Lasersaber strikes again. A joule thief king ?

[MarkE]

But at 60Hz a suitable receiving antenna ?  quarter wavelength is 4,100,000 Feet :/ 


Ya I saw that mosfet version; was thinking of playing with that. ( http://www.overunity.com/13175/25mv-joule-thief-powered-by-peltier-merely-using-our-body-heat-free-energy-247/msg410419/#msg410419 )  I don't have any JFET..I think... just power mosfet; but I think that just changing the core winding ratios can overcome that... most of the fets I chose for low gate capacitance

Most MOSFETs are enhancement mode devices:  current does not flow at zero gate bias.  The minimum voltage that you can work down to will be limited by your ability to get the thing started so that it can boot strap itself.  JFETs conduct at zero bias, which gives them a big advantage at very low bias.

[d3x0r]

As far as tuning goes... it is theoretically possible to tune to a given frequency with arbitrary combos of caps and inductor values that satisfy the resonant tank formula, but I have found that there is a "balance" of sorts between capacitance and inductance that seems to work best. I don't know how exactly this optimum balance can be calculated. Maybe it is "pF ~= nH" or something like that. In any case I think the capacitors are more lossy than the inductors we usually play with, so a larger cap will waste more of the power in the tank than a larger inductor will. But the larger inductor will lose more by RF radiation than the large cap will.

Okay; well there is a factor of capacitive and inductive reactance; where they are equal they are resonant...

frequency 1200000 (1200Khz)
8.7mH & 2pF  reactance 65596

decreasing inductance and increasing capacitance decreases reactance
1.76mH & 10pF reactance = 13270

and to go to the extreme
0.0176uH & 1uF reactance = 0.13270

(further search for 'ideal AM radio tank'); then there's Q... Q=Z/R (so a low reactance yields a low Q... but a high reactance starts getting a larger R from the wire)
Hmm.. so maybe if I start with the wavelength of wire and reverse coil length from target inductance and arbitrary radius, and turns from wavelength and radius....
slightly lower inductance than the highest frequency so I can add a core for tuning
I guess I have to further check internal capacitance...

[d3x0r]

Crank flashlight I found.... Not a tesla torch.


https://www.youtube.com/watch?v=RsKGjAc_mnM
Is kind of a nice form factor for the generator...

[TinselKoala]

Okay; well there is a factor of capacitive and inductive reactance; where they are equal they are resonant...

frequency 1200000 (1200Khz)
8.7mH & 2pF  reactance 65596

decreasing inductance and increasing capacitance decreases reactance
1.76mH & 10pF reactance = 13270

and to go to the extreme
0.0176uH & 1uF reactance = 0.13270

(further search for 'ideal AM radio tank'); then there's Q... Q=Z/R (so a low reactance yields a low Q... but a high reactance starts getting a larger R from the wire)
Hmm.. so maybe if I start with the wavelength of wire and reverse coil length from target inductance and arbitrary radius, and turns from wavelength and radius....
slightly lower inductance than the highest frequency so I can add a core for tuning
I guess I have to further check internal capacitance...

Yes, at resonance inductive and capacitive reactance are equal. This does not mean that one set of values only can be resonant at a given frequency! Nor does it mean, as some people evidently believe, that these reactances cancel to produce zero reactance.  Use this calculator to insert any two of the four values (L, C, Fo, and X) , and the others will be calculated for you.
http://www.sengpielaudio.com/calculator-XLC.htm

The issue of the receiver antenna matching the wavelength or 1/4 wavelength of the desired pickup frequency is an interesting one. Obviously you do not need a 1:1 match in physical length, or AM radios would be doomed to unworkability. What needs to match is _electrical length_ and the tuned resonant tank circuit does that matching for you. If your little loopstick/capacitor arrangement is resonant at, say, 300 kHz, that means that electrically it forms a cavity that "echoes" internally at that frequency. Like holding a conch shell up to your ear, vibrations at the tank frequency can be picked up from outside and will "amplify" within the tank to the point where they can be detected, rectified, and presented to an amplifier for your audio listening pleasure.  Losses exist in this process, though, so the very _best_ receiving antenna is still going to be some kind of 1/4 wave resonator that is physically of comparable size to the wavelength of interest.

[scifi123]

I simulated lasersaber's schematics version 3 found here:

   http://laserhacker.com/?p=420

using LTSspice (see the attached image).

Unfortunately, it doesn't work: the oscillation dies quickly.
Also, the schematics seems wrong since the secondary of the transformer (L2 in the schematics) is connected to the rest of the circuit in one point (should be 2 points).

Can lasersaber or someone else tell me what needs to be changed to make it work ?