Constant Current buck/boost driver for LED / Supercapacitor

[spec]

Here are the recommended minimum voltages for the common rechargeable battery types. The figures in brackets show an aggressive regime which scarifies battery life for the sake of maximum power extraction- often used in power tools:
NiCad/NMH 1V (0.9V)
Lead Acid: 1.75 (1.45)
LiPho: 2.7V (2.45V)
LiIon 3V (2.7V)

**broken link removed**

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[spec]

2v is the PIC cut off voltage. After that it won't work (so shouldn't discharge the battery any further). I now agree if the li-ion isn't charged to it's max then it's easier to charge, straight voltage across it.

Afraid that is not true Andrew. 2V is not the PIC cut off voltage; it is the voltage below which the PIC will not work. In fact is is quite possible that the PIC could take more current below its minimum operate voltage. The PIC and associated circuitry will almost certainly drain any battery down to 0V given time.

For the UVLO, a darlington and a zener to the base should do it, I think. That'll leave 3v to the PIC, which would give 2.4v at the output pin, which is acceptable (just!) as a 2v LDO exists with a 0.23v dropout. (LED's are 2v rated)
**broken link removed**

That would be one way to do it, but this is a common situation and there are many precision chips to give a battery a cut off function quite easily. Rest assured, it simply is not an issue. In fact you can buy batteries with the cut off function built-in.

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[ACharnley]

I wonder how much time it would take for a lithium coin-cell (40mAh) to drop from 3.6v to 3.47v given a 70mA load.

At least it would have a fair amount of time before it began to dim.

Perhaps another option would be to swap the ldo for three resistors and three zeners inline with the LED's. That would offer a .4v range. Undervoltage lockout at 3.2v, the led's would never go dim.

 

[spec]

I wonder how much time it would take for a lithium coin-cell (40mAh) to drop from 3.6v to 3.47v given a 70mA load.

That is far to narrow a voltage range to be practical and far too higher load current. A 40mA coin cell terminal voltage would drop below 3.47V immediately I would imagine.

With a coin cell the discharge rate is typically 0.2C, so you would really need a 350mA/H battery.

The maximum current from a coin cell is typically 1C, but at that current the capacity would be drastically reduced. Coin cells are not designed for producing power.

If you discharged a coin cell at 1.4C it would not last long. 0.1C and below is best for battery life.

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[MrAl]

Hello,

If i understand this project correctly you wanted to charge a super capacitor at first, using what is really a generator.
To do that you dont need current control unless the current can go too high for the super capacitor. Only then do you need current limiting.
You could do some tests to see how much current you get from the generator.
You usually have to do some tests for these kinds of projects to see what you really get.

If you switch to Li-ion then you need current limiting if the current can go higher than the max charge current for the cell.
You also need voltage limiting so the voltage cant go over 4.2v (for most cell types).
The coin sized Li-ion cells i have found dont last very long, but the normal sized cells like the 18650 last pretty long. As they get older, they do not hold a charge as well so the battery runs down much faster.
I would expect a super capacitor to last longer, but the current draw can not be too high or it will run down too fast.

If you intend to use a microcontroller chip then yes the lowest voltage will be that of the limits of the chip. Consider using a separate battery/super capacitor for the voltage source for the chip alone as they draw low current and when put to sleep they draw almost no current at all. You could manage the power quite easily that way and keep the chip up and running even when the LED power dies down.

 

[ACharnley]

Probably another reason why supercaps are used - the working life.

 

[ACharnley]

Researching I found this chip which does 0.7-5.5v up to 6v (perfect for 3x red leds), https://www.ti.com/product/TPS61220

Supercapacitor is back in play!

 

[dr pepper]

I made a couple of these for my and my wifes push irons, but I'm afraid they are a lot simpler than your thinking.
While riding the dyanamo powers a 10w red led, but its not driven at 10w, its just nice and bright, and when stopped 2 ultra bright red leds flash for a minute or so.

 

[ronsimpson]

ACharnley,
The IC (TPS61220) regulates voltage. You should use a "LED" driver that regulates current.
Here is a boost LED driver that measures current not voltage.
This is a very old part that we have used millions of. There are better newer parts.
Because it is boost only, the input voltage should not get higher then the LED turn on voltage. So if you added a third LED then the input voltage can go up another 2 volts.

 

[ACharnley]

Looks better ... if I can get one working down to 0.7v. I am assuming of course that the supercapacitor will be able to supply enough current at ~1v.

 

[ACharnley]

This looks promising!

 

[Nigel Goodwin]

Probably another reason why supercaps are used - the working life.

Not at all, the performance, and working life so (so called) 'super' caps is far inferior to batteries.

It was a 'nice little earner' replacing faulty super caps in VCR's - not that NiCd's didn't fail as well, but probably not as much.

 

[ACharnley]

Perhaps they've moved on since VCR's. I know I have.

 

[Nigel Goodwin]

Perhaps they've moved on since VCR's. I know I have.

I wouldn't have thought so - they are generally pretty useless devices, but people are 'conned' by the term 'super'

 

[ACharnley]

I read about considerably higher lifespans than any other battery type. I had no idea they existed all the way back to VCR's..

 

[Nigel Goodwin]

I read about considerably higher lifespans than any other battery type.

First off, they aren't a 'battery type', they are just a capacitor - and vastly inferior to a battery.

Longer lifespans seem pretty imaginary, and generally they aren't good at supplying any decent levels of current either.

Normal sort of use would be to keep a clock going when the mains fails, as long as current requirements are only a few uA's.

 

[ACharnley]

I've bought some to play with. I also bought one of the competitors products and will take it apart to see what they've done.

Current requirement is 60mA @ 2V, plus a bit of overhead so let's say 0.1A once it's half discharged (less above the half voltage point, more above). Quite a bit for a small supercap and I have to admit, it seems unlikely it can do that for 4 minutes.

I'v read reports of these devices failing after two years, but then that's the same with li-ion.

 

[ACharnley]

I took apart the Germany device from above to see how they went about it. It's a single LED and as soon as the power goes off (rider stops) the light dims significantly. It is then retained for a couple minutes by 4 x 0.47uF 5.5v caps.

The PCB is double sided and way more complex than I expected with further IC's on the rear. Can anyone identify the 0256p component? It's unusual, not seen one before.

 

[spec]

I've bought some to play with. I also bought one of the competitors products and will take it apart to see what they've done.

Current requirement is 60mA @ 2V, plus a bit of overhead so let's say 0.1A once it's half discharged (less above the half voltage point, more above). Quite a bit for a small supercap and I have to admit, it seems unlikely it can do that for 4 minutes.

I'v read reports of these devices failing after two years, but then that's the same with li-ion.

Assume a voltage drop of across the super capacitor of 2V with a constant 100mA drain for 4 minutes the capacity, in Farads, required would be:

Q=CV=IT

Thus C=IT/V= 0.1A * 4minutes * 60/2V = 12 F

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[ACharnley]

That'll be why they dim it so much, it's a single LED and I doubt it's even at 20mA.