Joule Thief

[Magluvin]

  What I want to learn about is what is under those gray blobs in my solar garden lights.  We have decided (as a group here) that under that blob is the chip that makes up the JT circuit. 

Those globs have been around since i was a kid. Near 50 now.

Seeing some being put together, back then, it was like looking in the window of an old ROM chip, you can see the tiny wires that connect the chip to the IC pin runners. Little gold wires.  These were done the same back then, but on the board instead of an IC housing.

Today they may be SM soldered, havnt looked lately, but still possibly done like back then.  Then the glob is applied to protect the chip and fragile connections. The globs are hard like plastic, no flex or the connections could be severed by some bending of the board.

So removing the glob may ruin connections, and may reveal a 'whole' chip, but just the chip. No numbers or identifiers. These are usually custom jobs. Made on the cheapest scale, well, so they are cheap.  Disposable, and all together(the end product) made for pennies. And usually are one off designs for a particular device and not made available for other purposes. Also, usually used in items that are never intended to be repaired.

Mags

[Pirate88179]

Mags:

I don't doubt what you say at all.  Yes, they are cheap and just designed for a single purpose.  You can get those led garden lights at the right time of year for like $2 (US) ea.  And, you get an led, nice solar cell, and a decent rechargeable AA or AAA battery.  I just wish we could get those chips and then we could make nice efficient JT's without winding inductors and would not need an additional transistor either.

I have many 555's here but don't use them because of the volts required.  Maybe I need to think a little bigger?

Bill

[MileHigh]

Bill:

There is an interesting design for a 555-based JT that I can suggest to you.  It's all about "thinking outside of the box."  I have been told that I suck at that!  lol

For starters, it's pretty easy to find a 555 that will run down to 1.5 volts, so that's a start.  The spec sheet is attached.

But I am going to throw a curve ball at you and I will design something from scratch.  It just popped into my head, the Mother of Invention thing.

Why not separate the timing power from the coil power?  Let's say you have a "timing battery" and a "coil battery."  If you do it right, let's say you use three new alkaline AA batteries for 4.5 volts as the timing battery.  If you do things right that should last for several years.  For the coil battery, you keep on using your single used AA batteries.

If you use a CMOS 555 timer and for the timing components you use the highest resistance and smallest capacitance possible, then you will have a 555 timer circuit that sips just a hundred or so microamperes.  Also, don't use a transistor if possible, use a MOSFET. You don't want to have to supply base current to a transistor because that will drain your timing batteries.

So now the problem of a 555 timer that doesn't work properly at low voltage goes away completely - for years.  Yet you are still using a 555 and there is a big advantage here.  The 555 uses much much less power than the JT feedback mechanism to implement the timing part of the circuit.  It might use 1000 times less power.  Big savings there.  Plus you can't forget that when you separate the timing circuit power from the coil power, the actual overhead to implement the timing is "zero" - the coil battery only powers the coil to light the LEDs, it does nothing else than that.  So your used AA batteries should last longer.

There are a few things to check out with respect to the MOSFET on resistance and the LEDs and stuff like that.  When the transistor is on, typically the collector-emitter voltage can get down to 0.2 volts, which is pretty good.  If one MOSFET or possibly two in parallel can give you a drain-source voltage of something close to that (when the JT is running) then you should be in pretty good shape.

If you can configure the 555 to run at a desired frequency and have a single pot adjust the pulse width to the MOSFETs, then you have a brightness control.  That's a nice thing to have.  Also, as the main coil battery starts to croak and the LED gets too dim, then you can turn up the brightness to compensate until you suck the last bit of juice out of the battery.

Can you visualize that?  A little box with three batteries inside for the 555 timing circuit that you might only have to replace in three or four years.  You have a potentiometer for a brightness control and a place to drop in one or two used AA batteries or batteries that you recharge with your solar charger during the day.  Perhaps you could even change the paradigm and use a big fat rechargeable D cell that you charge with a solar panel during the day.

Once you have the CMOS 555 timing circuit worked out to your satisfaction, then you can do whatever you want on the "power side" of the equation.  i.e.; you could for example have the same 555 circuit power three sets of MOSFETs with three power batteries and three small toroidal inductors with ferrite cores and three strings of LEDs for perhaps a full blown room light.

It could be a fun project.  Take it from a guy that can't think outside of the box.

MileHigh

[Pirate88179]

MH:

With all sincerity, this just might be GENIUS!!  By dividing the two functions of the battery, you can allow each part of the circuit to do what it does best....no compromise.  Let those batteries run the 555...that's all they ever have to do.  The other, as you said, can be a "dead" AA or whatever.

I have never read of nor heard of this approach before...this is a first.  I am not sure I have the needed skills to make this, but I will give it a go.  I hope that others with more knowledge will try this as it is an entire new way of looking at our humble little JT circuit.

I am very impressed Sir.  Don't settle for this, or settle for that, let each part do what they need to do.   This is a fantastic idea!

I really hope others on here take a look at this.  I will post my failures on Youtube to hopefully get others on board.

Very well done MH.  You destroyed my argument against using the 555's.

I know that mosfets can handle decent power, so no probs there.  Do you think, depending on the inductor, that I could get 350-400 volts out of such a circuit?  Maybe more?

This is a great idea!

Bill

***EDIT***

Here is a 555 that will run on 2 volts.    http://www.digikey.com/product-detail/en/TLC555CP/296-1857-5-ND/277502


***EDIT***

I see that the PDF you posted shows a 555 running at 1.5 volts.  I had no idea of this.

[MileHigh]

Bill:

I agree that the project will be a challenge.  Perhaps you could get others involved.  It's definitely doable with the people around here.  I honestly don't think I will have the time to follow and contribute that much.  Consider me the "hardware architect" and take it from there.  That's supposed to be part of the fun also, to develop it as a group effort and create the schematic and define the timings and select the parts yourselves.

You might actually need more than 4.5 volts to power the 555 because it has to generate sufficient gate voltage for the MOSFET you end up using.  The answer is in the spec sheet for the MOSFET.  Of course the batteries will still last years.

Then of course you have complete freedom to choose your inductor and also to define the timing, as was discussed a few postings ago.

In the second revision you may be able to experiment with over-driving the LED for a very bright flash of light.  The spec sheet may state something about an extra bright flashing mode and recommend the current level and the pulse length.  I haven't read a modern power LED spec sheet so I am just speculating.  This would be for a "beacon" function, just for fun.

MileHigh