Author Topic: Joule Thief with Zero Voltage Depletion Tranny  (Read 1213 times)

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Offline UndweeberTopic starter

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Joule Thief with Zero Voltage Depletion Tranny
« on: August 14, 2019, 12:33:57 am »
I’ve decided to revisit the topic of joule thief’s and while reading through some lists I’ve found some suggesting the use of zero depletion mosfets (transistors?).


The explanation if I have understood correctly (tell me if I didn’t) — that depletion mosfet would be allow the joule thief circuitry to fire up at low voltages, for example my joule thief circuit required 1V to fire up but could stay on down to 0.4V, with depletion or zero volt depletion it would allow the circuit to fire up at lower voltages — HOWEVER there was some sort of disadvantage, cannot recall which one, but maybe the one where it cannot operate at higher voltages, so you’d then need two circuits with a logic switching mechanism to start the circuit with one and run it on the second one(???).


If you can recommend one of those LED chip boost converter drivers — it needs to exceed capabilities of the simple joule thief, like automatically regulating resistance to provide constant voltage, and being able to fire up at low voltages ~0.5V and handle 2.7V


Goal is to run it on supercapacitors, I just ordered a few 10F ones because my 350F was way too big.
« Last Edit: August 14, 2019, 12:39:29 am by Undweeber »
 

Offline T3sl4co1l

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Re: Joule Thief with Zero Voltage Depletion Tranny
« Reply #1 on: August 14, 2019, 02:59:08 am »
Right, you can run a JFET oscillator from thermocouples (<< 1.0V).  Depletion MOS may work as well.  You can't really get any power from it though.  Mainly because no one makes such a transistor wide enough to handle appreciable current.

If we're talking batteries, it's less than pointless as the amount of energy remaining in the already-dead cell is minuscule.   Ditto with supercaps; from 2.5V down to 0.7V, you've consumed fully 92% of the total energy.

TEGs (including thermocouples, or Peltier modules in a thermal gradient) are somewhat useful*, and may generate low voltages in the process, for which a converter is required.

You're better off using such a circuit for a bias supply, then using that to power a controller that does the heavy lifting with a conventional approach (e.g., controller IC + switching MOSFET + output diode).

*Mostly if you happen to have access to a few pounds of Pu238 oxide pellets, as NASA does.  RTGs are very robust and reliable power sources, if terrifically inefficient (a few percent or so).  Of course, here on Earth you're better off, say, hooking up a Stirling or steam engine, or where fuel is involved, any of the usual types of combustion engine.

Tim
« Last Edit: August 14, 2019, 03:00:59 am by T3sl4co1l »
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