I've been trying to make one of these darn things for a couple of months but the transistors I had available were apparently not suitable. So I went out today and bought one which was. I'd like to find a way to make the thing more efficient however so it will run at or below 0.73 volts. Any suggestions?

 

Feels good, I bet! A schematic would help.

 

Feels good, I bet! A schematic would help.

It does feel good! The schematic can be found here:

http://farm3.static.flickr.com/2327/1828270262_b76c867954.jpg

 

I may be wrong but that circuit can't work as you approach 0.7V because the transistor would never turn on.

 

I may be wrong but that circuit can't work as you approach 0.7V because the transistor would never turn on.

I appreciate your input. So, given that fact what could I add to cause the transistor to turn on as I approach 0.7V? Perhaps a capacitor of some kind?

 

How about using a Germanium Transistor - if you can find one.

 

How about using a Germanium Transistor - if you can find one.

Yes, you could use an old 2N404 germanium transistor (from Ebay and others) which should turn on at about 0.35V or so.
But note that germanium transistors are PNP so the circuit polarities would need to be modified accordingly.

 

(probably stupid) question, why not use a Darlington?

 

use a germanium transistor like AC127..

 

(probably stupid) question, why not use a Darlington?

That would add another Vbe, and require ~1.4V to turn on.

 

That would add another Vbe, and require ~1.4V to turn on.

stupid question... as I had suspected... if I remember correctly, darlingtons require less current at their gates, not less voltage to fully turn on.

 

That would add another Vbe, and require ~1.4V to turn on.

I have a question for you Wayneh. I've got this little homemade battery that is a consistent 0.73 volts with no load. Mind you, I don't know what the ah might be. But what if I hooked it up to the Joule Thief I made the other day and zapped it with a 1.4V charge from an AA battery in order to kick start the transistor? Might it then be able to use some of the 0.73? I know, I'll probably have to give it a try and see what happens and I probably will but depending upon your reply I may figure it is or isn't worth a try.

 

I suppose you have little to lose from trying? Placing the 0.73V battery in series with the 1.4V cell may be the best way to get energy out of it.

 

I suppose you have little to lose from trying? Placing the 0.73V battery in series with the 1.4V cell may be the best way to get energy out of it.

Thanks for the input Wayneh. Very much appreciated!

 

Can you connect two of your homemade cells in series?

 

if you use a germanium transistor like AC127, the vbe will be 0.2V and may work better.

 

I've been trying to make one of these darn things for a couple of months but the transistors I had available were apparently not suitable. So I went out today and bought one which was. I'd like to find a way to make the thing more efficient however so it will run at or below 0.73 volts. Any suggestions?

In the textbook blocking-oscillator joule thief, the feedback part of the winding boosts the voltage so you could theoretically run the battery down to about Vce-sat. That's typically 0.4V, but you can get low Vce-sat transistors that go down to about 0.2V.

The sticky wicket is getting enough energy in the base to switch the transistor on in the first place. Most non-rechargeable cells recover a bit while standing, so starting up usually isn't a problem - once its going, you can finish off the cell good and proper.

If you want it to start up at less than 0.7V - you may need to find a source of germanium transistors.

There's an off the shelf chip you can buy for running a white LED from a single cell; PR4401 - just add an inductor and supply decoupling cap and you're good to go.

 

In the textbook blocking-oscillator joule thief, the feedback part of the winding boosts the voltage so you could theoretically run the battery down to about Vce-sat. That's typically 0.4V, but you can get low Vce-sat transistors that go down to about 0.2V.

The sticky wicket is getting enough energy in the base to switch the transistor on in the first place. Most non-rechargeable cells recover a bit while standing, so starting up usually isn't a problem - once its going, you can finish off the cell good and proper.

If you want it to start up at less than 0.7V - you may need to find a source of germanium transistors.

There's an off the shelf chip you can buy for running a white LED from a single cell; PR4401 - just add an inductor and supply decoupling cap and you're good to go.

Thanks for the info Ian!

 

Here is how you reverse the polarity from an NPN transistor(left) to a PNP transistor (right).

 

Here is how you reverse the polarity from an NPN transistor(left) to a PNP transistor (right).

I see! Thanks for taking the time to help me out this evening.