Joule Thief not working correctly

[landuchi]

Hello,

I have constructed a Joule Thief device following the instructions at:

YouTube - Make a Joule Thief
**broken link removed**

It did not work. After checking the wiring and the components I realized I was sold a 2N3906 transistor instead of the 2N3904 I had asked for.

Some google searching told me the transistor I have is PNP and the one in the Joule Thief diagrams is NPN. I am not an electronics expert, but I can see one of the diodes is reversed in the PNP type, I guess that's why it doesn't work.

Now I could (and probably will) buy the correct transistor, but I was wondering if there is a way to make it work with the 2N3906 transistor, maybe with a different wiring.

Thanks in advance!

 

[crutschow]

If you reverse the polarity of the battery and the LED, then it should still work.

 

[Hero999]

That will work but I think you should take the transistor back to whoever sold you it first or at least make a complaint.

 

[mvs sarma]

That will work but I think you should take the transistor back to whoever sold you it first or at least make a complaint.

While the logic of complaining helps correct the sales person, going to the shop for a complaint, might cost Op dearly.

 

[Hero999]

How would complaining cost him dearly? Maybe the shopkeeper is a psychopath who'll loose it and beat him to a pulp.

 

[HarveyH42]

Depends on the shop. If it small and the only one in town, the sales person is likely the owner or a relative. Depends on how you present your complaint, and the sort of person the shopkeeper is, but if you are known to be a picky/difficult customer, you might be limited to the items in plain sight, and the prices as marked. Where as someone else might be able to get parts ordered, or the shopkeep to search around a little for a suitable substitute. I need a magnifying glass to read those markings on those transistors myself. If it were only one or two pieces (dime each?), wouldn't bother complaining, I use both, and just be more observant in the future. A large order, from a big company, I'd raise hell though. I don't need a hundred of any wrong part sitting around.

 

[Hero999]

I suppose he has to first make sure that it isn't his mistake.

I do agree about it not being worth it if you've only bought one.

I remember buying some LEDs from Rapid, one of them didn't work but I didn't bother complaining because it was so cheap, there again maybe I should have complained because if I'd been a business and had put the faulty LED in a product and shipped it, it would've cost me more.

 

[landuchi]

I apologize for the delay, I was really busy during the week.

I switched the battery polarity, changed the wire from the emitter to the collector and it started working. However the light of the LED was very dim.

A few days after I went back to the shop, the idea of yelling, getting all mad, and punching the guy in the face crossed my mind but then I thought it was too much retaliation for 30c . Anyway I bought a BC549 transistor, and built a second Joule Thief. This one worked like a charm, the led is as bright as it is with a 3v battery.

What I'm trying to build is a LED garden torch powered by an earth battery.

Searching how it works and how to make it more efficient I found this modification, again I am rather new to electronics, i know he is adding a regular capacitor but what does the 10n stand for?

**broken link removed**

Img source: TALKING ELECTRONICS LED Torch

Thanks again!

 

[crutschow]

Why did you reverse the emitter and collector on the unit with the PNP transistor? You just needed to reverse the LED and battery polarity as I stated. Reversing the emitter and collector means the transistor was trying to operate in the inverted mode where it has very low gain. That's why the LED was dim.

10n is 10 nanofarad or 0.01µF.

 

[landuchi]

As I told you before my electronic knowledge is basic. I did that modification before I got to see your post (it seemed logical that it would work that way) and I thought the LED was going to be dim regardless of how I wired it. I didn't know the transistor has lower gain working in the inverted mode.

For the capacitor any voltage will do ?

Thanks!

 

[Hero999]

Yes, the voltage of the capacitor is the maximum rating so you can use any capacitor because the voltage is only 1.5V.

 

[3lectrokid]

isnt a resistor needed to limit the current going through the LED?

 

[Hero999]

No.

..........

 

[transistor495]

At least I'll tell you to get any general purpose NPN like 549,548,547 or even a 3904 from some old circuit boards. Search and you'll find tons of them!