Joule Thief

[TinselKoala]

I almost always put an ultrafast diode from D to S, reverse biased, to supplement the body diode in the mosfet. I don't know if this is "superstitious" or not but it does seem to help keep the mosfets cool.

My favorite 2n3055 JT circuit:

(No, the heat sink isn't needed, it's just for show!)

[allcanadian]

1st beer...On a side note yet another year has almost come to an end and this old dog has leaned a few new tricks which is kind of amazing when I think about it.


My favorite, the zero theshold detector or diode, I learned of quite a few years ago but was always on my mind since I first read the literature on Tesla's quenched arc gaps decades ago. A zero theshold diode is simply a mosfet with the gate pin tied to the source pin and if the applied voltage reverses the mosfet turns off... simple enough. The gain is in the fact a mosfet can have an extremely low resistance (Rds On) compared to a diode as well as a ridiculous conduction or "turn on" voltage to the order of 0.0002v. In the real world this means we can replace a $0.20 diode with a $0.80 mosfet and show a very real gain in efficiency.


As another year is about to transition from one to the next I would also like to introduce a neat little mostly unknown circuit I developed about a decade ago which applies here. I'm sure we all have had issues with the interface between a very high secondary voltage and a lower voltage switch on our primary. The issue being resonance which means nothing in itself as fundamentally we are speaking of time constants. Now the problem I have always had with this form of logic is that everyone would seem to be chasing something which is elusive, that is it is always changing because the world we live in is dynamic or changing and yet we devise static contrivance.


To me the solution was obvious which I see as a precursor to MPPT in that the time constant should be autonomous, one a variable a reflection of the other. The answer, my answer was stupid simple and I simply introduced a switch which was comprised of two mosfets back to back with a common gate. The input is #1 drain tied to #2 source the output #1 source tied to #2 drain the gates connected together. The internal body diodes are obviously back to back which may or may not constitute a negative resistance oscillator in in itself, I digress. The common gate conductor or wire is then connected to the high voltage secondary output by simply wrapping a few turn of the wire around the insulation of the high voltage secondary.


2nd beer, At which point one might understand we have created a simple capacitor through the insulation of the HV secondary driving our mosfet gate(s) and the time constant is determined by the final voltage, the capacitance of the insulation and the distance of the connection from the secondary coil output. The moment the HV rises to a given threshold it induces the gate conductor(s) with a charge some call a displacement current others capacitance some others parasitic capacitance and this charge conducts through the conductor which turns the mosfets off and the duty cycle is the delay in which the change in the charged state at the gates occurs.


It is stupid simple, it is dynamic in that the whole of the apparatus will automatically seek it's own self-resonance regardless of a change in parameters and most important it solves more problems than it creates.  3rd beer, You know you guys have so much damn potential, so much creativity and yet you fight each other tooth and nail to make your point, to reinforce your own beliefs in a world which we know is fluid where our beliefs must change. I know one thing... if my beliefs are not changing then I am not changing and if I am not changing then I am not evolving....there it is...evolution.


Merry Christmas and a happy new year


AC

[MarkE]

Back to back MOSFETs form bilateral switches.  They are very useful where one needs to block current from going either direction between two ports.  If you still have a rotating disc drive in your computer, then you have such a switch.  They are used to connect a reserve energy capacitor to unload and park the heads upon power loss.

If you are going to use MOSFETs in any environment with voltages greater than Vgss max, it is a really good idea to provide voltage protection.

[Pirate88179]

If you still have a rotating disc drive in your computer, then you have such a switch.  They are used to connect a reserve energy capacitor to unload and park the heads upon power loss.

Mark:

What do you mean..."still"?  What are you using over there...solid state drives?  The price is still too high for me on those but, I hear the read/write times are scary fast.

Bill

[MarkE]

Mark:

What do you mean..."still"?  What are you using over there...solid state drives?  The price is still too high for me on those but, I hear the read/write times are scary fast.

Bill

Yes, I mostly use solid state drives.  Yes, they are wicked fast.  They are down to about $0.40 - $0.50/Gbyte from high quality suppliers like Samsung.  If you need something to store a video library on then use redundant rotating drives.  B&H Photo offers a 250Gb Samsung 840 for $120.