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Wire guage for Faraday effect generator

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Fnord

Tue Nov 10 2009, 11:05PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39

Hello all,

I recently bought some beefy neodymium magnets with the intent of making a faraday generator (as found in shake flashlights).

What I need to know if there is some type of formula I can use to find the most efficient wire guage and coil shape based on the magnet I have.

I did some googling around and found that almost no one makes these things. I found a couple instructables, though they didn't have much in the way of math behind them. The only reference I found recommended around 34-36 guage wire.
I know that thinner wire = more turns/higher coil density = higher voltage, but there must be some point of diminishing return as far as efficiency and hassle of winding. Also, I only need to charge 5.5v rated caps, so really high voltage might no be desireable(?).

Here's the specs on the magnets:

Grade: n50
Shape: cylinder, .875 diam x 1" thick
magnetized thru axis
core field strength: 14k gauss
surface field strength: 4k-ish gauss.

Thanks!

rp181

Tue Nov 10 2009, 11:24PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Did you get them from ebay?
Someone sent me 8 2"x2"x1" Nd 52 magnets. I sent it in, and got guass readings, indicating the magnets were infact grade N32. Beware of scammers!

Im not sure about the coil. I would try for the wire gauge you mentioned, the length of the magnet. For thickness, I would see where the magnet got weak, I would probably go 3/8-1/2" thick.

Fnord

Tue Nov 10 2009, 11:34PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39

No, I got them from "Magnet4less". I know, the name doesn't exactly inspire confidence, but they have a huge selection and seem to be about the largest supplier on the web.

How are things on SF rp181? I haven't been there for a really long time and don't really want to go through 50,000 new topics right now. (Not to say I've given up the hobby; I have several things in the works right now).

Oh and how is DYI?

Bjørn

Tue Nov 10 2009, 11:44PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

In my experience charging capacitors is not a very good option. You need to do significant charging at low efficiency before you get a usable voltage without complicated DC/DC converters.

I have tried both and there is no comparison, lithium-ion is better at everything. Have a look at something like this:

You are right, you don't need a very high voltage, the longer the wire the more loss there will be in resistance in the wire. If you are interested in efficiency you should also think about your rectifier because at these voltages a 0.6 V drop over a diode is a 10% loss.

mikeselectricstuff

Wed Nov 11 2009, 12:54AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

From first priciples, you want the source impedance of the generator to be similar to that of the load as that of the load for maximum coupling efficiency. in practice this can be hard to predict, at both the generateor and load ends. thin wire = more turns for a given winding volume but also higher resistance. The extremes are one turn giving high current at unusably low voltage, and a gazillion turns of very thin wire giving high voltage at unusably low current. There will be a useful range somewhere between these two extremes, which may take some experimentation to find.
One thing that is very important is to get the coil windings as close as possible to the magnet - this will generally dictate a cylindrical coil with fairly low profile, and a winding length comparable to the magnet length.
The better quality shake torches use small repelling magnets at the ends as magnetic springs to preserve energy unused on a stroke and transfer it to the next.

MinorityCarrier

Wed Nov 11 2009, 01:55AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

There are schottkey barrier recitfier diodes with a Vf of ~.32V which should improve the efficiency a bit.

3l3ctrici7y

Wed Nov 11 2009, 05:07AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

BjÃ¸rn; where does the inefficiency in charging the capacitors come from?
Do the capacitors themselves have low charge efficiency?

Thanks.

Bjørn

Wed Nov 11 2009, 05:25AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The high capacity capacitors tend to discharge themselves very fast so when you need light they are almost always discharged. Then you have a 6 V charger charging a 0 V capacitor and a lot of your spent energy go to waste while waiting for the capacitor to get up to working voltage. How much goes to waste depends on the resistance of the coil and the ESR of the capacitor.

The lithium-ion batteries not only keep their charge for years they are also up to voltage in matter of seconds after you start charging them so you get instant light and very little waste.

Hazmatt_(The Underdog)

Wed Nov 11 2009, 07:25AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You ought to do some energy comparisons between the super capacitor and a battery pack. I was doing a small solar project sort of like what you are trying to accomplish here and when you calculate out the energy storage comparison you'll see why we're still using batteries despite fantastic numbers like 200F.

Consider a single AA battery is 1500mAh for one hour. Compare that storage to 100F @ 5.5V and there is a massive difference in energy storage.

It's a fun idea, and it should come someday, hopefully soon, but we still have to remember that to replace batteries we need even more capacity, and capacitors that are designed to source high current at these huge values, instead of tens of mA.

Fnord

Wed Nov 11 2009, 05:11PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39

Anyway...

The caps aren't really an issue since I'll be experimenting with both (also, I'm not necessarily looking for HUGE capacitance values, at least not yet...).

And efficiency is nice, but I'm not going to go to extreme lengths to get every last drop in this case. I thought about the diode voltage drop and may try Minority's suggestion.

Anyone have a rough guess as to the guage and turn number for a reasonable voltage/current output? If not I guess I can just experiment.

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