Author Topic: Joule Thief (Read 6372353 times)

Pirate88179 · « **Reply #16455 on:** November 29, 2013, 12:51:46 AM »

Quote from: MileHigh on November 26, 2013, 04:33:09 AM

Bill:

I can envision what you are talking about. There is a technical explanation for that that could apply here. You can excite various components at high frequencies, not necessarily high voltages, and see very fast pulse trains across them. The AC impedance of the physical part itself comes into play. So it's almost a "fake" voltage. You see voltage spikes but that's due to the inductance of the wire in the part. The actual diode itself will have an AC impedance. So the LEDs light up, and voltage spikes are kind of "sprinkled on top" of the current flow through the LEDs. So the assumption is that this process can take place at high voltages also.

I am not challenging your observations about the apparent brightness, just saying that there is a voltage "fuzz" on top of the signal that is actually powering the LEDs.

You would probably need a proper ultra low inductance current sensing resistor to see the actual current flowing through the circuit.

With respect to the question of what waveform makes the LED appear to be bright, there are other issues at play like the longevity of the LEDs. It may be that a "high spikey" current going through the LED makes it appear to be brighter, but at the expense of the premature shortening of the life of the LED. I am just speculating of course. You have to assume that there are diminishing returns as you increase the current flow, and of course eventually heat dissipation becomes a problem. It makes me curious about what the current waveform output looks like for a "real" LED power supply used for architectural lighting. There are fixed and dimmable versions. Will there be tell-tale inductive discharge spikes or will it look like DC? What about a dimmable LED power supply? I cheated and peeked at a chip. It fancier than and better than my suggestion for the previous chip that Farmhand pointed out. They do variable PWM, pulse width modulation, of a continuous fixed-frequency control signal that goes to the transistor base input. So there is always a current pulse stream going to the LED, and the size of the pulses is controlled by the PWM signal. That makes sense, whereas my suggestion for Farmhannd's chip was a bad kludge.

What's interesting is that there is a good chance that the inductor has continuous current flowing through it. It either goes into the LED or goes into the transistor. When it goes into the transistor the coil is being energized. So the LED sees a full-blown pulse train that looks like a square wave going from no current (like "ground") to a certain level. The LED never sees the current/voltage decay to zero. That's because below a certain current level, the LED is useless and not doing its job. So to be more efficient they avoid the decay to zero alltogether.

Even though I am talking about current all the time here, in this case you can look at the voltage across your LED on your scope. You should (I hope) see a nice pulse train with sharp rise and fall times and a slightly slanted "hat." That's telling you that there is a sharp on-off current pulse going through the LED at sufficient current level to light it up and no current is being "wasted" with the useless decay to zero.

The key to all of this is to use a relatively small inductor, not sure if there will be a core, and switch that sucker at a relatively high frequency, perhaps between 30 KHz and 60 KHz. It's small and cheap and does the job.

MileHigh

P.S.: You would have to be worried about your scope here and make sure that it's not grounded if you were going to peek at the waveform across the LED if in any way the LED was deriving its power from the mains. Disconnect the third prong. I assume that all scopes have their own isolation transformers for their main AC power input. Honestly, I would still buy a real isolation transformer to keep handy all the time.

MH:

Thanks for the reply. I am glad to see there might be a classical explanation for my observations. As you know I have not put any of these flash circuits on the scope but, I do check amp draw and the Cree 60 watt equiv. draws 210 mA's, which is really not bad for the amount of light. It is rated to require much more than that from the mains, of course, and, I am not getting mains brightness for sure. BUT, given the power used and the voltage supplied by a "dead" AA battery, there is a lot of light here for many, many hours, so, while not OU or anything,a worthwhile effort for me.

I like your explanation here because it seems to fit with my crude description of "fooling" the leds that they have what they need to light up. Maybe like a surface charge on a battery to show more voltage than is really available. We all know about duty cycle and how our eyes are fooled into seeing constant light when it is pulsed, so, that is routine with any JT circuit.

What I guess I am trying to say here is that I am lighting led modules with so little power from their rating that, they should not even turn on, but they do and are pretty bright.

TK has some of my circuits and a little led module I sent to him so maybe he can add his thoughts to this if he chooses to do so. I would be happy to send one to you as well if you would like to see what it can, and cannot do. I was wanting to market these on my website but, after seeing TK getting shocked by one in a short video he made, I might have second thoughts. I get nailed all the time by these even though I "know" better. I can just see the lawsuits, ha ha.

I just think it is funny when I go to Hackaday and see some guy lighting 10 leds using 12 volts, and they think that is great. They also seem to think the JT is just for lighting 1 led. I can light hundreds on an AA but they don't seem to be interested.

Thanks for the reply.

Bill

PS PM me if you would like one of these circuits to play with.

MileHigh · « **Reply #16456 on:** November 29, 2013, 02:04:25 AM »

Bill:

Thanks for the offer but I am fine. Let me describe the "Cadillac" Joule Thief "prototype station" or "test bed" for fun.

When you strip a JT to the bare bones what you have is a transistor switch that energizes an inductor, then the switch switches off and the inductor discharges through the LED.

There are several variables that you can change to experiment with the magnitude and shape of the current/voltage pulse generated when the inductor discharges through the LED or LED array.

The coil energizing voltage
The inductance of the coil
The initial current when the coil discharge starts
The amount of time the transistor is on *
The amount of time the transistor is off *

(* aka the frequency and duty cycle of the pulse train that controls the transistor)

In addition to this, naturally you would want to measure the average current consumption so you would know your input power.

You could use a signal generator or a dual 555 setup to have full control over the signal that controls the switching on and off of the transistor. For sure there are dozens of Arduino programs that will do that for you.

Then you can just play with the variables to your heart's delight, looking for the most bang for your buck. The whole thing could be done on a single breadboard. You check the spec sheet for your LEDs to check the recommended operating current and maximum current vs. time for the LED to make sure you don't blow it.

Supposing you find a sweet spot. Then comes the real interesting challenge. The challenge is to create a Joule Thief circuit that emulates the coil and timing configuration that you got from your test bed. So you have to engineer a Joule Thief. At least you have the coil and the power source already done. So perhaps by playing with the number of turns in the secondary coil that goes to the transistor base input resistor you will be able to get the timing to match. Then you would have engineered your very own optimized Joule Thief.

MileHigh

Pirate88179 · « **Reply #16457 on:** December 01, 2013, 12:23:19 AM »

MH:

I like your idea. I actually have a test rig I made for testing the HV/JT circuits in parallel, which shows a gain of light output for adding the second circuit, the third circuit added actually hurts the light output. Now I am going to see if I can run two of these circuits in series...I do not know if that is possible but, we will see.

With your testing system, you are right that you could alter each of the variables and hopefully obtain the optimum configuration. The only thing I would add, as I am now doing, would be to have a trimpot in the circuit so once you run on battery power, you can adjust the circuit to stay in the best conditions as the battery depletes. I purchased a bunch of these trimpots (20 turn) for this purpose.

Once again, I appreciate your input. I still believe that we have not reached the end for designing and building the "perfect" JT circuit. I still think that more research needs to be done using JT's that output at least 350 volts. I would like to play with the frequency and the duty cycle as you have suggested and your test rig would fit the bill.

Thanks,

Bill

crowclaw · « **Reply #16458 on:** December 01, 2013, 08:50:59 PM »

Hi Pirate, MH has answered the question exceptionally well. finding the ideal sweet spot by experiment is certainly not an easy task to accomplish, bearing in mind circuit characteristics vary so much and change considerably with applied voltage. In essence providing the spikes are of sufficient amplitude and the current is not allowed to cause thermal runaway led's will provide some degree of brightness. The art is as mentioned finding the ideal circuit balance and efficiency becomes the ultimate challenge. The JT circuit however is not necessary the best circuit for the task from many angles. MH mentioned the use of the popular 555 timer or alternative an arduino. For some time I had outdoor led lighting using switched inductors producing narrow high voltage spike pulses driven from a 555 astable incorporating inbuilt thermal compensation for outdoor temperature changes.
Crow

Pirate88179 · « **Reply #16459 on:** December 03, 2013, 03:05:31 AM »

Quote from: crowclaw on December 01, 2013, 08:50:59 PM

Hi Pirate, MH has answered the question exceptionally well. finding the ideal sweet spot by experiment is certainly not an easy task to accomplish, bearing in mind circuit characteristics vary so much and change considerably with applied voltage. In essence providing the spikes are of sufficient amplitude and the current is not allowed to cause thermal runaway led's will provide some degree of brightness. The art is as mentioned finding the ideal circuit balance and efficiency becomes the ultimate challenge. The JT circuit however is not necessary the best circuit for the task from many angles. MH mentioned the use of the popular 555 timer or alternative an arduino. For some time I had outdoor led lighting using switched inductors producing narrow high voltage spike pulses driven from a 555 astable incorporating inbuilt thermal compensation for outdoor temperature changes.
Crow

I agree with your description of MH's input. Excellent advice for experimentation. my problem with 555's and the like is that they need too many volts to run. (Not for testing mind you) 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. The green resistor looking component is an inductor I have been told. So, I want to know what chip is under there that will operate down to .6 volts and perform like a JT circuit to light an led requiring 3 volts from a 1.2 volt rechargeable battery. If I could get some of these, I would make circuits using them. I would also try the things that MH has suggested to perfect it. I can not find any chips out there that might do this. Is this why they are hidden under that glob of gray goo? Possibly I can do some surgery with a dremel to see what it might be.

Anyway, I have nothing against the chip approach except that the 555's all want like 5 volts or more to run.

Anyone know what that garden light chip might be?

Bill

Magluvin · « **Reply #16460 on:** December 03, 2013, 03:35:37 AM »

Quote from: Pirate88179 on December 03, 2013, 03:05:31 AM

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 · « **Reply #16461 on:** December 03, 2013, 04:14:28 AM »

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 · « **Reply #16462 on:** December 03, 2013, 05:50:19 AM »

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 · « **Reply #16463 on:** December 03, 2013, 06:20:27 AM »

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 · « **Reply #16464 on:** December 03, 2013, 07:18:15 AM »

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

SeaMonkey · « **Reply #16465 on:** December 03, 2013, 08:02:05 PM »

There are a number of low voltage (single cell)
LED Driver Chips available such as the ZETEX
ZXSC300 which have surprising capabilities.

Discussion here.

Datasheet here.

It does require an external transistor so
one of the other chips with integrated
switching transistor may be more convenient
to some experimenters.

Search "single cell led driver."

Pirate88179 · « **Reply #16466 on:** December 04, 2013, 02:57:59 AM »

Quote from: MileHigh on December 03, 2013, 07:18:15 AM

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

MH:

Well, I was hoping for at least a crude schematic. I would be lost without one. I know there are others here that will do this and, hopefully, they will jump on board. You see, I thought I was following your idea but, I am not sure why we need an external transistor, mosfet? There are many on that 555, are they not isolated for our use? (showing my ignorance here) Is there a low voltage 555 that is separated that we could use? I was hoping to see where the 555 pinouts and the mosfet leads would be connected. I have no idea how to do this. I suppose I am smart enough to learn, which is, afterall, part of the fun but, it may be beyond me.

I still love your idea though, and maybe this will force me to learn all about 555's and mosfets. It will be a while before I can buy some of those in your pdf. Money is tight this month so I have some time to study. I still think this could be a breakthrough in the JT experiments we have seen here. I am still dedicated to giving it my best efforts.

Bill

@Seamonkey:

Thanks for the links. It was funny that today, an electronics guy where I work told me to go to the same forum. Interesting.

Thanks,

Bill

MileHigh · « **Reply #16467 on:** December 04, 2013, 06:49:01 PM »

Bill:

You need an external transistor or MOSFET to do the power switching to drive the LEDs. The 555 can't do that, it's just a timing device. All that you need is the cheapest CMOS 555 so they should be cheap. You don't need a fancy low-voltage 555.

I can't remember if you have a scope, but ideally you would have one for this project. You can simply Google 555 circuits and pick one. But I realize doing the whole thing is outside of your comfort zone.

It would be a nice simple (relatively) Arduino project also, and for sure there has to be software already written that you could hijack for this project. There are also ways to make some micros, probably including models in the Arduino family, to run at ultra low power. So the whole timing setup could be done by a micro and probably run at about the same nearly insignificant power level.

Sorry I can't be directly involved if it were to happen but it's not the time for me and it's also a big commitment.

MileHigh

MileHigh · « **Reply #16468 on:** December 04, 2013, 07:18:43 PM »

Bill:

Necessity is the mother of invention so I dreamed up a great consolation prize for your consideration.

I can suggest a simplified version of the project that you could do and it gives you a bootstrap or launching pad for doing the real project. Just get an Arduino board, I think they are about 20 bucks? Find the software. Search on something like, "Arduino software oscillator" or "Arduino PWM oscillator" or "Making the Arduino programmable I/O into a square wave generator." (There might be hundreds of them out there.) The Arduino board will have a few input-output bits for driving external logic. So the software will make one of those I/O bits in to an output bit and turn it into a square wave output where you can control the frequency and duty cycle.

So you connect that output signal to the base resistor of your transistor. The external battery ground is connected to the ground pin for the digital output. Then you just do the usual circuit: battery -> coil -> transistor collector. And coil -> LED -> ground.

Then you are ready to roll. You can use all of your existing coils and transistors. But you have the freedom to try any of your coils or any battery voltage, and the Arduino program allows you to dial up any frequency and any duty cycle for the energizing of the coil. It's basically a quick and easy test bed setup.

Note of course the "test bed" is a very short step away from an actual usable project. You just flash the Arduino board with the program and give it it's own button cell or cells. Then connect up the "power train" and you are done. Some of those Arduino boards are the size of a USB key. So the USB key and the two button cells could be tucked away inside a project box and you add the power train with the replaceable AA cell, the coil, the transistor and LEDs and you are done.

MileHigh

MileHigh · « **Reply #16469 on:** December 04, 2013, 07:56:09 PM »

Bill:

I can't resist finishing off the software for the simple design. The Arduino board will have those programmable input-output bits. Some of them are used as inputs, and they are used for reading the momentary push buttons you see on some Arduino boards. Also, almost certainly you can program one of those pins to make the Arduino go to sleep. Then the next time you push it the Arduino wakes up. When the board is "asleep" it consumes almost no power. So you don't even need a physical on-off switch, the batteries are always powering the board but the board consumes no power when it is asleep.

So you can imagine this: Your JT-style LED light sitting on top of some sort of project box. Perhaps the holder for the single AA battery is conveniently accessible and open and flush with the top surface of the box. And on the front of the box there is a little tiny panel-mount momentary push-button switch. It cycles though off -> low -> medium -> high -> off. All very simple and elegant.

Wouldn't that be nice? Suppose the two button cells powering the Arduino board last a year. That's not too bad. Who knows they might last a lot longer than that. You can use a bigger battery to power the Arduino board if you want.

MileHigh

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Author Topic: Joule Thief (Read 6372353 times)

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