Solid States Devices > Joule Thief

Joule Thief

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Can anyone of you electronics guys answer a question for me concerning the Joule Thief? OK my question is this:
The plans I have call for a 1k resistor.  From everything I have learned in electronics thus far, resistors limit the flow and dissipate overage as heat.  This seems wasteful to me since the idea of this device is to improve efficiency, as in use all or most of the available energy in a battery or similar system.  I am wondering why the resistor is needed?  If it gets "warm" at all it is wasting power correct?  Is this required to protect the transistor?

I would really appreciate any input here as this is my first Joule Thief.  I know they have been around a while and I have a pretty good idea on how they work, at least in relation to the lighting of LEDs.  I am also going to experiment with these using supercaps as the power source both with, and without batteries.  This will all be tied into my work with earth batteries from which I can already light an LED.

If I need the resistor to protect the transistor, then fine, that's the way it is.  It just seems a little counter intuitive from my limited electronics knowledge at this point.  Thank you.


I=V/R  :  Amps = Voltage / Resistance

A small battery (AA, A,C,D cells) can push over 1.0 amps.  An LED or transistor often can't handle 1.0 amp.  (check the rating on the package) So you can either: reduce voltage or increase resistance to lower your amps and protect your component.  Increasing resistance by adding a resistor is easier (and usually more efficient) than reducing voltage.  Your resistor value will be determined by your voltage and how many amps you can safely draw using the I=V/R equation.  Resistors are in fact lossy components, but the losses are minuscule in low-power operations and required to keep your other components from frying.

Not familiar with the joule thief - sounds like it's a voltage booster?

If you are trying to increase the voltage of your output, you could design a simple voltage doubler (4x, 8x etc) by using a circuit like the attached pic. and/or google voltage doubler.

Hi Bill,

You should get good performance using 1 Farad supercap and a joule thief. I have a few 1 Farad 5.5V caps and just running straight into a white 3V LED it lights for quite a while.

I think I´ll have a go at building one too, it may make a good quick chargeup minilamp for nightime paper doc reading etc.

I´m not sure which circuit you´re referencing but I´ll build something like this (I´ll probably end up using different tranny and core):

circuit from that page:

I think the 1K resistor is just there to limit the base current on the transistor, which should only be a miniscule current, certainly not enough to cause any noticable heating. So yes it´s as you first thought to protect the transistor base from getting a heavy hit from the trigger coil.

I´ll be posting pics and run times for my circuit it will be interesting to compare results with you.


Hi Bill,

Attached is a Joule Thief circuit that uses less current to the base of the transistor.
The resistor can be very high in value, thus less energy is dissipated through the resistor.



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