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LAN_403

Conundrum

Mon May 14 2012, 07:03AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Post your favourite Joule Thief variants here

Extra credit for V-on of less than 0.5V, without using germaniums.

I wonder if an LED would work as the bias source?
Illuminate from outside and use the photovoltaic voltage of around 1V at 0.2mA to boost up
the circuit and make it start.
Variant of this is to use an IR LED driven off the output to keep it working even in the dark.

Also, using the JFET from an electret microphone as a current limiter means that it won't waste power
when the battery is fully charged.
Connect gate to source, Ilimit is around 5-30mA depending on the JFET type.

Pinky's Brain

Mon May 14 2012, 04:35PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Has no one made a jfet+transformer oscillator joule thief yet? They don't need germanium to operate at low voltages, the jfet is lossy ... but if you really wanted to you could use it to bootstrap something more efficient.

Neet Studio

Thu May 17 2012, 03:26PM

Registered Member #4037 Joined: Fri Jul 29 2011, 03:13PM
Location:
Posts: 86

I have made a version using a hybrid BJT+MOSFET blocking oscillator. There is a voltage based negative feedback to limit the output via controlling the bias.

Essentially a MOSFET is wired in parallel with the BJT. For each cycle, initially the BJT starts to conduct (tiny blue blip in picture 4). When the positive feedback builds up enough, the MOSFET (RON=0.08 ohms) starts to conduct (green curve)and takes over. This vastly improve on the efficiency. According to LTSpice simulation, the efficiency is 86%. I could further improve the efficiency by another 7-8% by running the LED in AC, but the complexity is beyond what I can fit on a circuit board within the diameter of the "Maglite" barrel for an AA cell.

My last one was for driving a LED with 2 AA cell in a Chinese knockoff Maglite that TI was giving away. Since it is a knock off, the normal LED upgrade "kit" won't work. Picture 1 shows the toroid on top of a PCB with SMT components. The 3 black shadows at 4, 6 & 7 O'clocks are 2 BJT, 1 MOSFET. I made it only with parts that I have and a $0.60 Chinese 1W LED from online. There are much better ways of doing this but hey what's the fun of doing it the easy way?

Picture 2 shows the 2.4V flat part of the NiMH discharge curve. 2.4V * .18A = 0.432W of input power. The BJT & MOSFET are cold to touch. It is a good trade off point between battery life and brightness. The circuit is optimized for my retired of NiMH AA cells that have too high an internal resistance.

Note: I pointed the LED away from the camera as it over-saturated the CCD causing black lines on my camera.

The startup voltage is 0.7V and it would work all the way down to 0.3V (picture 3). That low end has no practical values as it might extend the battery for that extra few hundreds of milliseconds and is mostly only for bragging points.

brandon3055

Mon May 21 2012, 05:34AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

What exactly is a joule thief some sort of small boost converter?

Neet Studio

Mon May 21 2012, 12:10PM

Registered Member #4037 Joined: Fri Jul 29 2011, 03:13PM
Location:
Posts: 86

It is a simple blocking oscillator for driving LED from a battery. The claim is that it runs all the way down to some low voltage and thus can let you suck the juices out of the battery that no other circuits would do. In reality, it is not that efficient due to the saturation voltage across the BJT, so the battery runs down faster.

The simplistic aspect and the LED are what attracts the newbies like a bad internet meme.

brandon3055

Wed May 23 2012, 05:43AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

can you please post an example circuit

edit: nether mind i just found it

Edit will the circuit in the link work with n-chanel mosfets I have 2 09N03LA and 2 06N03LA the 09N03LA is rated for 50A not shore about the other one I would like to run a 3.3v 1000ma cree XP-G led off of 1 or 2 4A rechargeable D cells what are your thoughts?

Ben Solon

Mon May 28 2012, 02:48AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

I have never built one before, mostly because as said: led's draw noobs like bugs to a light. It's a simple little thing, and I never had the need to light an led from a single battery cell. Why not just use one of the many dc power sources I have sitting around.

Anyhow, inspired mostly by Tim Williams and reminded by this thread, I have built two versions. I have no idea about effeciency, but one lights 9 bright white led's fully with a single aa. Brighter than the original flashlight did with 3 aaa cells. The other lights one bright white led fully, and can't drive more than the one led because of the inductor I wound- 25:50 turns of 30 awg wire.

Note: both aa cells are "dead" from my brothers Xbox controller, and can hardly light a single led when put in series. When measured unloaded(blocking oscillator) the peak voltage on the 1 led version was well past 50v. All parts I used where common. No high tech low sat's going on here- just a 2n3904.

single led version

9 led version electronics- led's omitted because of camera saturation even if pointed away

stuffed in the barrel

9 led version fit inside the old flashlight

Neet Studio

Mon May 28 2012, 03:13AM

Registered Member #4037 Joined: Fri Jul 29 2011, 03:13PM
Location:
Posts: 86

My first MOSFET one uses 20N03 (from old Dell motherboard) for a AA to 9 LED flash light from the Dollar Store. It used to run off 3AAA. Might even be the same ones as Ben's! (Coin Diameter 21.2 mm)

The 2nd one (my previous post) uses Si2307 for 2AA to 1 LED because I have almost a full reel of it and $0.70 1W LED from China was too tempting. Everything was shrunk down to a PCB the diameter (14.5mm) of an AA cell. Funny that the PMOS in SOT23 have 1/2 the RON of the D-PAK NMOS.

Sipex part is actual simplier than my design.

Mine need to run Spice to calculated the values to be for the particular MOSFET / transistor. i.e. Not going to spoon feed and play tech support here.

hboy007

Mon May 28 2012, 09:17AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Seeing all those fancy energy harvesting ICs from Maxim and LT, I needed to try something simple. This one is a blocking oscillator built around a current compensated filter choke. Oscillation starts at around 170mV (well within the voltage range achievable with body heat).

Dri0m

Mon May 28 2012, 01:00PM

Registered Member #4497 Joined: Thu Apr 19 2012, 12:53PM
Location: Behind you
Posts: 62

hboy007 wrote ...

Seeing all those fancy energy harvesting ICs from Maxim and LT, I needed to try something simple. This one is a blocking oscillator built around a current compensated filter choke. Oscillation starts at around 170mV (well within the voltage range achievable with body heat).

schematic should be nice :)

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