We have seen how transformers can move voltages and current up and down. They also have interesting functions in providing feedback loops. If you do a thought experiment you might wonder what could happen if you send secondary current to the primary.


The transformer is important as a voltage and current changer. It also can have a critical role in creating an oscillation. Think of an oscillation as a wave form that we can either hear as a sound or can be transmitted as a radio wave.

If we do a thought experiment with a transformer we could imagine how a voltage produced in a secondary could influence the primary.

We can capitalize on the idea of this feedback if we provide a couple of components that will properly time the feedback to coincide with the primary.

Let’s first confirm that an LED will not illuminate with a 1.5 volt cell:

LEDs require about 2.2 volts, so our cell cannot produce enough voltage to start it up. We can do some manipulation with a transformer, however.

The circuit we will use is simple, it is called a joule thief.  A joule is a measure of energy. The name seems to indicate we are trapping or finding lost energy somewhere. Let’s see!

Materials Required:

• The transformer you made with nails and wire.

• Any 1.5 volt cell you have—good or bad!

• A 1000 ohm resistor (brown, black red)

• An LED

• An NPN transistor

• Connector block

• The speaker from a previous experiment

• Wire connectors

• A capacitor

Everything you need should be in a labeled baggie.

Here is what the circuit look like with the parts and connectors.

If you cannot see how each pin is connected, let’s look at the pictorial:

There are a couple of things you might get wrong so check that the positive side of the cell is attached to the transformer connection. Also be sure that the long side of the LED wire (one wire attached to the LED is longer than the other) is on the bottom side of the little black transistor.  The transistor should be flat side up.  And, the only other potential problem could be that your transformer connections are out of sync. If that is the case, you may have to exchange one the blue and red wires that are connected together.  Exchange the other red wire for the one you have and that will arrange the windings properly.

If all goes well the LED will show brightly.  Now how can that be. We only have 1.5 volts. How can we be getting 2.2 or more volts?  You might say the transformer stepped up the voltage, and that would be a good starting point, but the problem is that we have battery current which means the transformer will only work if the current is always on and off.

That’s where the transistor comes in. It acts like a switch and turns the current on and off giving feedback to the primary. We have created an oscillator that is like alternating current or AC.

One Step Further.

We can hear this oscillation if we exchange our LED with a speaker. In the diagram below I have replaced the LED with a speaker and added a capacitor in parallel with the resistor.  This acts to delay the feedback to the primary. 

See the video below:

So, is the joule thief an example of generating something from nothing?  We used a pretty bad AA cell and it was able to drive a speaker and an LED. Where did all the joules come from?  

A cell like our failing AA still has potential to do work and has a supply of electrons. What our circuit has accomplished is rearranging the voltage of the cell so it is high enough to light the LED. The total power or wattage of the cell is the same so we haven’t “created” anything. There is no magic here, just physics. : )

RESPOND:  Did you give it a try? What was your experience? mjaeger@eou.edu

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