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can a joule thief be used for a 6 volt incandescent lantern? Answered

I want to use a joule thief to increase the lifetime of a lantern battery. I want to use the incandescent bulb, it has more power than leds. I plan to have one lantern for new batteries, and one for used batteries with the joule thief.



Best Answer 7 years ago

The statement "Incandescent bulbs...show flicker very effectively" is incorrect. The filament cannot cool quickly enough to show the flicker.

There are very powerful LEDs, up to 5 watts, or even higher. You can buy the replacement bulbs that have LEDs that put out as much light as the bulbs they replace and use much less current. I saw the ones that replace the "PR" prefocus bulbs at Fry's electronics store recently, but you can order them online. Here is a link to one Chinese company that makes or sells them. You can see what they look like.

The Joule thief is a circuit designed to operate on very low battery voltages, to extract the last, well, Joule from the cell. It is not appropriate for high current LEDs - which will kill the almost dead battery stone dead very quickly. There are other, better, topologies for driving LED loads when you have available power.


You're right about other circuits for driving LEDs. However the Joule Thief design is not "limited" to low currents. It depends on the components not the design. Here is one guy's version of a high power JT. He doesn't say anything about the amount of power but I suspect it puts out several watts with a fresh D cell.


Joule thief circuits only allow you to squeeze out the remaining portion of battery life between a cell  voltage of ~0.9v and ~0.65v. If you look at the battery mAh curves from the manufacturers, you see that less than 5% of the potential battery life is contained in this region. High current loads will quickly exhaust this little extra mAh. Joule thiefs work best on low current circuits where the little extra lasts a long time.

There is nothing that "only allows" you to use a half dead battery in a Joule Thief. A Joule Thief works even better on a brand new fresh battery.

There is nothing in the circuit design that makes it only "work best on low current".

By the way, if you go to the link I gave above, you have to scroll down the page to see the high power Joule Thief.

The Joule Thief is a boost type power supply, that lets you use a lower voltage source to get a higher voltage.

In the case of lighting an incandescent bulb, using a fresh battery it will make the light brighter because of the higher voltage. The price is shorter life because you are using the battery's available mAh at a faster rate. If you wanted the brightness and battery life, you use more batteries. Also, make sure your bulb is rated for the higher voltage.

You are right that for an incadescent bulb, you can run the battery completely up. It will  be dim after you expend the engery to the point that the cell voltage is ~.9v. 

If you use a Joule thief the .9 voltage point will be lower down the mAh curve because it is a boost type power supply.  The down side is you will get there faster (shorter life).

The Joule thief circuit is not magic. You can only the energy that is stored in the battery. For AA batteries this is about 2500 mAh, and for D batteries about 20000 mAh. 

The reason it works so well on LEDs is that the Vf of a LED is about 2v for red  and about 4 for blue at normal currents. That is why a single cell alkaline will not light an LED. Using the Joule thief the voltage of a single battery is boosted to about 3-4v. This higher voltage allows the LED to shine until the battery cell drops to about .7 volts or an output voltage of about 2 volts.

Ah, don't forget the O/P WANTS to drive an ordinary bulb. I did.


No. A joule thief works on two principles: LEDs take very little current to function, and LEDs can operate on intermittent (pulsed) power and still appear continuously lit (due to POV). Incandescent bulbs require a lot of current, since they operate by resistive heating, and they show flicker very effectively.

Of course, you could always just build one and prove me wrong.

Thanks for the answers, guys. I can buy a new lantern at Home Depot cheaper than a new battery at Ace Hardware, so I end up with a pile of half-dead lanterns. If there is an LED that I can fit right into the lantern, that's great. There is room in the lantern for the joule thief, I'm pretty sure.
So now, what is a guy to do with a bunch of left over 6 volt bulbs? (grin)