Inductor, ferrite bead wire coil for a "joule thief"

Discussion in 'General Electronics Chat' started by Garoad, May 7, 2012.

  1. Garoad

    Thread Starter New Member

    Apr 24, 2012
    Other than the amount of inductance, what is the difference between an inductor component like this.
    ...and a wire coil around a ferrite bead or magnetic torroid? (Something like what this guy has on this page.)

    For some background (why I ask) - I'm starting out learning basic electronics (my day job is software developer) and my first long term goal is to create a string of battery powered multi-color LED christmas lights which fade in and out gradually. In the process of researching relevant topics, I came across this concept of a "joule thief" which allows an LED to run off of a nearly "dead" battery. Given that I'd obviously like to maximize battery life, assuming this concept can be scaled up to many more than 1-2 LEDs, it "sounds like a good idea" (at this time :p ).

    I haven't actually started doing any actual work on THIS project yet (I realize the importance of starting small and learning the fundamentals first) but in parallel (no pun intended) I'm doing some research to provide myself some direction and a goal to work towards.

    Still not sure about the feasibility of the overall project ("battery powered multi color fading-blinking 30-50 LED joule thief"... whew), but any side comments or suggestions are welcome too.
  2. Bernard


    Aug 7, 2008
  3. acmefixer


    Aug 4, 2011

    Working backwards. Assume 30 LEDs, at 10 mA apiece. On continuously. That's 300 mA or about 1 watt. to the LEDs. A single AA cell is going to last for a very short time at that power level, without including the wasted power of a Joule Thief, which is only about 50 to 60 percent efficient.

    Fading LEDs are on for about half the time, so the battery is still not going to last for very long. Here's a vid of my LED fader. It needs a few volts more than the LED forward voltage, say 6 to 9 volts.

    If you use something like my Blue Blinky which flashes the LED on for only a small percentage of the time, the battery life will be greatly extended. My Blue Blinky will flash for more than a year. With 30 LEDs on a single fresh AA cell, maybe for a week or two.

    I suppose one could connect multiple LEDs in parallel to the Blue Blinky, and divide the current between them. Then the battery life would be extended at a sacrifice in brightness. I would try paralleling 4 LEDs, and use 8 Blue Blinkys for a total of 32 flashing LEDs. One could also wind a lotta turns onto the Blue Blinky's toroid and feed the output directly to a string of LED xmas tree lights. That would save a whole lot of time wiring them in parallel or whatever. But I guess this depends heavily on what you're going to do with the LEDs. If they are going to be installed in a heart shaped piece of hardboard, for Valentine's day for example, then obviously you're not going to want a whole bunch of xmas tree light wiring hanging down from the back.

    In any case, you're going to have to make a choice between battery life and light output. I would go with the flashing LEDs since they can be much easier on the batteries. I would avoid Joule Thief circuits because they waste a lot of power. I would use a simple two transistor flashing circuit and several AA cells in series to get the required LED voltage, and battery lifetime. But all this is up to you.

    The answer to your Q about inductors is: the difference between the choke which has a single winding and the toroid which has two windings is that the choke can't be used with the conventional JT because it lacks the second winding. I have wound a second winding on the outside of a choke, but the wire is very thin and difficult to work with. It's a lot easier to wind a toroid. You can use a coil made with two 16 foot or 5 M lengths of wire wound around a AA cell, then remove the cell and tie the coil with wire ties. Visit for some interesting circuits like that or check out his Instructables.
  4. Garoad

    Thread Starter New Member

    Apr 24, 2012
    Thanks for the great answers. I had no idea a joule thief was much less efficient, but I guess that makes sense since it'd probably be used all over the place if there were no downsides. So yeah, I'll just plan on a regular string of LEDs that either blink or fade.

    For my project idea, it was going to be used for the office at work, so I was planning on using at least 4 D cells @6V (or as many as I need in series/parallel to get the voltage/life I need). Basically, we aren't supposed to plug in Christmas lights without approvals (which is a hassle), so I get around that rule by using battery powered lights.

    I actually have a few commercial battery light sets (some are LED), but they're kinda boring and I'd like a longer run of lights. The non-LED sets also don't last long enough, of course. Some of the LED sets last surprisingly long (about a month) since they're only on when I'm around, so I figure I should be able to do at least as good with a longer run of fading or blinking lights running off larger (or more) batteries.

    The LED fader looks awesome - that's exactly the effect I'm going for! (Now, hopefully I can stick to this goal and not give into the temptation of "upgrading" to alternating blinkers or random fader LEDs or something more complex...)