High Efficiency Regulated Joule Thief




Introduction: High Efficiency Regulated Joule Thief

About: Thank you all for following me.

     Normally the Joule thief produces output voltage, which value is difficult to predict. Without load (the LED) I have measured voltages over 30 V. I wanted to create a Joule thief, which can be used to supply some small electronic devices, but having well defined and stable output voltage. There are known some solutions in which instead the LED load, a one-diode rectifier is used, and the output voltage is stabilized by the use of Zenner diode. I did not like this solution, because through the Zenner diode flows always a constant DC current, what  reduces drastically the efficiency of the device and empties fast the supply battery. I was looking for other, better solution of the output voltage stabilization (limitation).
     To try my solution, at first, I needed to build the standard working Joule thief. How to do this - there are a lot of articles and internet sites (for example this). How to find the needed parts? - I knew that inside the high efficiency lighting bulbs are some parts, which could be re-used. I had a defect bulb and I carefully cut the plastic box. From there I extracted the voltage converting board. On these PCB's can be found some very useful stuff : HV diodes, chokes, HV capacitors, HV transistors..etc (HV means high voltage ~ 400V). I took the ferrite toroidal transformer, cut and removed all its wires. After that I disassembled the choke. I took around one meter enamelled wire from it, and winded it around the ferrite bead. Because the wire was fold, I winded simultaneously two coils having ~ 50 turns. Having the main part of the Joule thief (the transformer) ready, the remaining work is not much.. The only tricky in the design is to connect both coils in the correct way. (see the mentioned link for additional information). So designed the Joule thief was able to produce 34 V voltage measured on the collector node of the NPN transistor (2N2222) without any load, when supplied by 1.2V AAA battery (filterd with 2.2uF capacitor).

Step 1: The Regulating Circuit

         After finishing the original Joule thief, I have created the additional circuit, which controls the output voltage. It is based on the use of bipolar transistor Schmitt trigger. Its input is connected to the middle point of variable resistive voltage divider placed between the regulated supply voltage and the common ground node.The output of the Schmitt trigger connects to the gate of NMOS switch transistor. The drain terminal of this switch connects to the base of the Joule thief oscillator transistor. The principle of work of the control circuit is following: After start up, the supply voltage increases with the time. This voltage is produced by simple half wave Schottky diode rectifier with capacitive filter (the 3.3 uF 350V capacitor). The input voltage at the Schmitt trigger input (which is part of the supply voltage) increases also proportionally to the supply voltage and the value of the variable bottom resistor of the voltage divider. When the supply voltage reaches a programmed by the voltage divider value, the Schitt trigger switches to other state thus changing its output voltage from low to high. This closes the NMOS switch, which respectively shorts the base terminal of the oscillating transistor to ground, blocking in this way the further oscillations. When the oscillations stop, the increasing of the supply voltage also stops. The energy storing (filtering) capacitor discharges slowly through the load. The output voltage starts to decrease. This process continues until the "high to low" threshold of the Schmitt trigger is reached. Then it changes again its state and the NMOS switch opens. The oscillations start again the supply voltage starts to increase again...until the threshold voltage "low to high" of the Schmitt triggers is reached...and this sequence can continue until the supply battery is empty. The supply voltage is kept in range which can be narrow (depends on the Schmitt trigger hysteresis and ratio of the used resistors in the voltage divider). Because two phases of regulation exist - active (when the oscillator works) and passive (when the needed current for the load is delivered only by the energy storing capacitor), the efficiency of the regulated Joule thief is high - it consumes energy from the supply battery only during the first phase.
      To help you better to understand the principle of work of the device, I have attached its schematic file which can be simulated by the Linear LTspice circuit simulator. Its is easy to use, free Spice simulator, which can be used for electrical simulation of different analog and digital circuits. You can download it from here. The circuit is ready for simulation, without any additional changes. You can look at the transient voltages and currents in the schematic, to change the values of the devices...and to play a lot...
      On the pictures I have presented the circuit and some voltage transients signals.

Step 2:

The list of the used parts:
  • 1x ferrite bead
  • 1m copper wire
  • a piece of veroboard 
  • 3 x 2N2222 NPN transistor - can be other NPN BJT with Beta>50
  • 1 x 2.2uF(3.3uF) 400V capacitor
  • 1 x 240 KOhm resistor
  • 1 x 200 KOhm resistor (150-250 is OK)
  • 1 x 57 KOhm resistor
  • 1 x 10 Ohm  (10-100) resistor
  • 1 x 30KOhm potentiometer 
  • 1 x BSS123 NMOS transistor (other NMOS fast switching transistor should be OK)
  • 1 x Schottky diode ( for 200mA, 40V...small signal should be OK)
How the circuit works, can be seen also here : http://www.youtube.com/watch?v=_IjBHpc2PjY

Step 3:

       Common feature of the Joule thief, supplied by a 1.5V single battery, is that is not able to deliver much of energy. It can be used to supply a LED diode/s , some low consumption electronic device...etc. It is possible to complicate the circuit, adding 3-rd coil, which can have more turns and can produce higher voltage.
       My regulated Joule thief I supply with one AAA battery. It is able to work with battery, which is ~ 0.35 V. The output voltage can vary from 1.5 to 16V. If a supply source, which must work with low supply voltages, but must not deliver big currents, is needed - the proposed circuit is ideal solution. If the supply noise (oscillations around one value) is not desired, additional LP (low pass) filter or LDO regulator can be placed. The output voltage of the regulated Joule thief can be adjusted so, that it is enough high, to allow the LDO to work in the proper regime. In this way over the pass transistor of the LDO the voltage drop should small and the energy efficiency shall be kept again high.
       To estimate the power driving capabilities of the proposed circuit, I did the following experiment:
        I have loaded the circuit with a  potentiometer. I have adjusted the output voltage to be 5V when the load resistance is 10 KOhm (0.5mA current), and measuring both the output voltage, and the flowing through the resistor current for different load resistance values, I made the following graph. There can be seen that the circuit deliver up to 3mA current at relatively stable output voltage, and further the output voltage drops down. Of course this curve is valid only for my particular implementation, and can differ a lot for different one, but it shows a common behavior, which this circuit implementation will have. The bend point ( in my case ~ 3 mA ) is the moment, where the whole energy inserted in the storage capacitor is delivered as power in the load, and the toggling of the Schmitt trigger stops.

Step 4:

After some discussions I decided to try to simplify the circuit. I found the following solution - simply removed the Schmitt trigger and connected the gate of the NMOS switch directly to the middle point of the resistive voltage divider. As mentioned before, this divider can be replaced by a single potentiometer ( 270 KOhm). With its help the division ratio can be changed, thus adjusting the output voltage. The voltage at the gate of the NMOS switch stays close the the Vth (threshold voltage) of the NMOS switch. The new schematic and some simulations results can be seen on the pictures. There can be seen that for some resistor values, the low frequency oscillation, which was caused by the Schmitt trigger transitions around its threshold voltages disappear. The output voltage is more smooth. May be with the time it will drift more than the voltage generated by the previous version (with the Schmitt trigger). For some resistor values, in the simpler implementation, can be seen also low frequency oscillation, which amplitude is not so stable, as the Schmitt trigger version.
There is a way to increase the efficiency of the Schmitt trigger version. It can be used when higher power will be needed, what means that the base resistor  will be relatively small and the DC current flowing from the supply to the ground node through this resistor and the NMOS switch during the non-active phase could be high. The solution is to break the path of the current by the use of additional PMOS switch placed in series with the base resistor,  with source connected to the supply, drain connected to top terminal of inductance L1, and gate connected to the output of the Schmitt trigger. Thus: When the base of the BJT is grounded by the NMOS switch, the PMOS switch will be open and no DC current will pas through. When the output of the Schmitt trigger has low logical state the PMOS will be closed and will connect the bottom terminal of the inductance L1 to the supply source, what corresponds to the active phase of work. To be able to use small supply voltages (<1.5V) in this case the PMOS switch must he chosen so, that it has low Ron (switch "ON" resistance) and low Vth ( threshold voltage). Normally the DMOS devices satisfy these requirements. Could be used also JFET transistor. In this case the base resistor could be even omitted. 
As conclusion - both versions can be used for applications, where the supply noise is not important. The output voltage can be set to some defined value by the use of the potentiometer and will have some oscillations around this operational point. Because the oscillation frequency is relatively high (more than 50 KHz), the regulated JT could be used also for supplying even of audio devices. If better noise performance is needed, then filtering of the output voltage should be implemented.



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    35 Discussions

    I think if using step up transformer 1.5 volt could easily hit 300 + volts and capable of powering 220 v 3 watt LED bulb...and with the highest efficiency even at 1.1 volt it still powering

    1 reply

    I don't think this counts but I made a modified darlington pair joule thief. I got a voltage drop of 0.005 after 13.2 hours. So is that good, I also used it to recharge another AA cell a little bit.

    1 reply

    That is the primary cells dropped 0.005v. but I did cheat a bit. I had it run a solar led that has it's own joule thief. I added a secon lc circuit to the base and a 1200uf cap across the battery. That and I had both bases connected and collectors connected in the circuit. The lc circut had one or 2 10 pf caps across it going through a 10 pf cap to the bases. Transistor one was a 2N2222 or equivalent going into what I think is a tip 31.

    The 1. was cut of haha. Thanks ill try that. I have never measure over 2v off an inductor that has no primary coil. This i dont understand.


    2 years ago


    I do not see big difference between my circuit and yours - try to replace these 3 resistors (100K,470K,560K)with single potentiometer 500K, with tap connected to the NMOS gate. Try with lower input voltage - not 5, but 1.5V. I have done JT, which were working with 300 mV. Another solution could be to increase the number of the turns of the coils (or at least their ratio)

    Ive made several joule thieves but i cant seem to get them to put out more than a couple volts :/

    I hope you can see this


    Not sure what you mean i checked to see that i was getting a voltage across my inductor.. I did put a 560k ohm and a 470k ohm resistor in parallel for an ~ 255k resistor but for some reason it was reading as only 1k ohm (i made sure they were only connected to R7 through the positive rail with the other lead connected to the gate of my irf640 nmos transistor and a 100k pot) any idea what happened there? Sorry I know this must be monotomous for you- i greatly appreciate your responses thank you!

    2 replies


    Sorry but I do not understand your .explanation.
    I would wait for the circuit.
    But in all cases I would recommend that you first make only the standard JT working (something like this : https://www.instructables.com/id/Joule-Thief-Explained/) and after that add (connect) the regulation circuit. This will allow you to track easier the errors if exist.

    Also build*- i dont have spice setup yet on my pc waiting on a power supply :X ill upload a diagram of my circuit in the morning

    Hello! Sorry in advance if this is a dumb question but i'm having some trouble with this circuit-- where do the output leads go? I assumed they would be on the end of the positive and negative rails on either side of R7- is this my problem? Does my inductor have to be about 10 micro henries?

    1 reply


    You are right - the output is the top connection point of R7 with reference to the common ground.
    Are you trying to simulate the circuit or to build it?
    For the simulations you can use the attached files.
    If you try to build it and it does not work - try to sweep the leads of one of the inductances.

    Hey Milen,

    Can you post or send me some pictures of the board itself. I'm quite new to reading schematics and if I know how the phisical thing looks like I would do much better.



    1 reply

    Sorry Nikolai,

    but I do not have this board more - I have used the parts for other projects.

    But, do not worry - you will find a lot of instructables how to make a Joule thief using perfo-board or breadboard. You should have in mind that this project is only

    for education purposes If you want to use it for supping of some other electronic device - better to buy some step up converter from ebay, which price now is very low.



    Sorrr, but I did not use a breadboard.
    May be there are some software to import a spice netlist in whatever you want...

    Thank you (Krb686, Milen) for this discussion. I learned a lot from your comments.

    Hey this is very cool! I love JTs and have messed around with some myself. I am curious why your circuit is as complex as it is. You have potentiometer controlled shut off, and your storage capacitor discharges through R2 and the potentiometer so that your JT must turn on and fire every so often to recharge up to the necessary level. Here is a very similar circuit I built that instead uses a zener diode runover turning on a Q2 to shutoff the JT.


    Notice how the transistor leaks much less current than your potentiometer, and the JT turns on less frequently so that it wastes less current.


    Hey I'm not trying to bash your design just so ya know, I've never built yours and maybe it has advantages I don't know about, but you could try mine out and see what you think! Good work by the way

    1 reply