Moving air without fans or blowers

[bobkoure]

When I was in high school I built a toy hovercraft-like-thing that used high voltage and two parallel charged pieces of hardware cloth to move enough air to lift itself off the ground (not counting the power supply which was pretty heavy. Basically the charge differential pushes/pulls ions from one grid to the other and lots more air molecules get dragged along through entrainment - same thing that sucks your shower curtain in when the shower is running.
Thinking that fans (or air movement in general) without moving blades or vanes might be an interesting thing - all be it more interesting to the silence guys than here (and, frankly, thinking that there must be a "fatal flaw" keeping it from being used) I did a web search and came up with Corona driven air propulsion for cooling of electronics.

I guess it's time for me to get a high voltage power supply...
Anyone already work with high voltage and got pointers for me? Other than "don't get zapped"...? Seen anything appropriate on ebay lately?

[JamesAvery22]

I thought the ionic breeze stuff was what you just described?

[Breach]

you might try putting this is the right forum section

[bigben2k]

No mention of pressure; I doubt that this thing would be able to handle any kind of flow restriction.

[MadHacker]

wouldn't the highvoltage interfere with the circuits in the PC it self?

[bigben2k]

No, as long as it's isolated nicely.

[Butcher]

Use a current limited EHT supply - that way if you do get a shock you're unlikely to kill yourself. That paper suggests you only need ~120uA which is way below the current required to kill yourself. If you get a supply limited to a few mA then it's not that dangerous (still try to avoid shocks though!)

I don't think it needs saying that an EHT supply contacting any piece of electronics will instantly fry it, so be careful.

[bobkoure]

Found a more appropriate (for what I'm thinking of) link - The Lifters Experiments. Lots of good info and "how to" stuff.
I'm guessing that using the radiator as a ground electrode grid would cause me all sorts of problems, both electrochemical and just plain frying everything if the HV supply ground floats.
So... if I don't want the rad to be part of the circuit, I suspect my challenge is going to be making sure there isn't an interplay between the radiator and the "air capacitor".

[Brians256]

Whoa! I guess it fits with the group style, though. The next logical step after adding water to the electronics is to simply add REALLY high voltage to the computer.

I'm thinking this is no longer about cooling things, but in simply making the computer more and more dangerous.

However.... if you get it working, I'll be in line trying to do it too! Should be very quiet if you can get any decent airflow. Certainly no turbulence there.

[DrMemory]

Sigh, here we go again. Any current greater than few micro-amps at voltages greater than ~40 VDC or 40VAC RMS (at low frequencies) CAN KILL YOU. DEAD. If you are determined to play around with high voltage then the least you can do is use the one hand rule. Keep one hand in your pocket while doing anything that might cause the other hand to come in contact with the high voltage. It is also a very good idea to wear shoes, preferably with rubber or plastic soles.

[redleader]

Sigh, here we go again. Any current greater than few micro-amps at voltages greater than ~40 VDC or 40VAC RMS (at low frequencies) CAN KILL YOU. DEAD.

Err no it can't. Not if its really only a "few micro-amps". You can't even percieve that.

[DrMemory]

Originally Posted by redleader

Err no it can't. Not if its really only a "few micro-amps". You can't even percieve that.

Uh, Yes it can. If that amount of current passes through the nerves that control your heart it will fibulate (beat chaotically) and you will die unless a defibulator is nearby and someone else is there to use it. Your right about not feeling the current though.

[Razor6]

The established threshold for fatal electrocution is 6ma through the heart, far more than a few micro amps. GFI outlets trip at 5ma.

[bobkoure]

Originally Posted by Brians256

I'm thinking this is no longer about cooling things, but in simply making the computer more and more dangerous.

Maybe you're right - certainly wasn't my intention. I was just looking at a PWMed nexus 120 and wondering "what could be quieter than that?"

[Butcher]

Originally Posted by Razor6

The established threshold for fatal electrocution is 6ma through the heart, far more than a few micro amps. GFI outlets trip at 5ma.

Usually listed as 30mA from what I've seen, it varies per person though.

Micro-amps aren't that dangerous, it's when you get above a few mA that things get risky. My physics teacher back at school shocked himself on an EHT supply at 5kV, it was current limited to 5mA and he survived (just said "ouch" loudly). As long as you use a current limited EHT supply it's not more dangerous than dealing with the AC line (probably much less dangerous in fact).

[redleader]

Originally Posted by DrMemory

Uh, Yes it can. If that amount of current passes through the nerves that control your heart it will fibulate (beat chaotically) and you will die unless a defibulator is nearby and someone else is there to use it. Your right about not feeling the current though.

Techically you're correct microamps will do damage to the heart (though you need tens to hundreds not a few), however how exactly do you propose to deliver this current directly to the heart without difuseing it through the rest of the body? Cut your chest open and push the wires in?

Cutting the heart out in order to better electrocute one's self is not commonly associated with electrical accidents.

[DrMemory]

The body is conductive. The (dry) skin has the highest resistance. Inside the body there is fairly low resistance due to the amount of (salty) fluids. However, as in all cases, the amount of voltage needed to produce a given current flow in any conductor (including the human body, through the dry skin) is given by Ohms law. With enough voltage, a current will flow. No cuts, punctures, Aztec rituals, or other silly suggestions are required.

[DrMemory]

Originally Posted by Butcher

Usually listed as 30mA from what I've seen, it varies per person though.

Micro-amps aren't that dangerous, it's when you get above a few mA that things get risky. My physics teacher back at school shocked himself on an EHT supply at 5kV, it was current limited to 5mA and he survived (just said "ouch" loudly). As long as you use a current limited EHT supply it's not more dangerous than dealing with the AC line (probably much less dangerous in fact).

If he was smart (and still alive), your teacher probably did not allow the current to flow THROUGH his chest.

Edited to add this:

My digital mutli-meter gives my body resistance (arm to arm, dry skin) as about 1.4M Ohm. When the DC voltage being measured exceeds ~ 40V the High Voltage indicator comes on as a safety warning. The amount of current that would flow through my body at this voltage is about 28 uA.

It is not about damaging the heart. It is about confusing the nerves that use electrical impulses to control the heart beat. These nerves work at considerably less than a few mA.

[Butcher]

I draw to your attention:

http://web.mit.edu/environment/ehs/e...echanical.html

In particular:

CURRENT IN MILLIAMPS   EFFECTS OF 60 HZ CURRENT PASSING THROUGH THE BODY  
1 or less              May not be felt - Maximum harmless intensity 
1 to 8                 Sensation of mild shock, can let go at will  
8 to 15                Painful shock, muscles contract, may still be able to let go  
15 to 20               Painful shock, can NOT let go  
20 to 75               Intense pain, breathing may be paralyzed  
100 to 200             Ventricular fibrillation; holds unconscious victim to the circuit, could be fatal  
200 or more            Heart stops, muscles contract intensely & could break bones, severe burns, breathing stops

Please note that according to this data a shock under 1mA (such as one in the microamp range) is considered harmless and is unlikely to be felt.

I'm more inclined to believe MIT's safety page than you frankly. Especially given that even the briefest of google searches turns up numbers in the 20+ mA range as the lethal current for humans.

I would like to add that the typical finger to finger resistance of the body excluding skin is 1000 ohms, hence the 30 (or 40) volt warning as a safety precaution - it is the minimum voltage to get a lethal shock at (outside of jamming electrodes into the heart). 30V, 1000ohms gives 30mA current, though to actually get the resistance that low you would need wet hands or some other means to massively lower the skin's resistance.

[DrMemory]

Originally Posted by Butcher

I draw to your attention:

http://web.mit.edu/environment/ehs/e...echanical.html

In particular:

CURRENT IN MILLIAMPS   EFFECTS OF 60 HZ CURRENT PASSING THROUGH THE BODY  
1 or less              May not be felt - Maximum harmless intensity 
1 to 8                 Sensation of mild shock, can let go at will  
8 to 15                Painful shock, muscles contract, may still be able to let go  
15 to 20               Painful shock, can NOT let go  
20 to 75               Intense pain, breathing may be paralyzed  
100 to 200             Ventricular fibrillation; holds unconscious victim to the circuit, could be fatal  
200 or more            Heart stops, muscles contract intensely & could break bones, severe burns, breathing stops

Please note that according to this data a shock under 1mA (such as one in the microamp range) is considered harmless and is unlikely to be felt.

I'm more inclined to believe MIT's safety page than you frankly. Especially given that even the briefest of google searches turns up numbers in the 20+ mA range as the lethal current for humans.

I would like to add that the typical finger to finger resistance of the body excluding skin is 1000 ohms, hence the 30 (or 40) volt warning as a safety precaution - it is the minimum voltage to get a lethal shock at (outside of jamming electrodes into the heart). 30V, 1000ohms gives 30mA current, though to actually get the resistance that low you would need wet hands or some other means to massively lower the skin's resistance.

I guess I'll have to quit taking off my skin before I work with electronic circuits from now on

[bobkoure]

To get back to air movement, ionic wind with sufficient air movement to actually cool something may not actually be very quiet. I found an article titled Force on an Asymmetric Capacitor (US Army Research Lab).

When the asymmetric capacitors have an applied DC voltage, and they are producing a net force
in air, they all emit a peculiar hissing sound with pitch varying with the applied voltage. This
sound is similar to static on a television or radioset when it is not tuned to a good channel.

There's also a real video of a lifter flying here and it's pretty loud, but I think most of that noise is the ground electrode shaking, which won't happen if weight is not an issue.

[Butcher]

Originally Posted by DrMemory

I guess I'll have to quit taking off my skin before I work with electronic circuits from now on

Wet hands or a cut would suffice to remove pretty much all the skin's resistance actually.

[Brians256]

If I recall correctly, there was some idiot that was trying to measure his internal resistance was. So, he punctured his skin to let the ohmmeter get past skin resistance. 9V battery going through internal resistance caused a mild heart attack. Unfortunately, he was alone and without means to reach medical assistance. Result? Darwin award. The story goes that this is why the Navy has safety precautions (associated with their ohmmeters I think).

[redleader]

Originally Posted by DrMemory

The body is conductive. The (dry) skin has the highest resistance. Inside the body there is fairly low resistance due to the amount of (salty) fluids. However, as in all cases, the amount of voltage needed to produce a given current flow in any conductor (including the human body, through the dry skin) is given by Ohms law. With enough voltage, a current will flow. No cuts, punctures, Aztec rituals, or other silly suggestions are required.

Yes all of that is correct, but think about the problem more. All the conductance in your body does not lead directly through the heart. So if you jab an electrode in each hand, most of the current will not be going through your heart. Some will pass through your lungs, others through your muscle tissue, some through the fat around your ribs, some will dip down into your lower body cavity and then come back up. In short its spread out all over your body with only a tiny fraction actually passing through the few inches of cross section represented by your heart.

In terms of circuit theory, your whole body is a big n-way current divider. So you need to put in a substantially higher amount of current at the outside to direct a significatant amount through the heart.

Though during heart surgery, shock is a much bigger problem since the heart is directly connected to instruments, some of which are actually electrified.

If I recall correctly, there was some idiot that was trying to measure his internal resistance was. So, he punctured his skin to let the ohmmeter get past skin resistance. 9V battery going through internal resistance caused a mild heart attack. Unfortunately, he was alone and without means to reach medical assistance. Result? Darwin award. The story goes that this is why the Navy has safety precautions (associated with their ohmmeters I think).

One of my textbooks has an example of someone who died licking a 9v battery. I wish it had details, but evidently he managed to fribrillate his heart doing it.

[Butcher]

Originally Posted by redleader

One of my textbooks has an example of someone who died licking a 9v battery. I wish it had details, but evidently he managed to fribrillate his heart doing it.

That's quite crazy given the shortest path is through the tongue and nowhere else.
Wonder if he had some congenital heart defect or such?