"MIT Wristband Could Make AC Obsolete"

[WCKda]

I do this with cold paper towels. Still doesn't beat A/C especially during summer in a small room with your devices generating heat.

[ionbasa]

No shit. That doesn't mean the diodes are in the correct place to prevent someone from putting a voltage across the output. The semiconductor devices on the pwm circuit are there to change the duty cycle. Do you know what the last block of a DC-DC converter is? A capacitor. Do you know what happens when you invert the current on a electrolytic capacitor? It explodes. It literally explodes. Having a diode somewhere inside the circuit doesn't mean crap. And even then there should only be a single diode in a pwm circuit and it's the free-wheel diode to avoid discontinities in the load current, everything else is done with thyristors and transistors, so I have no idea where you even got the notion that there are "rectifying diodes" in a pwm circuit. Not to mention that the pwm circuit is just on that single pin to provide the modulation, the other two pins have nothing to do with pwm, so why I have absolutely no idea why you even brought up the pwm circuit.

Sorry to bother you, but look at post #13 in this thread, you brought up the idea of applying a 12v voltage to a PWM Fan header, that your Motherboard will "fry". Also when I was referencing the diodes, I was referring to the +12V pin on the header itself, not the pulse pin. As per: 

The diodes I mentioned have nothing to do with the PWM branch itself, but the actual source of power maybe you have heard of "reverse bias." If I try to apply current through a diode on the "N junction" (cathode) it will not allow current to pass through in reverse.

[dyent10]

here's an idea. install an alternator on a stationary bicycle. connect the alternator to a battery bank. have twenty it made. market the devices as a new workout regiment in a local gym. make the gym free from the city's electrical grid, using the bicycles. save money on electricity and pocket the money coming from the people who pay to use the bicycles.

[Canoas]

Sorry to bother you, but look at post #13 in this thread, you brought up the idea of applying a 12v voltage to a PWM Fan header, that your Motherboard will "fry". Also when I was referencing the diodes, I was referring to the +12V pin on the header itself, not the pulse pin. As per: 

The diodes I mentioned have nothing to do with the PWM branch itself, but the actual source of power maybe you have heard of "reverse bias." If I try to apply current through a diode on the "N junction" (cathode) it will not allow current to pass through in reverse.

Nope, read the post, I never said anything about PWM. You're the one who started talking about the non-existant "rectifying diodes" in the PWM fan circuits. Also, I know what a diode is, you keep saying diode over and over again as if it is some magic word that will magically fix all the problems. Do you even know what you're talking about? Maybe that "tactic" works with other people, but I'm finishing my masters on electrical engineering, copy-pasting random sentences from wikipedia about diodes and images of 4 wire headers is not going to cut it.

 

With that said I'm not going to reply to you any further. You clearly don't know anything about what you're saying, this is not a worthwhile discussion.

[ionbasa]

Nope, read the post, I never said anything about PWM. You're the one who started talking about the non-existant "rectifying diodes" in the PWM fan circuits. Also, I know what a diode is, you keep saying diode over and over again as if it is some magic word that will magically fix all the problems. Do you even know what you're talking about? Maybe that "tactic" works with other people, but I'm finishing my masters on electrical engineering, copy-pasting random sentences from wikipedia about diodes and images of 4 wire headers is not going to cut it.

 

With that said I'm not going to reply to you any further. You clearly don't know anything about what you're saying, this is not a worthwhile discussion.

 

First you tried to tell me that if I apply a 12 V current to a fan header, the motherboard would fry:

 

Yes, that is true, but if you start spinning your fans fast enough to produce a moderate voltage you'll end up frying your motherboard.

 

I then said:

Why bring up a motherboard? This has nothing to do with that. Also this will never work to power a motherboard using your fans because of rectifying diodes to protect the pwm fan circuits.

 

I messed up, when I said pwm fan circuit, I never meant the actual portion of the circuit that produces the PWM signal. I meant to reference the PWM Fan "Header", which happens to provide both +12v and Logical Ground. Also I was referencing the rectifying diodes on the +12 V circuit provided at the header, not the actual PWM pin/signal. Also they aren't non existent as you speak of. As you are almost finishing your studies in Electrical Engineering (I am in university for Mechanical Engineering btw) I wanted to ask you if there is anything wrong with using a rectifier on a DC input? Now there is some food for thought. It can also be used on DC sources to provide reverse current protection:

 

That is:

 

As you can see in the last two pictures, I tried applying the current backwards in the rectifier, and thus very little current  was able to reach the "source" voltmeter. I believe that some manufacturers do use this method for reverse current protection, but it is a very crude setup, and isn't perfect as some voltage drop will most likely occur under load. 

 

 

Also I wan't blindly copy and pasting from Wikipedia.

 

I also owe you an apology as I did state pwm in my post, but that was not what I was trying to say, I meant to say the +12V lead on the pwm fan header, my mistake, bad wording and trying to reply too fast without actually reading my post to make sure it is sensible.

 

Also regarding one other thing: Earlier in the tread you said that you can't have a cooling device and a generator in one with out it being efficient, and that in practicality it would never work, but in my Chemistry II class we we taught the exact opposite regarding this issue with thermodynamics.

 

In order for this device developed by MIT to cool, then it would have to be through an endothermic process, so it has to absorb heat from your body. That heat then has to transferred to electricity, otherwise the bracelet would just warm up to body temperature and it wouldn't cool as all of the energy (Delta H) would have been absorbed, but now what? What are you going to do with that energy, you cant store it in the device, so you convert it to electricity. By doing so you create energy transfer, so the bracelet could keep cooling as the endothermic process would continue.

 

Now if you wanted to heat the bracelet you have to grab energy from an outside source, thus electricity. Provide electricity and the bracelet converts electricity to heat though an exothermic process, as now your body starts to absorb the heat given off the device. 

 

So it really is a 2 in 1 device, it can generate electricity when needing to cool (endothermic) and converts electricity to heat (exothermic). Or at least it technically should when thinking of the bracelet itself as merely transferring energy from one form to another.

 

 

Also sorry if I managed to irritate you, I know I got frustrated, but after reading back through the thread we ended up arguing over a misunderstanding in my wording.  :unsure:

[Canoas]

@ionbasa

Rectifying diodes are used on AC-DC converters or when you want to "mirror" a sine wave. There's no need for it if you just want to protect the circuit from a reverse current, just one diode will do. And what you put there isn't a rectifier, the diodes are facing the wrong way. If you had implemented the rectifier properly then what it'd do is that it'd reverse back any reverse voltage, so if you put -12V it'd turn it into 12V, whereas the single diode would just stop it from flowing altogether. Either way unless you really need it to be protected, which isn't the case in motherboard fan headers since they don't expect anyone to put any voltage there, you shouldn't even try to use such systems as there is a voltage drop across the diodes which means wasted energy. The more diodes the current is passing through the more energy is wasted, so a single diode is more efficient than a rectifier if you just want to protect the circuit.

Though it should be possible to eliminate the voltage drop, you just need to make a circuit that will let the current flow freely without passing through any diodes when it's correctly polarized and it only needs to pass through a diode when it's reversed. Right now I can't think of anything, but I'm sure it exists though I don't know complicated it will be.

Either way this is not topic to discuss this. Open a new topic in the off-topic section if you want to discuss this further.

 

 

In order for this device developed by MIT to cool, then it would have to be through an endothermic process, so it has to absorb heat from your body. That heat then has to transferred to electricity, otherwise the bracelet would just warm up to body temperature and it wouldn't cool as all of the energy (Delta H) would have been absorbed, but now what? What are you going to do with that energy, you cant store it in the device, so you convert it to electricity. By doing so you create energy transfer, so the bracelet could keep cooling as the endothermic process would continue.

When the device is cooling then the absorbed heat is dissipated by the heatsink, that's why there's a heatsink in his wrist. Also, like I said before, the cooler and generator would need different circuits and only one can function at a given time or they'll simply be destroyed if there's no protection or just won't work if there is (the current from the cooler will just flow to the generator). That means that when it is cooling it cannot in any way generate energy from the heat. After it is done cooling then it can indeed turn on the generator, but if I remember correctly there's less than 10 seconds between each 5 second pulse, and the thermoelectric generators have an efficiency of much less than 10%. That means that in the very least, assuming that the cooler has a 100% efficiency and the generator an efficiency of 10% which they do not, you'd need 50 seconds to generate enough power for the 5 second cooling pulse. In a real scenario you'd probably need close to 5 minutes or even more for just that 5 second pulse, assuming that the convertion ratio stays the same even at very low temperature differences which i doubt it does. So in the end they're putting another circuit in there, making the overall cost higher and the device bulkier, for a few % of extra battery life at best.

 

A thermoelectric generator would make sense in an electric car or something of the sort, where even a 1% extra battery life will make a difference. In a device like a watch? Nope, even 20 more minutes of battery life won't prevent you from having to charge it at the end of the day.

[ForsakenLive]

What if it only makes you think you are cold, while it's still extremely hot, and all the internal heat triggered processes that happen inside your body don't start?

Things like different way your organs work, sweat, and other heat protection mechanism the body uses, will not start, causing damage to the user.

This wouldn't be a problem if there is only 27°C and people instead of using the AC, use this. But where I live on summer temperatures may rise further than 45°C.

Are this wrist bands safe on that environment?

[Stonep11]

It looks kind of stupid, but you would be wearing it around the house anyways...most of the time. It is definitely a neat invention though, and if the price is right, I would definitely buy one. Hopefully they make it look a bit less like a...well...computer heatsink.

I was thinking the same thing. I could see this, in the future, combined with Smart Watches. I think in 15 years or so everyone will only carry 1 item that works like a smartphone, wallet, ID, keys, entertainment, AC cooler, everything on their wrist or some thing like that.

[Stonep11]

What if it only makes you think you are cold, while it's still extremely hot, and all the internal heat triggered processes that happen inside your body don't start?

Things like different way your organs work, sweat, and other heat protection mechanism the body uses, will not start, causing damage to the user.

This wouldn't be a problem if there is only 27°C and people instead of using the AC, use this. But where I live on summer temperatures may rise further than 45°C.

Are this wrist bands safe on that environment?

There is a difference between making you feel like the room is 5 degrees or so warmer. This thing is not going to give you the ability to stay cool in a fire or walk around a snowstorm in a t shirt. What you are thinking could be dangerous, and I bet this gets banned for sports as that could cause injury, but for just normal wear, no problem.

[xRAGExSounDxRIOTX]

 

 

 

That was a university in nigeria or something, not MIT

My mistake.

[TheYoungEnthusiast]

Pretty cool technology. btw this thread went to shit and is overrun by an argument/debate between two guys....