The Joule Thief is a deceptively simple circuit of enduring popularity. With just a handful of components, a transistor, a resistor, and a small transformer, it can coax enough useful energy to light an LED from batteries that would normally be considered “dead”.
But behind that simplicity is a surprisingly interesting bit of physics and electronics. The Joule Thief is essentially a self-oscillating boost converter. In this post, I’m going to take a closer look at what’s really happening inside the circuit with a particular eye toward circuit efficiency.
For those only interest in the results. I suggest you jump to the summary.
All data used gathered from a real joule thief circuit used in this article is available here. (LibreCalc + raw CSV data)
What You’ll Need
- 22x14x8mm MnZn Toroid (AliExpress | Amazon)
- Assorted Resistors (AliExpress | Amazon)
- Assorted Transistors (AliExpress | Amazon)
Schematic
Here I’ve replicated the standard Joule Thief circuit with a few minor modifications. I’ve introduced a 1Ω sense resistor (Rsense) to monitor the current through the primary coil, and replaced the usual LED output with a Schottky rectifier diode (D1) and a fixed 200Ω load. C1 has also been added to smooth the output.
The circuit will be powered from a fixed 1V supply to approximate the voltage of an almost-empty AA battery.
Transformer
I constructed the transformer on a green MnZn core with dimensions:
- Outer Diameter: 22 mm
- Inner Diameter: 14 mm
- Height: 8mm
The winding consists of 10 bifilar turns of 0.5 mm enameled copper wire. The measured electrical characteristics are:
- 0.657 Ω resistance
- 274.5 μH inductance
Primary Coil Branch Measurements
With the circuit operating, the first quantity to analyze is the current through the transformer’s primary coil. Most of the key performance metrics can be derived from this value.
This current through the primary coil can be observed by measuring the voltage across RSense, where 1 V corresponds to 1 A of current.
In the above chart, CHAN1 (Red) is the voltage across Rsense, and CHAN2 (Blue) represents the transistor’s base voltage.
The primary coil is charging when CHAN2 is high, and discharging when it is low.
A number of key measurements can be extracted from this chart:
| Measurement | Value |
|---|---|
| On Period (μs) | 33.650 |
| Off Period (μs) | 13.500 |
| Duty Cycle | 0.714 |
| Switching Frequency (Hz) | 21,208 |
| Peak Primary Current (mA) | 120 |
| RMS Primary Current (mA) | 65.833 |
| Average Primary Current (mA) | 56.730 |
| Average Primary Voltage (V) | 0.920 |
| RMS Primary Current On (mA) | 70.667 |
Most of these measurements are extracted directly from the above chart, but a few require further explanation.
RMS Primary Current
This is the current through the primary winding, calculated by taking the RMS of every data point over a full switching cycle—from zero primary current back to zero primary current.
RMS Primary Current On (mA)
This is the root mean square (RMS) current calculated over the 33 µs interval during which the switch is conducting. This value will be useful later when calculating losses that occur only during the on portion of the switching cycle.
With the primary branch measurements complete, I can now turn to the feedback branch.
Average Primary Voltage
This is the voltage applied to the primary winding branch. It is slightly less than the 1 V supply due to the voltage drop across Rsense. To account for this loss when calculating input power, the voltage after RSense is averaged over a full switching cycle.
Average Primary Current
To accurately calculate the power drawn from the battery and the resulting efficiency of the Joule thief, I use the average primary voltage and average primary current rather than their RMS values. While RMS is useful for determining resistive heating, it can overestimate the energy delivered when voltage and current are not perfectly proportional — which is the case in the inductive, pulsed waveforms of the primary winding.
By using the average voltage and current, I capture the true net energy per cycle supplied by the battery, ensuring that my efficiency calculation reflects the actual power available to the circuit. This approach guarantees consistency with energy conservation: the total energy drawn from the battery equals the energy ultimately dissipated or delivered to the load.
Feedback Coil Branch Measurements
The feedback coil branch is analyzed in much the same way as the primary coil branch, except the 1.5 kΩ RBase resistor is used to determine the current. RBase is positioned after the feedback coil, but it is an equally valid location to measure the current, since Kirchhoff’s current law states that the current through a branch is the same at every point along that branch.
For readability, I’ve normalized the following chart so that 1 V corresponds to 1 A, just like in the previous chart.
As before, I extract and calculate the RMS current through this branch. The RMS voltage is simply 1 V, since this branch is connected directly to the supply and contains negligible ripple.
| Measurement | Value |
|---|---|
| RMS Feedback Current (mA) | 0.601 |
| RMS Feedback Current On (mA) | 0.711 |
| Feedback Voltage (V) | 1.00 |
| RBase(Ω) | 1500 |
Collector Emitter Junction Measurements
The situation becomes more complex at the collector because the current has two possible branches:
- Through the schottky diode D1
- Through the collector emitter junction of Q1
However, the collector voltage waveform provides a useful simplification:
When the transistor turns on, the collector voltage is pulled below the forward voltage of D1 for nearly the entire on-interval, except for a brief moment during Q1’s turn-off transition. This means current flows through both paths simultaneously only for a very short switching interval. With this assumption, the collector–emitter losses of Q1 can be analyzed during the on-interval and the diode losses of D1 during the off-interval with negligible error.
The following data better captures the area of interest:
Again I calculated required measurements from this chart
| Measurement | Value |
|---|---|
| RMS Collector Voltage On (V) | 0.140 |
Diode Junction Measurements
Next I need to determine the forward voltage of both the base–emitter junction of Q1 and the anode–cathode junction of D1 while they are forward biased.
Q1’s Base-Emitter junction is only forward biased when Q1 is On, and D1 is forward biased when Q1 is off, so we measure the RMS voltage over a different period for each.
| Measurement | Value | Comment |
|---|---|---|
| RMS Q1 BE Forward On (V) | 0.742 | Vfwd during the Q1 on part of the cycle |
| RMS D1 Forward Off (V) | 0.337 | Vfwd during the Q1 off part of the cycle |
Output Measurements
Lastly, I measure the output voltage. The RMS value is used to account for the small amount of ripple remaining on the output, even with C1 providing filtering.
| Measurement | Value |
|---|---|
| RMS Output Voltage (V) | 2.580V |
| RLoad (Ω) | 200 |
Measurements Summary
Now that I’ve performed all the measurements. I can gather all of them in one place ready to do some calculations
| Measurement | Value | Comment |
|---|---|---|
| Primary Branch | ||
| Average Primary Voltage (V) | 0.924 | Used for input power |
| RMS Primary Current (mA) | 65.833 | |
| Average Primary Current (mA) | 56.730 | Used for input power |
| RMS Primary Current On (mA) | 70.667 | During Q1 On Period |
| Peak Primary Current (mA) | 120 | Peak inductor energy |
| RMS Output Voltage (V) | 2.580 | |
| RMS Collector Voltage On (V) | 0.140 | During Q1 On Period |
| RMS D1 Forward Off (V) | 0.337 | During Q1 Off Period |
| RLoad (Ω) | 200 | |
| RWinding (Ω) | 0.657 | Same on both windings |
| Feedback Branch | ||
| Feedback Voltage (V) | 1.00 | Supply voltage |
| RMS Feedback Current (mA) | 0.601 | |
| Average Feedback Current (mA) | 0.495 | Used for input power |
| RMS Feedback Current On (mA) | 0.711 | During Q1 On Period |
| RMS Q1 BE Forward On (V) | 0.742 | During Q1 On Period |
| RBase (Ω) | 1500 | |
| Frequency | ||
| On Period (μs) | 33.650 | |
| Off Period (μs) | 13.500 | |
| Duty Cycle | 0.714 | |
| Switching Frequency (Hz) | 21,208 |
Total Power Supplied
The total power supplied to the circuit from the lab PSU is simply the power delivered to the primary branch plus the power delivered to the secondary branch.
Primary Supplied Power (mW): 52.419
Feedback Supplied Power (mW): 0.495
Total Supplied Power (mW): 52.914
I probably could have ignored feedback power here, its impact on total efficiency is very small.
Resistive Losses
Next I’ll calculate resistive losses, these are the simplest.
I’ll start with the power delivered to the load — after that I can calculate our overall efficiency
Power dissipated in the load is an I^2 * R calculation so needs to use RMS Current values.
Load Power (mW): 33.282
Finally I can calculate the total efficiency:
Efficiency (%): 62.90
Next I’ll calculate losses for other resistive elements in the circuit the same way.
Primary Winding Loss (mW): 2.852
Feedback Winding Loss (mW): 0.000
Looks like I could have ignored feedback resistive losses. This isn’t too surprising as the current through the feedback winding is small.
RBase Loss (mW): 0.542
We can safely ignore Rsense here, since the voltage drop across this resistor is already accounted for in the RMS Primary Voltage, which was measured after Rsense.
Junction Losses
The junction losses are a little trickier because they only conduct during one part of the cycle, I need to be careful to convert RMS values if required between partial and full periods.
Q1 BE Junction Loss (mW): 0.377
(Multiplying by the duty cycle D converts the power during the “on” period to an average power over the full cycle)
Q1 CE Junction Loss (mW): 7.061
(Multiplying by the duty cycle D converts the power during the “on” period to an average power over the full cycle)
D1 Loss (mW): 2.326 mW
First I calculate RMS current through D1 from:
This converts the total RMS current to the RMS current during the off-time only, accounting for the duty cycle D:
Finally:
After all that, I’m finally in a position to summarize all the sources of inefficiency in this circuit.
Efficiency Summary
The total efficiency of this joule-thief when at 1 V sourced 60.5mW from the power supply and delivered 33.28mW to the load.
For a total efficiency of ~62.90%.
The table below summarizes the calculated losses in the Joule Thief. Most of the power is delivered to the load, while resistive and junction losses account for smaller fractions.
| Loss | Remaining (mW) | Percent Total (%) |
|---|---|---|
| Total Supplied Power | 52.914 | 100.00% |
| Primary Winding Resistance | -2.852 | -5.39% |
| D1 | -2.326 | -4.40% |
| Q1 CE Junction | -7.061 | -13.34% |
| Q1 BE Junction | -0.377 | -0.71% |
| RBase | -0.542 | -1.02% |
| RLoad | -33.282 | -62.90% |
| Feedback Winding Resistance | 0.000 | 0.00% |
| Remaining | 6.474 | 12.24% |
The loss breakdown for the Joule Thief operating from a 1 V supply looks reasonable. About 63 % of the input power is delivered to the load, while most of the remaining losses occur in the transistor collector–emitter junction and the primary winding resistance.
Replacing Q1 and D1 with MOSFETs in a synchronous configuration could significantly reduce junction losses, potentially raising overall efficiency to around 80 %, which aligns with what one would expect from a well-designed boost converter producing 3 V from a 1 V supply.
There is also a relatively large unaccounted loss of about 12 %, likely due to magnetic core losses, including hysteresis and other magnetization effects. This could potentially be reduced by adjusting the winding ratio, limiting the flux swing, or choosing a ferrite core material better suited to the operating frequency.
Considering the simplicity of the circuit, the Joule Thief performs quite well. If higher efficiency is desired, reducing switching losses seems the most promising place to start.
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