2016-01-19 Weird Joule Thief With No Resistor

I found this on YouTube.  It has no resistors, no capacitors, just two transistors and two inductors.  And and a 1.5V cell and LED of course.

I redrew the topsy-turvy schematic that was shown at the beginning.  It was confusing with the minus on the left and plus on the right.  It makes more sense as I show in the picture (it should say p=10990 at the bottom).

When I look at this, I see a Joule Thief with a single transistor, the PN2222A.  The schematic says 2N2222A, but if it’s a plastic case, it is a PN2222A. The BC557B transistor, as shown, has the base connected to the collector with a very low resistance.  The 1 mH inductor is typically from less than an ohm to a dozen or so ohms, depending on its construction, so to the BC557 it acts as a DC short.  As shown in the original schematic, it is separate from the 220 uH inductor.  The BC557B has the base and collector tied together, so it acts like a diode between the emitter and other two leads.

This means that as the battery voltage goes above 1 volt, this ‘diode’ and the base to emitter junction of the PN2222A start to conduct.  The battery voltage goes up to 1.4 volts for a charged cell, and these two transistors conduct heavily, with an excessive amount of current.  There is no resistance to limit the current, so the two transistors may overheat and be damaged.  There should be a resistor between the BC557B and the base of the PN2222A to limit the current.

As shown, the two inductors are separate.  But there would be no feedback and no oscillation if this were so.  The two inductors must be close together and oriented correctly for it to oscillate and make light.  If they are not, and the Joule Thief refuses to oscillate, then there is nothing to prevent the circuit from drawing excessive current, overheating, and further damage.

I would connect a few hundred ohm resistor in series with the BC557’s collector and the base of the PN2222A. It might help prevent excessive current. And the battery voltage should never, ever exceed 1.5 volts.

5 Responses

  1. Themba says:

    Judging by your blog you’re a real expert on joule thieves. Could you help me with some advice on a hobby project I’m designing? If not, it’s okay to tell me off and ignore the rest of this message.

    My project will involve 27 separate nodes, each of which will contain a battery, an attiny mcu, an led, and a sensor (not important for this question). The attiny should wake up about every 15 minutes, read the sensor and determine whether the led should be on or off, toggle the led if necessary and go to deep sleep again. I know that in this setup the attiny and the sensor will consume negligible power compared to the led (which will often stay on for long periods of time), but they do require a stable supply of at least 2.7V but preferably something in the range 3.3 to 5V.

    I’m looking for ways to keep these things running for periods of weeks (months if possible but I realize that weeks is already going to be challenging).

    The main reason I’m looking into joule thieves is to bring cost down since batteries are going to be the most expensive part in this project. I know that I could put in 3 fully charged AA batteries and get a good 3.6V supply that will probably last for weeks if I use a high enough current limiting resistor on the led… but batteries are expensive and if I could achieve this with fewer batteries or get longer operating times out of the same batteries that would mean a big difference on the cost.

    I see many different joule thief circuits out there and it is unclear to me which is best in terms of efficiency and will also enable me to run the attiny for a few milliseconds once in a while. Do you have any ideas after reading this story?

    Thanks for your time,

    • admin says:

      There are several things that could help, but I don’t believe a Joule Thief is one of them. A good power source is a battery holder with 3 AA or AAA cells, which is 100 percent efficient. Joule Thiefs are much less than that, only about 50 to 60%.

      The LED is the problem. You can get a bright red or green LED that is visible with 1 milliamp of current. That should make the battery last a long time. But if you can flash the LED, say at 5 mA for 1/10 of a second on every second, that should be brighter and draw half as much average current.

      You say 3.6 volts for 3 cells, which I assume means they’re rechargeable. If so, conventional Ni-MH cells lose 1% of their charge per day, which means they become discharged after a month or two. If you use the Panasonic Eneloop cells you won’t have this problem, but they are more expensive. An alternative is to recharge them with solar cells.

      If you contact me at my yahoo email address acmefixer I can give you the info to contact a guy who may be able to give you more info, especially MCUs. I apologize for not seeing this sooner, but I check the comments about once a week and I can’t explain why this one didn’t show up sooner. I hope to hear from you.

  2. Kulwinder says:

    Ok, so I want to learn about how digital camreas, specifically mirror less or EVIL camreas. The step by step process in making a camera, such as which wires go where, how to physically construct the camera, where to attach the shutter button and how to attach it, how to install a sensor (advanced stuff)Where would I go to learn this information? (on the internet) I am in high school and therefore cannot take any college classes Thanks in advance!

    • admin says:

      The instructables.com website has a lot of fun projects, but I don’t know if they have any for cameras. Most camera makers will sell you a schematic, IPB (illustrated parts breakdown) and repair manual, but it may be expensive and they may only sell to a repair business.

  3. ramon says:

    one joule thief with battery 3.7v and a light bulb 220v

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