# Joule Thief Help

Discussion in 'The Projects Forum' started by Yaser43082, Oct 26, 2013.

1. ### Yaser43082 Thread Starter New Member

Aug 26, 2013
7
0
Hello Everyone,

I am trying to design a Joule Thief but a little confused as the where to start. I would like to be able to calculate the required Inductor, Capacitor and Resistor sizes. Also I am assume there is has to be some starting point Frequency to know what size of Inductor to use.

Can someone prove me the a starting point so I can perform the calculations instead of trial and error approach.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
Most low power switch mode power supplies run in the 10kHz to 1MHz range. You usually specify the ferrite core mix to match your chosen frequency. If you are starting with an unknown ferrite, then wind a few turns and measure the inductance...

3. ### wayneh Expert

Sep 9, 2010
13,435
4,272
Perhaps start here?

There also a detailed practical discussion here.

Last edited: Oct 26, 2013
4. ### Yaser43082 Thread Starter New Member

Aug 26, 2013
7
0
Hi Mike,

Thanks for the help. But what is the relationship between frequency and Inductor value? I know the Impedence of the Inductor = 2* pi * f * L
and that the Power = Energy * Frequency

How does this help me in sizing my Inductor?

Thanks

5. ### Yaser43082 Thread Starter New Member

Aug 26, 2013
7
0
Hi, thanks for the links. But I do not want use a torrid. Rather I want to know how to determine the size of the inductor given he following:

1) Input Voltage
2) Input Current
3) Output Voltage
4) Output Current

I realize a torrid is an Inductor. But I am not interested in calculating the number of required turns. Just in the inductor value

Thanks

6. ### acmefixer New Member

Aug 4, 2011
17
1
When you subtract the base to emitter voltage, you get about 0.9 volts across the resistor with a 1.5V battery. With a 1k resistor, this gives you about 0.9 milliamps of base current. Different transistors have different current gains, so if you choose a 1k resistor, you will find different LED currents and different brightnesses when you change the transistor. A 1k resistor will give you about 15 to 20 mA LED current with the typical 2N4401, PN2222A or BC337-25. You can change the resistor to a lower value, but then some transistors will not start with a 1.5V supply. If you drop the supply V and the LED lights up, then the resistor may be too low and you may have to increase it until the LED lights up when 1.5V is applied. All in all, the Joule Thief is very tolerant of changes in the parts values, so if you just arbitrarily choose a coil, resistor and transistor, the LED should light up if you have the circuit connected correctly. Then you can measure the supply current, which is typically 70 to 90 milliamps for the typical JT when it's lighting the LED brightly. Play around and have fun!

7. ### acmefixer New Member

Aug 4, 2011
17
1
One other thing: I've built a lot of JTs and the frequency may range anywhere from 10 kHz on up to the Am broadcast band; a MHz or more. Typical JTs are in the 30 to 100 kHz range, but the frequency is not critical, as long as the coil's primary winding is low resistance to keep losses low.

Also, the inductor value does not make much difference in the performance as long as the windings are roughly equal in inductance. You can use windings with inductance of about 100 microhenrys and it will make a good compromise in winding resistance and coil and wire size.

8. ### wayneh Expert

Sep 9, 2010
13,435
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Who said anything about a torroid?

9. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
There are a lot of variables that effect the frequency. For example:
With 100 uh coils the frequency with a 2N2222 is around 30 KHz. With a BC547 about 37 KHz. But the frequency is kind of linear with inductance.. So 30 KHz with 100uh 15Khz with 200 uh, 60 KHz with 50 uh.