Supercapacitors

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rbond9

New Member
I have two questions,

The first is to ask if anyone has any experience using a 5 or 5.5V supercapacitor to power their PICAXE circuits (for a short period of time obviously).

The second is concerned with getting my hands on cheap supercapacitors, I am looking for a minimun of 1F rated at either 5 or 5.5V.

I will leave the question open at that but I have been trawling the interweb looking for a soloution. The best value capacitor I can find is 1F 5.5V for about 50 pence each if I buy 250 of them.

Rich
 
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manuka

Senior Member
I've been a great supercap. fan for years, & all manner of electrical party tricks are possible with them. A classic involves connecting a small efficient DC motor, spinning it with your fingers as a generator, then having the stored energy (re)drive the motor.

Why their PICAXE appeal however? They certainly will run PICAXEs, but their stored energy is pretty small compared with say a Li coin cell. Of course (compared with Li cells), that 30 Ohm internal resistance may be a bonus, as it limits the current should some stupid short develop. Are you really most after their recharging ability? How many do you need? Have you rescued any, from say an old VCR, for trials?

Capacitor maths - Q=VxC =Ixt, means a 5V 1F cap could sustain (at say 10mA) a load for 5/0.01 =500 seconds before fully discharging. Of course the usual cap. exponential drop off means a PICAXE load would stop working when ~3V is reached, so less time will be possible. All up, with a typical PICAXE circuit of simple sounds/LED pulses, you might get a few minutes run time?

The Ah capacity of such a supercap. is (at best) just 10mA x 500/3600 = ~1.39 mAh This is orders of magnitude below 3 V Li coin cells, which typically are rated ~100mAh. In the right places (here in NZ anyway!) I can pick up such Li cells for ~10p each. Coin shops often sell 6 packs of them (approaching their use by date) around these prices too.

I've had a lot of fun with the tiny polyacene types (rated 3V 0.6F) from US firm Pager Motors. See => http://www.pagermotors.com/index_fil.../Polyacene.htm They're so tiny & thin (~Aspirin tablet) that you can lose them for good if dropped on the floor!

EXTRA: FWIW -the supercap/battery field is evolving rapidly,especially with combo "bacitor" or "cabattery" types in e-vehicle load smoothing/regen. braking. So called ultracaps. (2.5V/100F range) are already available, & unlike supercaps these can be charged/discharged at blistering levels.
 

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retepsnikrep

Senior Member
I used a 15v 56F supercapacitor in my old solarvan to buffer the output from the 120v dc-dc 13.8v converter when supplying the 13.8V normal vehicle auxillary loads.

It frightened me to death at is had virtually no internal resistance and when charged was a bomb with two big connectors!

It sat there looking at me saying "Short me out at your peril, go on I dare you!"

I had a decent fuse right by the connector in case.

It could run the vehicle for about 15 seconds without dc-dc operating.

Interesting but fairly pointless, just meant I could say I had a supercapacitor in the van!!

Those little 5.5v 1f caps are quite good I used on in a couple of silly designs
 
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BCJKiwi

Senior Member
Thing to remember about supercaps is that they discharge like caps, not like a battery - i.e. the voltage falls steadily with charge depletion where batteries hold a (relatively depending on chemistry) steady voltage until close to depletion.
 
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rbond9

New Member
Andrew Cowan said:
Watch the postage - that's from the US.

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Click to expand...
The postage is the same for any quantity, thank god.

In response to some other questions, these are for a school/class project, the theme is sustainability hence the need to recharge. Batteries are very inefficient.

I was thinking about more like a 10-15F capacitor for the PICAXE, just wanted to know if anyone had done it yet.

I have still not seen any cheaper than in the ebay listing, especially when you consider that the cost does not increase with quantity. I almost considered ordering them and reselling in the UK.
 
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BeanieBots

Moderator
Yes, I've done almost exactly what you describe.
There are two pitfalls to look out for.
The first, as already mentioned is the exponential decay of the voltage.
When an 08M is driven from a capacitor, it will eventually fail as the voltage gets too low. Bringing the voltage back up will NOT bring it back to life. The voltage must be taken right down to <0.5v to effect a proper reset or it just hangs.
Also, there are two types of "super caps".
The memory backup type are much smaller and have higher internal resistance. They behave very much like a battery rather than a capacitor. A capacitor would be fully charged if placed across a battery for a fraction of a second. A memory backup capacitor will take 48Hrs to be fully charged.
A TRUE "super capacitor" will charge almost instantly just like a 'normal' capacitor. Be sure you know and understand which type you are getting.
 
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manuka

Senior Member
You really need to specify the PICAXE load! How were you intending to recharge the supercap? Dirt cheap AA NiMH cells/batteries in fact may be the best "greenie" option- they've a stable output as they discharge too

Perhaps hack a single AA solar garden lamp , both for it's ~2V 30mA PV & also it's electronic boost circuitry? These are capable of providing ~5mA at a smoothed 5V almost continually under ~4 hours of good sunshine. We've used them for low duty cycle 433 MHz wireless weather stations etc. Solar powered PICAXE-you can't get greener than that! Stan
 
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Dippy

Moderator
"I almost considered ordering them and reselling in the UK."

I suggest you buy a few and try them first. Some time ago, someone here posted a picture of a large electrolytic capacitor sourced from the cheaper-end-of-the-world, it was a large can with a smaller cap inside - as a result I am even more cynical about cheaper products.

BB raises a serious point re drooping voltages and clean resets.
esp. if you start fiddling around with BOR.
If you have a proper (good quality) adjustable PSU you can experiment as it may behave differently with different PICAXEs.
[It has to be good quality as a noisy pot could cause a droop/pulse which can mask the effect of a slow droop.]
(Of course, MY bench PSU uses opticals for its knob :) )

But, anyway, it would be a nice demo for the kids - for a few minutes at least ;)

I've got a better school project.
A 2 foot tall electronic scarecrow to keep the birds off my strawberries.
Movement sensing with waving arms and miiaaaaooww noises.
 
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rbond9

New Member
Dippy said:
A 2 foot tall electronic scarecrow to keep the birds off my strawberries.
Movement sensing with waving arms and miiaaaaooww noises.
Click to expand...
POST A VIDEO WHEN IT'S DONE, WE HAD A SCIENCE TECHNICIAN THAT MADE A SIMILAR PROP FOR THE JAIL IN YORK MUSEUM, BARS RATTLING AND SOME NOISE. (WHAT NOISE DO THEY MAKE IN JAIL?) :D I WOULDN'T LIKE TO FIND OUT! :eek:
 
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Tom2000

Senior Member
I use a little one -- 0.022 uF -- as a clock backup for a DS1302. Works like a champ. Keeps the -1302 alive for over a week. Also, it takes a whole lot less board space than a coin cell and socket.

Tom
 
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jglenn

Senior Member
I have a bunch of 1F, 2.5V, for free, maybe a couple dozen for you, if you
pay shipping from USA, about $5 postal. You can put 2 in series for 5V.

Note that over time the ESR may go up. Some dummy put them in a circuit
I have to service, wrong solution for the wrong problem. It turns out you
don't even need them for our system. I found they will power white LEDs for
a long time, there are now supercap flashlites out there, maybe I should make
one. Powered by a solar cell and picaxes! :cool:

johnf@apk.net
 
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CliveWilcox

Member
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BeanieBots

Moderator
Dippy said:
Just because you have a capacitor the size of a wheeliebin doesn't make it , by default, a supercap.
Click to expand...
I agree fully but had to laugh none the less because it reminded me of a project we had back in the late 80's long before 1F was a series consideration.
We needed to store a charge at 250kV.
Nothing was commercially available so we made our own. (waxed paper/foil)
It was the size of wheeliebin and only 1000pF:eek:
We DID refer to it as the "supercap".
 
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Dippy

Moderator
Oh dear.
"supercap".
- you sure you didn't leave an "r" out of there somewhere ? :)

(and I'm not referring to a large fish).

So, you had wheelie bin wasting your time then ... hahha.
 
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Dippy

Moderator
Have you still got it BB?
It may come in handy if there is a Dustbinman strike.

(Or maybe I should call them Official Waste Operatives or Refuse Mangement Facilitators or some other ars name).
 
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rbond9

New Member
jglenn said:
I have a bunch of 1F, 2.5V, for free, maybe a couple dozen for you, if you
pay shipping from USA, about $5 postal. You can put 2 in series for 5V.
johnf@apk.net
Click to expand...
Thanks John,

I have emailed you. This would make a fantastic resource for my sustainability project. There a lots of school projects that could make use of even just 2.5V.

Richard
 
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jglenn

Senior Member
Yes, got you covered. May be tossing a few 1.5F in the box too.

Don't short them out. Might blow up. Wear safety glasses. :D
 
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jglenn

Senior Member
U are rite, 2 one F caps in series will yield 1/2F. It was not my idea. We make
engine controls, a past design team was baffled by the battery droop when the
starter runs. They put the 5V cap cluster on the OUTPUT of the 5V cpu regulator, which was dumb. The way we do it now is a diode isolated normal
1000uF cap on the INPUT of the reg, that way it will create 5V all the way
down to 6V on the cap. We use a switching reg. The problem is that there
are thousands of the old design out there, and guess who gets to fix them? :(

It was a costly mistake. These caps are about $3 each, and wear out in 2 years. I developed a hybrid boost cap stuck on with RTV, a 10,000 uf electrolytic. This has worked out so well (none of these unit come back, and
work in the cold of canada with worn out supercaps, bravo) the boss insists
on new units getting it too. The few that still use that old design, that is.

We were the first to talk to CAN bus engines and have a patent on it. :p
 
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Rickharris

Senior Member
Many moons ago I worked on ground radar eqiuipment which used a PFN (Pulse forming network) of inductors and capacitors in an oil bath - It was about the size of a respectable suitcase, no idea of the values but it generated about 80Kv to drive a Magnetron.

One day the system ceased to work and the smell was akin to a burning chip shop.

The bank of capacitors had failed resulting in the steel case of the PFN splitting and spewing the oil over everything else. BIG bang aparently, several operators needed new underware.

Teaches you to respect capacitors!!
 
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