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Even though I'm not an EE, I have simulated the Joule Thief to death in LTSpice, but I have not yet made a prototype of this circuit. I have a friend who is an EE who won a contract to light some areas in the third world, like refugee, kind of areas, and I would like to propose my circuit to them, but I don't want to propose a bad circuit, or for it to have any glaring flaws, etc.

Unfortunately, I'm guessing that the circuit may be too expensive with the custom coupled-magnetics piece that will be required, but I don't know how much they cost. I see them in cheapo USB 120V=>5V,2A wall adapters / USB chargers, and bug zapper circuits, so I really don't know.

Please let me know if you think this will work. I somehow think this should have been invented already, but haven't seen this before, so perhaps there's something wrong with it? Thanks ahead of time for letting me know what you think, how it could be improved, are there any flaws, etc. Or I might need to add missing parasitics, or could benefit from using a different spice / other simulator.

enter image description here

Edit 1

I am aware that a linear solution may work here, and it's still one of the options being considered. We still may need this design in this, or other, circumstances.

To clarify a little...

Overarching goal is to make people feel safer, and be safer. This is just a small part, and only represents lighting the inside of the outhouses. We must light outhouses internally to make people feel safe, and be able to perform standard toilet tasks, like monthly menses, and go to the bathroom. Well-lit provides a feeling of safety in addition to some real protection. If they don't feel safe, they won't use the facilities, which creates problems. So, we must protect women and children from predators of all kinds, if you know what I mean. (Think refugee camp.) Therefore, the lighting must maintain brightness all night, summer and winter. And to protect women and children during midnight outhouse trips, lighting must be even and reliable (but also efficient and long-lasting and cost-optimized). While this is a specific design, please, if you would, also evaluate this circuit in the general sense, for other uses in other projects.

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    \$\begingroup\$ Is [almost] any circuit required? LEDs might simply run from LiFePO4 with nearly no resistance at all. The light output will be variable, and perhaps temperature runs high at full charge, or lifetime isn't so reliable, or charge use is shallow, but these might be acceptable compromises in such circumstances. \$\endgroup\$
    – Tim Williams
    Commented Jan 30 at 7:11
  • \$\begingroup\$ @TimWilliams -- Duly noted, and a linear solution has been on the table (for this configuration). But this design may be needed in other circumstances, and so the question still stands. Your comment now answered in question. \$\endgroup\$
    – MicroservicesOnDDD
    Commented Jan 30 at 9:00
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    \$\begingroup\$ What is it's benefit over other designs that do the same thing. It's that that is important. \$\endgroup\$
    – Andy aka
    Commented Jan 30 at 9:34
  • \$\begingroup\$ Looks very expensive compared to just a linear current regulator \$\endgroup\$
    – winny
    Commented Jan 30 at 10:28
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    \$\begingroup\$ I thought a Joule Thief was to drain a battery down to zero. That's a bad idea for a lithium cell you plan to recharge, and why the circuit should have a cut off below about 2.5V. I'm not seeing that in this circuit. \$\endgroup\$
    – user1908704
    Commented Jan 30 at 12:38

2 Answers 2

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Your application needs light at night and not at daytime so why not look at cheap solar garden light which have very simple circuit

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  • \$\begingroup\$ That's a great idea. But we'll want our product to last much longer than these products typically last. So we'd probably want to use one of the discrete versions that can be found, and then make it our own. Once again, great idea! \$\endgroup\$
    – MicroservicesOnDDD
    Commented Jan 30 at 13:46
  • \$\begingroup\$ The products that dont last have bad waterproofing .Also the Nicad cells have a short life .Adress this and you will be fine . \$\endgroup\$
    – Autistic
    Commented Feb 3 at 10:12
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You can make a self-oscillating buck converter LED driver very easily by using just one inductor + feedback winding.

The important part is to determine the value of C1. Its capacitance has to be such that the voltage on the right side of C1 measured against ground (Gnd) has a nice slope during the ON cycle (blue waveform), which means that the transistor is quickly turning off.

The green current through inductor L1 (and the LED) ensures low losses, since all current from the charged inductor (coil) is transferred to the LED.

Image of Self-Oscillating Buck Converter and LED driver

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  • \$\begingroup\$ Thank you. I was unaware of this circuit. Where did you find it / learn it? \$\endgroup\$
    – MicroservicesOnDDD
    Commented Jan 31 at 1:10
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    \$\begingroup\$ @MicroservicesOnDDD Nowhere. I just realized it can be done this way 4 years ago. I tried to build it, even with feedback, it worked. Maybe someone did the same, who knows. \$\endgroup\$
    – Michal Podmanický
    Commented Jan 31 at 1:32
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    \$\begingroup\$ I tested it by supplying from 2m2 standalone capacitor charged to 40V. It had a very good performance, i.e. it last very long time. The biasing was different of course. \$\endgroup\$
    – Michal Podmanický
    Commented Jan 31 at 1:39

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