Indoor solar powered joule thief - Page 1
Forum section: electronics / beginners

author: surveyranger
author_rank: Newbie
timestamp: August 27, 2021, 09:05:53 pm
content: First time posting here so I hope I picked the correct forum. I did a forum search for joule thief, but didn't find a similar circuit in the 220 search results. I'm a hobby electronics enthusiast and a big fan of the EEVBlog YouTube videos.I put together a solar powered joule thief circuit from through hole components I had (1st picture). It consists of 3 x BPW34 photodiodes, 100uF electrolytic capacitor, 2 x 470uH axial inductors, 2n2222, and a red LED. I measured the open voltage of a BPW34 at .4V, and the loaded voltage of all 3 combined under the same conditions is .463V (as measured with a Fluke 27 DMM). The intent is to use this for a small indoor project. I don't have an o-scope or function generator, but I have a healthy bench stock of components.The joule thief works in the sense that the LED blinks in low light, and is dimly lit in bright light. I would like for the LED to be brighter. I experimented with switching out components. I found that the circuit will not work with a capacitance below 47uF, and blinks slower as the capacitance gets larger. The axial inductors looked like they performed the same at 220uH, 330uH, 470uH, and 680uH. However, the base and collector inductors had to be the same size. The inductors are currently separated by an air gap because the LED will not light up if they are touching. I've experimented with arranging the inductors next to each other to see if that would change anything but with no luck.Also, I tried adding another capacitor in parallel with the LED along with a Schottky diode, but that caused the LED not to light up at all after I left the circuit in bright light for an hour. Then, I added a 1.2V 80mAh NiMH battery in parallel with the LED along with a Schottky diode to prevent battery discharge back into the circuit (2nd picture). The LED did not light up either in that configuration after I let it sit overnight.Any ideas how to make the LED brighter using minimal components? I'm trying to keep the footprint small so I'd like to avoid a toroid inductor if possible.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 28, 2021, 03:42:36 am
content: I thought a joule thief used a transformer in some clever manner, not sure how/if it's supposed to work with two inductors.

author: amyk
author_rank: Super Contributor
timestamp: August 28, 2021, 05:22:41 am
content: What's the maximum power you can get out of the photodiodes? That will be the ultimate limit of what you can supply the LED. A joule thief is just a self-oscillating boost converter, and unless carefully optimised, won't be very efficient.

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2021, 11:58:52 am
content: Yes, you need a transformer, rather than two inductors.If it's more convenient to use an inductor, two transistors can be used with one inductor.https://wiki.analog.com/university/courses/alm1k/circuits1/alm-joule-thiefhttps://circuits-diy.com/simple-joule-thief-circuit-using-bc547-npn-transistor/

author: surveyranger
author_rank: Newbie
timestamp: August 28, 2021, 02:19:18 pm
content: Quote from: amyk on August 28, 2021, 05:22:41 amFor each BPW34 photodiode, I measured the open voltage at .4V and the short circuit current at 9.5uA under the same conditions. The datasheet showed that the tested short circuit current was 47uA so I'm far from that.I built an oscillating circuit using 2 transistors and no inductors, but I couldn't get it to work with voltages below 1.4V.Quote from: Zero999 on August 28, 2021, 11:58:52 amLike I said originally, trying to keep the footprint small. Transformers and wound toroid cores are too bulky unless you know of some small ones. Thanks for the links. It'll take me awhile to make it through the in depth write up on the first one. I have read the second article previously.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2021, 02:57:48 pm
content: You can buy toroids down to a few mm, and much smaller have been made in the past, though I don't know how available they are these days (not that you'd want to try and thread them by hand, anyway!).You can get some pre-wound, like Ethernet transformers. You may even have some handy to salvage. They're often found in Ethernet connectors, or nearby on the board. Follow the traces or ohm out the windings to see where everything is.They do saturate at some 10s of mA, so don't expect much power before efficiency tanks. But that shouldn't be a problem here.Other pulse transformers are fine, and plenty abundant; they'll usually be on the larger side (10mm+ say).You do need at least enough bias current (from the solar panel) to bias the transistor into amplification. In my experience, some ~µA should be adequate, at least with a good transistor, and even that was with much smaller inductances. (Years ago, I made a highish power JT, using a single NiMH cell to drive a 1W LED. I can touch my finger across the power switch, charging the base with what should be some ~µA, causing the LED to flash once every few seconds.)FYI, the basic circuit is a blocking oscillator:Rbias and Cbb set a time constant, how often the pulse repeats. Operation can be continuous, burst or single pulses, depending on this time constant, and relative to the inductance and hFE, and how well damped the inductor is. Typically Rb is zero or nearly so; it mostly seems to perform worse with much value. The P:S turns ratio is given by the supply voltage; you need at least on the order of Vbe (~0.5V) at the base, from whatever full supply voltage is. As shown, the output is a spike waveform, which is rather dangerous for the transistor and of course you simply clamp that with a diode to obtain a useful DC output, as you've shown.In my case, +V = 1.2V (down to 0.7V or so, somewhere below which it stops working), Rbias = 100 (continuous operation; as high as some ~MΩ evidently still works, at a very low repeat rate), P:S I think is close to 1:1 I forget anymore; and the transistor was PBSS303NX or something like that.In your case, with even lower +V, a 1:1 ratio is probably fine, might even be better with 1:2 (higher base drive voltage), and I suppsoe Rbias doesn't matter much since you want it to start as early and often as possible.Note that, given the latter assumption, it'll tend to hold the supply at 0.6V or so, so match that to the maximum power point of your photocell(s). I suppose two cells might be better than three (or rather, as good as, because the extra voltage is wasted), but if your ambient is quite dim indeed, three might be necessary (in which case, they should also be amorphous type?).I think it should still pulse even for quite weak currents (i.e., ~µA), which should be available even from quite small photocells.Or, indeed, photodiodes in daylight-ish, I suppose...(Oh, a final note about voltage: Vbe(on) is variously maybe 0.4 to 1V+, depending on load current and temperature. At such low currents, you may find it's starting quite low, maybe 0.5-0.6V. It will be lower at elevated temperature, too; though leakage current, of the transistor, capacitor and photodiodes, will also be worse at high temperature.)(Oh, leakage might be a good reason not to use the electrolytic capacitor -- not much is needed, probably more than 0.1uF is a good idea but beyond that, it's probably equivalent to the Cbb above, where you'll get more of a tone-burst operation than a single-pulse operation at larger values.)Tim

author: JimRemington
author_rank: Regular Contributor
timestamp: August 28, 2021, 03:12:35 pm
content: Big Clive shows how to make a Joule thief with just about the tiniest possible transformer here:http://www.bigclive.com/joule.htm

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2021, 07:18:16 pm
content: Quote from: surveyranger on August 28, 2021, 02:19:18 pmA transformer is definitely smaller than two inductors. It doesn't need to be toroidal.If you really need to save space, use one of the single inductor circuits I linked to above and a dual transistor package.https://www.onsemi.com/pdf/datasheet/mbt3904dw1t1-d.pdfhttps://docs.rs-online.com/fd4a/0900766b8060b636.pdf

author: surveyranger
author_rank: Newbie
timestamp: August 28, 2021, 07:50:57 pm
content: Quote from: Zero999 on August 28, 2021, 07:18:16 pmThanks! Hadn't considered a dual transistor package. I'll look into it. I breadboarded the single inductor/dual transistor circuit and found it works down to .6V when I increased the inductor to 1mH instead of 100uH. Still too high of a supply voltage for the BPW34 photodiodes, but I have small-ish solar cells that I will try.As for the transformer, the smallest gauge wire I have is 24ga that I got from ethernet cable. I'll experiment with that to see if a couple turns on a solid core will change anything in the original circuit.

author: surveyranger
author_rank: Newbie
timestamp: August 28, 2021, 08:39:28 pm
content: Quote from: T3sl4co1l on August 28, 2021, 02:57:48 pmI'll keep my eye out for some. Googled some pics and they look pretty small.Quote from: T3sl4co1l on August 28, 2021, 02:57:48 pmI saw some pulse transformers on Mouser. They're about 10mm x 9mm.Quote from: T3sl4co1l on August 28, 2021, 02:57:48 pmI seem to have enough bias current with 2-3 of the photodiodes. I'm using a 2n2222 transistor that seems to well enough at low current.Quote from: T3sl4co1l on August 28, 2021, 02:57:48 pmGonna take some time to digest this part.

author: JoeRlDuncan
author_rank: Newbie
timestamp: May 18, 2022, 09:58:23 pm
content: Hey, I saw the red warning about 120 days plus, but this post isn't even a year old yet so it's hardly a necro.I figured the op might appreciate some answers though, if not and I am making a faux-pas just kindly delete this post (mod).Quote from: surveyranger on August 27, 2021, 09:05:53 pmYes, actually, a couple. I've built several Joule Thief circuits myself.First off to the naysayers: a transformer is *literally* just two inductors conveniently packaged side-by-side for you. Placing two axial aligned inductors (the little seafoam green dipped kind) side-by-side literally creates a transformer.That said, here's some advice that might help you Ranger:1) the impedances you are using are too high - for a simple AA battery JT, I've found 100-150uH is the range you need to be using. You're literally putting up too much resistance for your power source2) your switching activation voltage is also high, a 2n2222 saturates at like 1.3-2.6 volts, try a 2n3904 which has a Vbesat of like 0.6 to 0.8 or something. You can also try germanium transistors, which can theoretically operate on as little as 0.3v (although the one I built from an old soviet GT320B pnp peters out at about 0.4 v practically)3) try removing the smoothing cap entirely, it isn't really necessary in most cases I've found (at least with 1.2V AAs & AAAs - in your case as long as the light is relatively steady second by second though it shouldn't make much difference)4) although red LEDs have the lowest fwd voltage, red light is not very well perceived by the human eye, the best trade-offs of power to brightness perception is with green or white LEDs, fwd load voltage isn't the main problem though, as your mini transformer from the two inductors will step it up - you're eating too much current with your other specs being too demanding, not leaving enough for the LED, and spending what little you have on light it's harder for us to see.Good luck and let me know if any of that works!