Low voltage Joule thief - exotic transistors? - Page 1
Forum section: electronics / projects

author: ivoras
author_rank: Contributor
timestamp: August 27, 2018, 06:58:53 pm
content: Most of the Joule thief designs I've found seem have a lower voltage limited by the Ube voltage of a transistor, and that won't go lower than about 0.6 V if lucky. I've searched around and found that there are germanium transistors which have Ube at 0.3 V or lower, but they also seem rare and expensive.So, two questions:#1: since I've never even seen a germanium transistor, can I just plug in one instead of the silicon NPN transistor in the Joule thief and expect it to work? Some of the germanium ones seem to have a surprisingly low maximum limit on Uce - is that a general rule?And #2: can someone recommend such a transistor, or an alternative way to make a very low voltage Joule thief, preferably with through-hole components?

author: JimRemington
author_rank: Regular Contributor
timestamp: August 27, 2018, 07:10:39 pm
content: A MOSFET can oscillate with supply voltages of less than 50 mV, and this has been used to develop nanopower "energy harvesting" boost converters that work even with thermoelectric generators.For an example, check out the LTC3108 athttps://www.digikey.com/en/articles/techzone/2011/dec/ultra-low-voltage-energy-harvester-uses-thermoelectric-generator-for-battery-free-wireless-sensorsBreakout board is available at Banggood, but it is clearly missing the required transformer.https://www.banggood.com/CJMCU-3108-LTC3108-1-Ultra-Low-Voltage-Boost-Converter-Power-Manager-Development-Board-p-1321584.html

author: ogden
author_rank: Super Contributor
timestamp: August 27, 2018, 07:23:13 pm
content: Right. Start-up voltage shall not be confused with operating voltage. Some (voltage boost) circuits can operate down to very low voltages, but require significantly higher voltage for start-up.

author: mzzj
author_rank: Super Contributor
timestamp: August 27, 2018, 07:50:19 pm
content: Quote from: ogden on August 27, 2018, 07:23:13 pmTrick with the mosfets is to use DEPLETION mode mosfet that is conducting already with zero volts VGS

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 09:02:14 pm
content: 0. Most JT designs are crap. The most commonly seen scheme is a ferrite bead with some wire on it, feeding the base of a transistor through a 1k resistor. This is the worst way to do it, electrically speaking. Note that no one ever measures efficiency, they just make it light up an LED from a dead AA cell then throw it on the shelf to get dusty.1. And speaking of efficiency, and metrics like that -- what do you hope to accomplish with a much lower startup voltage?You can start from a heavily depleted alkaline cell, for example, but there's maybe 5% of full charge left in the poor thing. You're certainly not going to get much light out of it, by intensity or by duration. Better off throwing it out and putting in a fresh one (which will gladly start things at 1.5V, where you don't need to worry about Vbe).Or if solar cells, why not use more, smaller cells? Same size, same area, different voltage and current.Or if a TEG, you're barking up the wrong tree anyway, as you'll need quite high current capacity to do the job -- not something you'll find in BJTs.Yes, germanium will do a fine job, but mind that most parts you'll find (if you do go looking for old-stock parts) are either small-signal transistors that can't handle hardly any current, or power transistors that are dreadfully slow (which doesn't rule them out for switching duty, but you'll need a much larger inductor). Ideally, you'd find a late model part with, say, 100mA Ic, 30V Vce, and fT > 20MHz.2. Startup.Si BJTs will keep running to fairly low voltages if designed for it, but won't start up.JFETs can start in the 10s of mV range, but aren't worth a piss of power. Similarly with MOSFETs.The most general method is to build a small startup circuit that kicks the main circuit into operation, using conventional logic and gate drives and beefy transistors however it does. This is obviously a little more complex than a transformer, transistor and LED.Tim

author: ogden
author_rank: Super Contributor
timestamp: August 27, 2018, 10:09:39 pm
content: Quote from: T3sl4co1l on August 27, 2018, 09:02:14 pmIn single word: impractical

author: Wolfgang
author_rank: Super Contributor
timestamp: August 29, 2018, 07:21:11 pm
content: Hi there,I played a little with low voltage inverter circuits using different Germanium and Silicon transistors.https://electronicprojectsforfun.wordpress.com/energy-harvesting/In a few words, I found this:- Germanium has the lowest startup voltage *with almost no load*- Even at lighter loads and more with heavier loads, the best transistors areZTX851. They have 5A in a TO-92 case, high gain at high currents, plus anincredibly low CE saturation voltage. They were made for photoflash inverters.- Efficieny at 0.7V can be up to almost 70% at light loads (unregulated)I also did experiments with ultra low voltages (0.1 to 0.2V) . Here you need either depletion modeMOSFETs or JFETs. This works, but efficiency is terrible (a few %), and on my standardtransformer the output voltage was still too low to be useful (0.5Vpp).