We love close-up pictures of intricate work, and [w] hits the spot with a tiny joule thief in a fuse case (social media post, embedded below) powered by an old coin cell from a watch. It’s so tiny!
A joule thief is a sort of minimum-component voltage booster that can suck nearly every last drop of energy from even seemingly-drained batteries, and is probably most famously used to light LEDs from cells that are considered “dead”.
Many joule thief designs feature hand-wound coils, which is great for junk box builds but certainly becomes more of a challenge for a tiny build like this one.
We really like that [w] salvaged a miniscule coil from an Ethernet transformer, most of which look like blocky SMD components from the outside but actually contain tiny coils.
The joule thief has been the basis of plenty of hacks over the years, and it’s always nice to see new twists on the concept.
Miniature 'Joule thief' in a fuse case powered by a 10 year old watch battery 😈 pic.twitter.com/flJHggDRc8
— w (@WFrwrd) August 7, 2024
How efficient is a Joule Thief exactly?
Sure, it can run off of a “dead” battery, but the amount of energy remaining in the battery is less than 5-10% extra. If the running efficiency of the circuit is less than 90% you end up losing more energy than gaining.
I’ve heard 50-60%.
It doesn’t matter because you can us cells that were already “dead” in their original application.
Sounds about right.
Probably depends strongly on the transistor used and how much time it spends in the linear “I’m acutally a resistor in disguise” mode.
But the spirit of a Joule Thief isn’t about efficiency (the intensive margin) but instead about making use of something you wouldn’t have otherwise (the extensive margin).
When you have more pie, you don’t worry as much about each slice. :)
Back on topic: this thing is beautiful.
If you’re building a light, you’d be tempted to start with full batteries with the idea that the circuit would run for longer, but in reality it runs for a shorter amount of time.
I’ve seen some flashlight designs that employ the circuit to operate 3 Volt diodes out of two AA batteries, because the voltage soon drops below the 3 Volts needed to run the diode, which is a fair point.
only if you use new cells in it
But sure the wasted efficiency is better then 90% of nothing? Those dead batteries would end up in the trash otherwise is generally the thinking?
Or you could string the “dead” batteries in series and drive the LED just fine that way. No need for this circuit in particular unless you’re adamant in working with just one or two cells at a time. Four AAs in a pack would work down to 0.75 volts per cell, which is for all intents and purposes truly empty.
The point is that if you’re making a light, someone’s going to pop in fresh AA batteries because they’re out of dead ones.
Is this an US fuse or an European fuse? It looks like an US fuse (6,3×32), but I’m not exactly sure.
I’m wondering, because our European fuses (5×20) are smaller and more of a challenge to work with.
The US type is often being used in CB radio power cords, that’s were I know them of.
That is weird, considering the US fuses only need to work at half the voltage.
The length to diameter ratio suggests a US fuse, as its length appears to be more than 4x the diameter.
Fuses burns from joule effect. Joule effect scale on the square if the current. Since in US the voltage is halved, the current must be doubled for a given power. This is coherent with bigger fuses.
Your remarks remind me of a recent event. A storm knocked out power to my neighborhood, causing two fuses on a nearby pole to blow. After 4 hrs in the dark, the lineman showed up, replaced the fuses, and we had an interesting chat.
The fuses in question were open-ended ceramic tubes, about the diameter and length of a standard #2 pencil, with a thin, stranded pigtail—the extension cords at my house have more robust wire, mind you, and these fuses are only rated to 10 amps. So what gives?
Well, line-to-line voltage is 13.5 kv…. times 10 amps… times 3 phases… and you realize those tiny fuses can deliver ~400 kilowatts!
Indeed and the reason why high power lines have high voltage and low current is precisely to minimise line loss in the form of heat.
A few comments.
I live in a rural area and I build a spiral pump to move water from the creek up hill and over to to my shiitake mushroom bolt stack. I had a piece of well pipe over them with holes drilled in it that could drizzle water over the logs instead of either placing them in an old horse troth that had to be filled up with 5 gallon buckets, manually from the creek, or my second innovation was putting long stainless screw eyes in the end of the logs, running a chain through them and putting the logs in the creek. Nothing more fun that on a hot and humid day having to drag a couple hundred heavy, slimey, and slippery logs up the hill and stack them. So I was showing off my wonderful invention to a pal of mine who is an engineering student, and while he was impressed with the build he pointed out a lot of inefficiencies. I handed him a 5 gallon bucket and told him to go fill it with water, and repeat that about 60 times. He suddenly decided that the efficiency was not really that bad. Not to mention there were no expensive or hard to find parts in the build. 100% stuff anybody can get at a halfway good hardware store. The whole thing about efficiency and free energy only matters if you are space limited. Like a big solar plant may want to upgrade when they can get 5% more out of the newest panels. For a lot of us, if we can get the old ones for free or next to free, that is like manna.
Also, the neatest thing I have seem along the joule thief lines is a car jump pack that is powered by your dead car battery. You put it on and let it sit anywhere from a few minutes to a few hours and you get 5 seconds of cranking time out of a “dead” car battery.