efficiency of Joule-thief vs series cells to power Picaxe

[chipwich]

The Joule-thief (http://en.wikipedia.org/wiki/Joule_thief) is a small self-oscillating circuit which circuit which can use nearly all the energy in a battery. I've seen some posts (eg, http://www.picaxeforum.co.uk/showthread.php?14547-Joule-Thief-powered-PICAXE) discussing how a Picaxe can be powered off a Joule-thief in order to boost the the voltage from 1.5v to well above the minimum operating voltage, thereby both allowing operation from a single 1.5v cell, and using almost all the power from the battery.

But if my primary goal is to simply use almost all of the stored energy in the battery, is there really much to be gained by using a Joule-thief circuit, rather than just using a 3 cell holder? Since Picaxe chips seem to operate down to 1.8v or lower, they will function until the cells are at 0.6v each; this is *well below* their initial 1.5v state. Of course this doesn't have the neat voltage booster effect of the Joule thief. But when a cell is at 0.6v, how much energy remains that could still be harvested by the Joule-thief circuit?

 

[boriz]

The circuit itself uses power. It's not 100% efficient. So if you can use three cells, that's the way to go.

"But when a cell is at 0.6v, how much energy remains that could still be harvested by the Joule-thief circuit? "

Probably none. Once the voltage drops below the transistor Vbe, it stops oscillating.

However, if you use two (perfectly matched*) cells with a Joule-thief, then they would both be at about 0.3v before it stopped, and you would have gained that extra juice. But how much would you have lost in the Joule-thief circuit? And the voltage will need regulating some how, adding losses. If you gain 5% from the extra charge and lose 5% in the circuit, there's no point.

*Unlikely.

 

[boriz]

Maybe use three cells directly until they drop too low then engage a Joule-thief circuit to squeeze out the last bit?

 

[srnet]

But when a cell is at 0.6v, how much energy remains that could still be harvested by the Joule-thief circuit?

Probably only a fraction of a percent.

I can see the point of using the Joule Thief with a single cell setup, but it does not make much sense for two or more cells where you would expect the Joule thief to reduce battery life.

Why not run a real life test ?

 

[inglewoodpete]

I think a joule thief circuit comes into its own when it's sucking hard on an undersized or underilluminated solar cell.

 

[manuka]

Compact Joule thief circuitry is now acceptably cheap,compact & pretty efficient. However there are further factors to consider than just sipping the energy store dry-
Positive
* Physical size comparison: A single cell (such as an AAA) driven by a Joule thief may contain more usable energy than a similar sized multiple cell battery like a regular 9 V type.

* Recharging: A single rechargeable cell with output boosted by a Joule thief will be cheaper than multiple rechargeables

* Multiple cells may have greater physical size - perhaps too large to fit in the device

* Multiple cells may have contact woes,cell degradation & added cost for the battery holder.

Negative
*However Joule thief circuits may introduce unacceptable electrical NOISE.

* Current capabilities may be limited

* Failure mode - a no go inbuilt Joule thief may thwart normal operation of the device. (I once had this happen on a swish Yaesu radio...)

Stan.

 

[bluejets]

Wasn't the whole idea originally to drive LEDs off a single cell.

 

[AllyCat]

Picaxe chips seem to operate down to 1.8v or lower, they will function until the cells are at 0.6v each; this is *well below* their initial 1.5v state.

Hi,

Yes, the PIcaxe may well work down to 1.8 volts, but some/many of the external components will not. White or Blue LEDs for example require about 3 volts.

A particular hobby horse of mine is that NiMH cells are sometimes described as "not suitable" for a particular circuit design because their 1.2 volts is too low (compared with the "1.5 volts" of primary cells). The "truth" is that these circuits are poorly designed because, as you say, Alkaline cells, used efficiently deliver much less than 1.2 volts near their "end of life". However, the voltage does fall quite rapidly below about 1.0 volts, even ignoring the fact that a Joule Thief needs to draw increasing current to deliver the required power.

The link posted by Manuka in #6 of this thread is well worth a look, particularly for the voltage/time graphs.

Cheers, Alan.

 

[manuka]

Joule Thief voltage spikes can further enhance LED brightness under POV (persistence of vision) as well!

Versatile DC-DC charge pump voltage boost circuits arose decades ago however. At the hobbiest level I recall Jaycar used to sell a discrete based one to replace a 9V battery with a single AA cell. Specialised DC-DC boost ICs now abound -check the claimed 97% efficient AS1337.

 

[Jeremy Harris]

I had a similar problem when looking to power an O8M2 plus a 433MHz RF Tx from a single NiMH cell a while ago. The NiMH cell is charged from a small "6V" solar panel, via a current limiting circuit, so that it can pretty much always get some charge, even on a dull day. I opted to use the ready-built NCP1402 breakout board from SparkFun (https://www.sparkfun.com/products/10968) which accepts any voltage from 1 to 4 V on the input and delivers a well-regulated and fairly noise free 5V, at up to 200mA.

This has been working reliably for many months non-stop now and seems a pretty good way to power both a picaxe and any reasonably high current peripherals (like LEDs, for example) from a low voltage source like a single cell.

 

[srnet]

Negative
*However Joule thief circuits may introduce unacceptable electrical NOISE.

* Current capabilities may be limited

* Failure mode - a no go inbuilt Joule thief may thwart normal operation of the device. (I once had this happen on a swish Yaesu radio...)

Stan.

Well, single cells also need a battery holder, unless of course you use tagged cells.

So which option is likely to be more reliable, two connected tagged cells, or one tagged cell and all the extra components and connections of adding a Joule Thief ?