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Month: December 2010

Joule Thief

Joule Thief

A little info on the Joule Thief for those of you that have not heard about it. In the article “One Volt LED – A Bright Light” written by Z. Kaparnik from Swindon, Wilts, UK published in the November 1999 issue of the magazine “Everyday Practical Electronics” in the section “Ingenuity Unlimited”; described how to make a “Micro-torch circuit” using a very compact high frequency, high efficiency DC-DC converter design. The circuit consisted of a hand wound micro toroid, a 10K resistor and a ZTX450 transistor. The circuit was designed to run a LED that had a forward voltage drop greater than 1.8VDC from a single cell battery (1.5VDC) and could run as low as 750mV, this meant it could run from nearly dead batteries. This circuit has been propagated, experimented with, and changed at numerous sites and discussion boards and most significantly can now run white LEDs that have forward voltages greater than 3V. Someone nicknamed the circuit “Joule Thief” and it has stuck ever since.

A Joule Thief circuit is a simple three component, low Voltage DC-DC boost converter. The circuit can run on voltages as low as 300-400mV depending on the transistor used and windings on the transformer. The output voltage and current depend on the three components used in the circuit. As a minimum the transistor must have high enough gain and should have a collect-emitter voltage rating that is well above the maximum output peak voltage on the secondary winding ( I like at least a 25% margin ). The resistor is chosen so that it limits the maximum circuit current, by limiting the current to the base of the NPN transistor. The transformer can be wound 1:1 for simple operation or can be wound with more than two coils ( A third winding may be wound to create high voltages for running EL devices, Nixie tubes, neon bulbs, etc… ).

I decided to try and experiment with this circuit myself. I have made many variations of the circuit, some that could run to voltages as low as ~350mV and still produce 12V out (not much current though with such a low input voltage). In making the circuit, I decided it would be nice to have a PCB so that I did not have worry about problems that I was having with the air wired experimental circuits. The air wired circuit had several problems due to wiring shorts or opens, and was not robust enough to carry around. I thought it would be nice to produce some kits for other experimenters so that it would be easy to assemble and not have to worry about wiring problems. Visit the products page or see Joule Thief kits here: http://www.madscientisthut.com/Shopping/agora.cgi?product=Energy%20Harvesting&user4=Joule%20Thief%20Kits
Read the rest of the Joule Thief Blog for circuits, simulations, and experiments.

Using the Joule Thief with Stepper Motors at Low RPM to Generate Useful Power

Using the Joule Thief with Stepper Motors at Low RPM to Generate Useful Power

When ever I took apart old equipment like dot matrix printers that were heading for the trash heap I saved the stepper motors, slide rails, etc… I have tried using the stepper motors in projects like making generators, but you have to spin them fairly fast to get any useful voltage out of them, so I never did much with them other filling up a box in the garage.

Since I have been playing with the Joule Thief and low Voltage energy harvesting experiments, I thought well lets see what we can do with a stepper motor. So I took one of the stepper motors out of the garage this morning and made a circuit with a stepper motor that has both coils connected to two bridge rectifiers feeding a 8200uF 10VDC capacitor. The capacitor then feeds a Joule Thief circuit that is running 4 white LEDs in series. The generator is able to light the LEDs at very low RPM. ( I stopped at 4 LEDs because this is around the target voltage I would like to use this circuit at which is 14VDC, I am sure that this will run more LEDs in series.)

A Joule Thief is a perfect circuit for this application. The circuit is a simple 3 component, low voltage DC-DC boost converter. The circuit can run on voltages as low as 300-400mV depending on the transistor used and windings on the transformer. Specifically in this application I am rectifying the low output of the stepper motor and storing the energy in a capacitor then using the Joule thief to boost low voltage stored in the capacitor to about 14VDC. The final application for this Joule Thief circuit will be to charge 12V sealed lead acid batteries using a low speed windmill.

So here are some applications this could work in (be aware that these circuits would have to be redesigned with some protection to prevent failures):
* Low wind speed generator.
* Low speed water generator using a water wheel setup or a small Pelton wheel with low water head.
* A machine that converts linear motion to rotary motion could also use this type of setup (like the old sewing machines that were run from foot power pushing a pedal).

Here is the Schematic: (If you want to see a full size image click the image, and then click on the next page image)

Joule Thief Stepper Motor Generator
Joule Thief Stepper Motor Generator














Here is a picture of the Joule Thief Generator Circuit as built:(If you want to see a full size image click the image, and then click on the next page image)

Joule Thief Stepper Generator Circuit
Joule Thief Stepper Generator Circuit














And in the following short video of the circuit in operation, I am spinning the stepper at a very slow rate of less than 100RPM and you see the Voltage generated by the stepper never gets above 2VDC, but it is lighting 4 white LEDs in series at about 14VDC:

Joule Thief Simulation II

Joule Thief Simulation II

We have Joule Thief kits in stock. This also includes our new high power Joule Thief Kit that includes a Cree XLamp 1 Watt white LED. We ship all over the World.

For a fully interactive simulation; visit the forum at http://madscientisthut.com/forum_php/viewtopic.php?f=12&t=3 to download the simulation files, a link to download the free LTspice simulation software, and to see the joule thief schematic (the download is near the bottom of the page).

I did the Joule Thief simulation graphics again. I wanted to make them a little better this time. I added some arrows on the schematic to show where the simulation graph values are coming from, I hope this helps. Click on the image, then click on the next page image for a full size version ( I still have to figure out why you have to click the image twice to get a full size image )

Joule thief Simulation II
Joule thief Simulation II Graph and Schematic
Hardware debouncing

Hardware debouncing

I’ve been playing with debouncing signals that come in from mechanical switches, because I’m building a timer that’ll detect when a garage door has been opened, and close it again 10 minutes later, so several friends of mine won’t end up coming home to open, and emptied-out, garages. It’s possible to debounce signals in software. Here’s a whole bunch of stuff on software debouncing, concentrating on interrupt-driven intelligent debounce code, that doesn’t monopolize the processor with polling or delays. Good stuff, but somewhat complicated. I like hardware solutions because they give me more code space.
This page details a hardware debounce circuit. I’d come up with pretty much the same design, fooling around on my own. A lot of people omit the diode, and while it still usually works, I’ve run into testing situations where having a diode in there made a lot of difference.
First I messed about with switches to see how bad they were. I’d like some idea of the worst-case switch noise, for sizing the R/C filter on the front of the schmitt trigger.
So, here are some screen captures of some lousy switches.
A standard cheap 110v wall switch:

Oscilloscope trace of lightswitch noise
Oscilloscope trace of lightswitch noise

That’s about 200 microseconds of noise.

I grabbed a nice DPDT rocker switch, with a lighted switch, that I’d bought as a backup for the one I stuck in my girlfriend’s car, the day I surreptitiously swapped her horn for one that goes “a-WOO-GAH!” so that when she tires of the awoogah horn she can switch back to the boring little beep-beep.  It works great for that purpose.  It doesn’t work so well for interfacing to microcontrollers, because here’s how it looks:

scope shot of toggleswitch noise
Oscilloscope screenshot of toggleswitch noise

Note the time stamp in the bottom right hand corner: each division is 500 microseconds, so this switch is still thrashing about at 5 milliseconds.  Thankfully this is my worst-case scenario, of the switches I tried.

On the other end of the spectrum, here’s a cheap tactile switch.

Oscilloscope shot of tactile switch noise
Oscilloscope shot of tactile switch noise

Again, note the time: this switch has grounded in 200 *nanoseconds* with no excursions whatsoever.  So choose your switch carefully and you might not even need any debouncing.

However, you’re not going to trust that, right?

So let’s look at an actual use case.  I’m working with a SPDT toggle switch.  For the sake of completeness I checked the bounce when I turned it on, and when I turned it off.  They’re not anything like the same.

Here’s the signal we’re probably looking for: 5V switching to ground.

Oscilloscope trace of switch turning off
Oscilloscope trace of switch turning off

That could stand some debouncing.

On the other hand, here’s what happens when the switch opens, and the signal runs back up to 5V.

Oscilloscope trace of toggle switch opening
Oscilloscope trace of toggle switch opening

That could run as is.

But I’m not willing to rely on that.  Maybe next time it’ll have junk in the contacts.

So I put the hardware debouncer between the switch and the scope.  The debouncer has an RC filter on the front, and then a trigger.  The filter smooths out a lot of the junk we see in the signal, and the trigger digitizes it, choosing whether it’s a 0 or a 1.

Here’s a screenshot of the same timeframe as the previous two shots, as the switch opens.

Oscilloscope trace for debounced switch opening
Oscilloscope trace for debounced switch opening

The RC filter slows down the voltage rise, and smooths it out.  Here’s a shot with the time cranked out from microseconds to milliseconds.

Oscilloscope trace for debounced switch opening
Oscilloscope trace for debounced switch opening

And finally, the switch closing, with the debounce circuit.

Oscilloscope trace for debounced toggle switch closing
Oscilloscope trace for debounced toggle switch closing

That looks pretty good to me: my back-of-envelope calculations say it should be able to debounce noise out to about 50 milliseconds.  Now, the flip side of that is that it can’t see any *intentional* signals that have a shorter duration than that, so there’s a bandwidth limit, and I need to choose my RC filter pair to match the signal I’m trying to condition.