LED Joule thief type circuit help

Thread Starter

electronicsnewbie

Joined Dec 7, 2009
10
Hi,
first post here :) I was wondering if somebody could help me with this little circuit. It a joule thief type circuit that powers a 3V led from a 1.5 battery. I have built the circuit and it works almost as it should but it's not quite as bright as it should be,the LED is brightish, but not brilliantly bright like a white LED should be. I would like some suggestions for component values to change, to make the LED brighter.

Here is the circuit I used (I did not design it, found it somewhere on the Internet )

Thanks for any help.
 

whatsthatsmell

Joined Oct 9, 2009
102
What are the specs of the led you are using? (forward voltage, current draw and mcd)

Have you used this type led before in another circuit and was it brighter?

Have you measured the voltage in the circuit across the led?
 

Thread Starter

electronicsnewbie

Joined Dec 7, 2009
10
What are the specs of the led you are using? (forward voltage, current draw and mcd)

Have you used this type led before in another circuit and was it brighter?

Have you measured the voltage in the circuit across the led?

Thanks for your reply. At the moment I don't have any "constants", because I'm using an LED I got out of a torch(took two AA's) and I don't (currently) have a multimeter though that soon will be rectified. But I am 100% certain that the LED should be brighter than it is. Also I tried a differen't orange led and that isn't quite bright enough.

Cheers,
 

hgmjr

Joined Jan 28, 2005
9,029
What is the part number for your inductor? I strongly suspect that it has a significant DC resistance. Measure its DC resistance with your new voltmeter once you have it. First measure the resistance of your meter leads then measure the resistance of your inductor. The difference between the two readings is the DC resistance of the inductor.

For this circuit to work properly you need to use an inductor with a DC resistance of less than 1 ohm.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Here is a link to an inductor that would fill the requirements of low DC resistance.

This inductor or an equivalent one should make a big improvement in the efficiency of your LED driver.

All of the alternate inductor-based circuits that have been suggested will require a low DC resistance inductor for them to operate at their peak efficency.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Of course if you are really industrious, you may want to try winding your own inductor. All you need is some coil wire or magnet wire of 30 gauge or so and a toroid with an outer diameter of a half inch or thereabouts. The inductance is not overly critical in the circuit you have chosen. 30 to 50 turns on a suitable toroid should do the trick. Most Radio Shack stores has some toroids and magnet wire that I have found to work fairly well.

hgmjr
 

Wendy

Joined Mar 24, 2008
22,164
The last experiment I finished using a CMOS 555 shows how to make an inductor. It is also realistic in that I did a really crappy job!

CMOS 555 Long Duration LED Flyback Flasher

A question for BMorse, do you mind in I turn that schematic into a AAC experiment? As a suggestion I'd look into some plastic battery holders for your breadboard, I'm buying a small selection myself, they have the single battery versions.

It is definitely simple enough for newbies.
 

hgmjr

Joined Jan 28, 2005
9,029
Bill,

I agree with you that the joule thief is an excellent and elegant circuit for beginners. It is a very forgiving circuit with regard to the component values used. It also provides some very valuable experience in designing with inductors. If you understand how a Joule Thief circuit works you will have acquired valuable insight into the fundamental operation of a switched mode power supply.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Thanks! That must be the problem . I will try a better inductor

This is what I used:

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=260450729005&ssPageName=STRK:MEWNX:IT

So yeah, comparing the two it looks like that might be the problem.

BMorse Thanks for the circuit, I am going to try that one as well.
The inductor you have is most likely one that has a fairly high DC resistance although I can't see any actual details on the specifications in the link.

hgmjr
 
Last edited:

BMorse

Joined Sep 26, 2009
2,675
A question for BMorse, do you mind in I turn that schematic into a AAC experiment? As a suggestion I'd look into some plastic battery holders for your breadboard, I'm buying a small selection myself, they have the single battery versions.

It is definitely simple enough for newbies.
No, I do not mind at all..... go ahead if it will help others.....:rolleyes:
 

Audioguru

Joined Dec 20, 2007
11,249
My solar garden lights use an inductor like that. Their LEDs are fairly bright and have a very wide angle for the light beams.
 

tkng211

Joined Jan 4, 2008
65
Suggest you replace the resistor R2 with a 1K Ohm trimmer connected in series with a 220 Ohm resistor (or experiment with different values of resistor, 560, 680, 820, 1K, 1.2K, 1.5K) to see how it affects the performance of the LED. After you get a more satisfactory result, you can further adjust the value of R1 to get better result if possible.
Make sure that the transistors are connected correctly and the LED has not been damaged by the reverse voltage from the inductor. Maybe you can also add
a diode 1N4148 connected in parallel with the LED but in reverse direction to protect the LED from breaking down by the reverse induced voltage .
 

BMorse

Joined Sep 26, 2009
2,675
Suggest you replace the resistor R2 with a 1K Ohm trimmer connected in series with a 220 Ohm resistor (or experiment with different values of resistor, 560, 680, 820, 1K, 1.2K, 1.5K) to see how it affects the performance of the LED. After you get a more satisfactory result, you can further adjust the value of R1 to get better result if possible.
Make sure that the transistors are connected correctly and the LED has not been damaged by the reverse voltage from the inductor. Maybe you can also add
a diode 1N4148 connected in parallel with the LED but in reverse direction to protect the LED from breaking down by the reverse induced voltage .
The components selected for this circuit I posted are optimum for its efficiency any deviation from the circuit could result in poor performance/and or low LED luminosity.
 
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