Joule Thief Is Not Working

[geratheg]

Applying 3 V, the LED lights up, but applying 1.5 V, no light.
Green wire is 22 ga, blue wire is 26 ga.
Using a 2N3904 Transistor and a Ferrite Toroid.

Am I doing something wrong?

 

[KrisBlueNZ]

Can you post a schematic?

 

[geratheg]

 

[(*steve*)]

Did it ever work? I ask this because a simple blocking oscillator is very hard on the transistor, it may be damaged.

If not, have you tried swapping the connections over on one winding?

 

[geratheg]

Here is what I'm trying to replicate and also where I got my schematic from:

http://makezine.com/2007/11/02/make-a-joule-thief-weeken-1/

Tried swapping connections yes, didn't work. I don't know if swapping connections is important since current flows in opposite directions either way. I found some copper wire, but it's without insulation. Going to try it.

 

[(*steve*)]

the inductor must be made from insulated wire.

Swapping one of the windings makes the current through one coil opposite with respect to the other.

 

[geratheg]

I got it to "barely" work using another toroid found from an old computer motherboard, which had one thick wire around it, so I just wrapped another copper wire around it.

Used it the toroid with the 1.5 V battery and the LED turned on pretty dimly, and only lit up if I just touched the battery, but wouldn't stay lit. Didn't observe this before. Probably a toroid issue more than a transistor issue.

 

[BobK]

I have made a Joule thief myself with a similar sized toroid, but I had, if I remember correctly, 22 windings for each coil. I believe you do not have enough from your picture.

Bob

 

[Arouse1973]

How does it oscillate without a capacitor?
Adam

 

[(*steve*)]

How does it oscillate without a capacitor?
Adam

Google "blocking oscillator".

basically, the transistor gets base drive through one side of the transformer. This causes collector current to start to flow. The increasing collector current causes an increase in base current via the transformer. This increases collector current further, which causes the transistor to rapidly saturate. At this point the collector current stops rising. This causes the base current to fall. This then leads to the collector current falling. This falling collector current causes a further fall in base current. The process continues with the transistor pushed far into cutoff. As the energy in the transformer falls to zero, base current again begins to flow, turning on the transistor.

Essentially you have massive positive feedback where a change in collector current causes a change in base current which reinforces the change in collector current. The transistor thus switches very rapidly between saturation and cutoff.

 

[BobK]

About 66 KHz on the one I built.

Bob

 

[Arouse1973]

Google "blocking oscillator".

basically, the transistor gets base drive through one side of the transformer. This causes collector current to start to flow. The increasing collector current causes an increase in base current via the transformer. This increases collector current further, which causes the transistor to rapidly saturate. At this point the collector current stops rising. This causes the base current to fall. This then leads to the collector current falling. This falling collector current causes a further fall in base current. The process continues with the transistor pushed far into cutoff. As the energy in the transformer falls to zero, base current again begins to flow, turning on the transistor.

Essentially you have massive positive feedback where a change in collector current causes a change in base current which reinforces the change in collector current. The transistor thus switches very rapidly between saturation and cutoff.

Thanks Steve, I never heard of that before, how does it respond to component tolerances?
Adam

 

[BobK]

Most likely, by changing its frequency and amplitude! They are very uncritical with respect to the transistor's properties.

Bob

 

[geratheg]

I have made a Joule thief myself with a similar sized toroid, but I had, if I remember correctly, 22 windings for each coil. I believe you do not have enough from your picture.

Bob

Bob, which size wire did you use? And which type of wire? Copper/Magnetic?

My torroid is 7mm inside diameter and about 13mm outside diameter.

 

[davenn]

And which type of wire? Copper/Magnetic?

"magnetic wire " IS copper wire

really wish that term would stop being used all over the net, it confuses the hell out of new comers

DAve

 

[geratheg]

Lol, I'm a new and it confused me too.

Do you prefer "copper wire"?

 

[davenn]

magnet wire is copper wire.... the bad term magnet wire comes from the used of enamel plated copper wire being used to make electromagnets and other inductors etc like transformers ( the enamel plating is the insulation)

the wire you have used is tin plated copper wire that is plastic insulated

The advantage of enamel plated wire is that it isn't so bulky ( doesn't take up as much room) so you can have more turns of wire on a coil if it is needed

Assuming you have the correct number of turns on your ferrite core, for that low number of turns, it wont make any real difference what type of insulation.
If you needed more turns on the same sized core, then moving to the use of enamel plated wire would allow for more turns

Dave

 

[(*steve*)]

Thanks Steve, I never heard of that before, how does it respond to component tolerances?

Amazingly well.

It is almost an achievement to make a blocking oscillator that doesn't work.

Of course, that's gilding the lily quite a bit, but if you look you;ll find people who have made them work after winding some random number of turns on an iron nail.

There are two potential issues. The first is that the base and collector current are limited only by the combination of frequency and inductance. Generally though this is the smaller issue. The second and more major issue is that the base can be driven a long way negative. This is almost certainly going to cause breakdown of the base-emitter junction.

As I've previously demonstrated (after being a little skeptical), this will result in an inexorable loss of gain in the transistor, even at extremely low currents (google -- or even search this site -- for "hot carrier injection").

 

[geratheg]

Thanks for all the responses.

I think I should try copper wire.
I have a T50-7 toroid, with about 7mm ID and 13mm OD and 5mm thick.

Which size copper wire should I get? 24awg, 26awg, 28awg, or 30awg?

Possibly wire that could be used for many other applications?

Does the size of wires determine anything besides the amount of current? Does it for example determine the amount of induced current, or is it that amount of windings that matters more?

 

[geratheg]

Wrapped 30 gauge wire, which took a while since it's so thin. Made 29 turns.

Still didn't work. I double checked the circuit, everything appeared correct.
I thought maybe it's the toroid.
But then realized the 1Kohm resistor affected the performance.

I kind of connected the transistor without a resistor (shorted it basically) and the light bulb just lit up.
The transistor did get hot, so I'm sure a resistor is necessary.

How do I determine which resistor value I need?