Instructables

how does a toroid in Joule Thief circuit work?

sorry for the noob question, but i can't understand how a toroid can cause a LED with higher voltage than the source to be lit.. i tried to make it without the toroid (only the transistor) but it didn't work... any explanation is much appreciated.. tq..

NachoMahma5 years ago
. When there is relative motion between a magnetic field and a coil of wire, an EMF is produced in the coil. . When you send current through a coil a magnetic field is produced. . When you remove the power to the coil, the magnetic field collapses around the coil and produces an EMF "kick" in the coil. This kick is usually of higher voltage than what was applied. The transistor acts as the switch. . This does not supply constant DC to the LED, but instead pulses. If done fast enough, the eye doesn't notice. . A filtering capacitor is commonly used to reduce the ripple, but not really needed for many circuits.
Nacho has got it on the head. The toroid simply acts with wires wrapped around it as a transformer - it works in conjunction with an astable transistor oscilator which makes very fast pulses at the battery voltage. The transformer converts these pulses to a high enough voltage.
lemonie5 years ago
There's an explanation & circuit diagram here
Like Cartermarquis said it acts like a transformer, the transistor element creates some kind of oscillation / pulsing which allows the thing to work with a DC supply (it's not a steady state flow of current)

L
I think people don't understand how a transformer works. The ratio of the two windings determines the transformation. But in the Joule Thief the LED is connected to the same winding as the battery and transistor. Is there transformation when everything is happening in the same winding? NO! See my earlier reply above in this same thread.

The question is more than 2 years old, what prompted you to add another comment now?

L
acmefixer5 years ago
Saying that it's a transformer is a bit misleading. The 2 transistor JouleThief uses a coil with a single winding and has no transformer action - it doesn't transform one voltage to another. What the coil does is store electric energy in the form of a magnetic field, and when it is released it is at a higher voltage but lower current. The only thing the second winding does is invert the drive current to the base of the transistor, so that it will oscillate. You don't need this winding, you can use the second transistor to invert the signal. The transistor acts as a switch to break the DC up into pulses so that the coil can do its voltage conversion. If you want to do a similar experiment, try putting your fingers across the contacts of a small 3 to 6 volt buzzer. When the contacts open, the voltage rises high enough to give you a shock. In this case the magnetic field pulls the armature of the buzzer in to break the contacts.
qs5 years ago
A toroid is not necessary for a joule thief to work - see here.

In fact you can make a joule thief with 2 pieces of wire, each about 6 feet long, and wind both together over a 1/2" form, like the AA baterry. Remove from the battery and it'll work with most low or medium power transistors in a joule thief.

You can read more about it on my website.
From the way I understand it, it works as a kind of transformer, but i'm not totally sure of that.
firdaus_alizh (author)  Cartermarquis5 years ago
that's exactly what i thought, but come to think about it, a transformer works only in AC source. but the source in Joule Thief is a battery which is a pure DC source... how can there be a difference in the frequency that causes the induction? or am I missing something here?
The current does not necessarily have to be AC; it does have to vary with time. Pulsing on and off is not the most efficient use of a transformer, but it works.
i agree, but i can't see any device that can vary that DC source (or that's what toroid do?) in the circuit. it only consists of LED, NPN transistor, toroid, and a 1k resistor.. thanks..