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Handmade Toroidal Inductor

How does the number of turns in the toroidal inductor affect the performance of a Joule Thief?

I made another toroidal inductor and decided to try a different number of turns to see how it affected the electrical characteristics of the circuit.  Here is the comparison.  In my original Joule Thief circuit, I swapped out the inductor, leaving everything else the same. The Joule Thief was powering a white LED inside a pretend candle.

White LED lights up the liquid in this fake candle.

The “candle” was my daughter’s night light during Christmas, but the (3) button cells inside it died.  The Joule Thief powers it up nicely with a rechargeable AA battery.   Here is how much power the candle-light used with the two different toroidal inductors.

Toroidal Inductor #1:  20 turns

1.38V, 5.3mA

Toroidal Inductor #2:  23 turns

1.37V, 4.9mA

Notice how the current went down 8% with just 3 extra turns on the toroidal inductor.  This makes sense when you look at the theory.

The formula for the frequency is:

Image from Wikipedia.

Why do we care about frequency?  That’s the number of times the LED turns ON per unit of time.  The LED is actually blinking, but super fast, so our eyes cannot notice.

The formula shows us that the frequency (F) decreases as the inductance (L) increases.  The frequency of LED blinking goes down as the inductance goes up.  But how does the number of turns affect inductance?  We can see by looking at the formula for inductance:

Image from Wikipedia.

It looks complicated but we can ignore most of the formula.  We just want to know the relationship between the number of turns (N) and inductance (L).  As the number of turns (N) increases, inductance (L) also increases.  Combined with the first formula, we know that:

As the number of turns (N) increases, the frequency (F) decreases.

As the frequency of LED blinking decreases, the amount of current going through the LED decreases.  That is because the LED spends more of its time OFF, using less energy. Based on the two formulas above, we know that as we increase the number of turns on the toroidal inductor, the current consumption through the load should decrease.

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Everything else in the circuit was identical. Both toroids were salvaged from spent CFL’s. [ See my post on how to recycle CFL's.  That post also shows other sources of toroids. ]  The choice of wire was the same.  Everything else in the circuit was unchanged, including the battery.  I performed a quick hot-swap of the inductor on my breadboard and took fast measurements before the battery lost much charge.  I repeated the measurements and found the same results.

Conclusion

Increasing the number of turns in my Joule Thief circuit decreased the current consumption by 8%.  The LED should also be a bit dimmer, I would assume, but it wasn’t a noticeable change in brightness.  The circuit continued to work with 23 turns.  It would be interesting to try the opposite: use less turns and see if the current goes up.

New Questions

What are the inductance ranges that work in the Joule Thief?

What are the limits to the choice of turns?

What is the maximum number of turns and the minimum number of turns needed to get the Joule Thief to function?