this does not make things look any better for uBeam. I'd really like to know who did their technical due diligence. What's really bad to me - but I'm a nitpicker - is that in the video the lady refers to the volt meter as a power meter, which it definitely is not. It measures potential, not power, any power that it takes from the circuit can be chalked up to imperfect design and the laws of physics. Maybe the person doing the demo was not knowledgeable about the physics behind the equipment but that's betraying a complete mis-understanding of the basic concepts involved. The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny. What gets me about this whole saga is that uBeam makes it seem as if they invented wireless power transfer using ultra-sonics, but any sound is wireless power transfer and that includes ultra-sound. The whole crux is how much power can be generated and whether or not there is a practical way to overcome all the obstacles. To transfer some power doesn't prove a thing, when I whisper in your ear I'm transferring power using soundwaves...
  - ykl 11y ago
    I could be wrong, but I think the demonstrator in the video is the CEO.
    - jacquesm 11y ago
      In that case we might hope for an accidental slip or a deliberate simplification for the audience but she certainly should know better than to call that thing a power meter.
      - saintgimp 11y ago
        That's the point. It's likely that at the time, she honestly didn't know the difference (or didn't understand the difference well enough to realize that it would be obvious to other people). She did a Ted Talk where she bragged about how she didn't know anything about engineering when she started uBeam.
  - deleted 11y ago
    [deleted]
  - cnvogel 11y ago
    As I had remarked, a good RMS meter on a known impedance, on a frequency it's spec'ed for. The setup shown is neither. Also: For low-level AC measurements you invariably need active rectification. So your meter will never be directly connected (or through a series resistor) to the source.
    - jacquesm 11y ago
      Agreed. The guts of that meter register 37uA for full scale (I found some specs), at 10V that's an internal resistance of 270K so probably it really is driving the meter directly. For 8 V that translates into 0.25 mW of power produced, which I don't think will be charging any cell-phones.
  - avian 11y ago
    > The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny. Exactly. These transducers look just like low-cost piezos used for ultrasonic ranging in air (e.g. car parking sensors, alarms, etc.). I have not seen any in that package that would be made for any significant amount of power. High power transducers look quite different.

by ykl 11y ago

jacquesm 11y ago

A static charge from the carpet would be a lot more than just 8 volts.

deleted 11y ago

[deleted]

cnvogel 11y ago

More specifically, THIS voltmeter, which is a little bit "under-spec'ed", to say the least: Radio Shack (22-223) https://www.google.de/search?q=radio-shack+22-223&tbm=isch https://www.google.de/search?q=radio-shack+22-223&tbm=isch It's set to the 10V-AC range. I'd expect a VC backed company to invest in a little more elaborate measurement equipment than that. On the other hand, given a a good RMS meter operated in a frequency range where it's spec'ed to work and a known circuit impedance will give you a very good measurement of the power: P=Vrms²/R. And analog meters make for rather pretty demos, to be honest.

jacquesm 11y ago
Actually, that makes it more believable, not less. Let me explain why: A voltmeter can't just measure volts, it has to make a tiny current flow through a resistor. The value of that resistor will be lower for cheaper volt meters because the cheaper volt meters need more energy extracted from the circuit under test to function. A more expensive one has a very high internal resistance, and so loads the circuit under test far less. In this case being able to supply 8 volts into a meter with a low internal resistance is actually a plus ;) That said, the current is still tiny, even a crappy volt meter will pull no more than a few hundred micro-amps from the circuit that it is measuring. So let's be generous and say that it pulls a whopping 10 mA (very likely on the high side (edit: it turned out to be 37 uA)), in that case the power generated would be 0.08 W. Not enough to charge your phone by a long shot, but enough for a pretty demo with a moving needle. Finally, these cheap meters tend to output complete nonsense values when presented with high frequency AC, one would expect that they would at a minimum measure power transfered using a dummy load and some kind of dedicated power meter. All in all this does not make things look any better for uBeam. I'd really like to know who did their technical due diligence. What's really bad to me - but I'm a nitpicker - is that in the video the lady refers to the volt meter as a power meter, which it definitely is not. It measures potential, not power, any power that it takes from the circuit can be chalked up to imperfect design and the laws of physics. Maybe the person doing the demo was not knowledgeable about the physics behind the equipment but that's betraying a complete mis-understanding of the basic concepts involved. The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny. What gets me about this whole saga is that uBeam makes it seem as if they invented wireless power transfer using ultra-sonics, but any sound is wireless power transfer and that includes ultra-sound. The whole crux is how much power can be generated and whether or not there is a practical way to overcome all the obstacles. To transfer some power doesn't prove a thing, when I whisper in your ear I'm transferring power using soundwaves...
- ykl 11y ago
  I could be wrong, but I think the demonstrator in the video is the CEO.
  - jacquesm 11y ago
    In that case we might hope for an accidental slip or a deliberate simplification for the audience but she certainly should know better than to call that thing a power meter.
    - saintgimp 11y ago
      That's the point. It's likely that at the time, she honestly didn't know the difference (or didn't understand the difference well enough to realize that it would be obvious to other people). She did a Ted Talk where she bragged about how she didn't know anything about engineering when she started uBeam.
- deleted 11y ago
  [deleted]
- cnvogel 11y ago
  As I had remarked, a good RMS meter on a known impedance, on a frequency it's spec'ed for. The setup shown is neither. Also: For low-level AC measurements you invariably need active rectification. So your meter will never be directly connected (or through a series resistor) to the source.
  - jacquesm 11y ago
    Agreed. The guts of that meter register 37uA for full scale (I found some specs), at 10V that's an internal resistance of 270K so probably it really is driving the meter directly. For 8 V that translates into 0.25 mW of power produced, which I don't think will be charging any cell-phones.
- avian 11y ago
  > The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny. Exactly. These transducers look just like low-cost piezos used for ultrasonic ranging in air (e.g. car parking sensors, alarms, etc.). I have not seen any in that package that would be made for any significant amount of power. High power transducers look quite different.