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Batteries Or Super-Capacitors ?
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 Posted: Wed Mar 24th, 2021 10:19 am
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Steve Soar
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Hi again,

On second thoughts,
just a 2v7 Zener-diode across each super-capacitor,
no resistor.

Steve

:s:


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 Posted: Fri Mar 26th, 2021 04:19 pm
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Helmut
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When you look up the specs. of these super-capacitors,
almost every manufacturer recommends, if balancing is used,
a parallel resistor, that draws ~10x the leakage-current, of that specific capacitor.

For example, a 3F/2.7V green-cap. specified for 8µA of leakage-current.
For balancing, a 33K resistor in parallel to it will suffice.




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 Posted: Sat Mar 27th, 2021 09:51 am
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Steve Soar
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I agree that resistors do give a degree of voltage sharing.

However if one super-capacitor in the string, has 10% lower value,
then it will charge to a 10% higher voltage, this is not good,
and could cause a further reduction in capacitance.
 
When the string is discharged,
the lower value super-capacitor will then become reverse biased,
this too is detrimental.

I would suggest that if resistors are used,
then look at the voltages on each super-capacitor when charged,
and check all are 0V when discharged.

Steve


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 Posted: Sat Mar 27th, 2021 02:55 pm
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Helmut
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That's why one should use capacitors from one batch, ordered all at once.
Probability is very high that they will be taken from the same string.

I have made stacks of 10 in series,
and their charge voltages differed by ~1% which is negligible.


The drawback of common Zener-diodes is their undefined transient region.
Sharp-knee Zener-diodes are the only ones really making sense.

They are available only for 2.4V max. from 'On-Semiconductor',
which means you can charge an arrangement of 2 only at 4.5V


I think the actual risks in our applications are exaggerated.


Addition:

Here's a result of a 'torture' I applied to a Panasonic super-capacitor.
I used a 3.3F/2.3V Panasonic-HW that I charged @5V via a 10-Ohm resistor.

As soon as the voltage across the cap exceeded 3.3V, the leakage-current increased rapidly,
and the voltage rose to 3.6V to increase further with a rate of 10mV/5 seconds.

That is, the leakage-current changed from a few µA to ~130mA.

Taking the power away resulted in a quick drop back to 3.3V,
and an asymptotic decrease to 2.5V within 10 minutes.


My conclusion is that it might be possible to apply 140% of the rated voltage,
without any long-term problems under our circumstances.

The usable charge will NOT change, however.
The capacitor voltage still read 2.3V after 15 minutes and 1.1V after 12 (!) hours.


Still, if you have $5.38 to spare,
you can buy an ALD810026 to balance four caps.




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 Posted: Sat Mar 27th, 2021 05:56 pm
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Steve Soar
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I agree.

If the super-capacitors are from the same batch,
then ok, resistors are good.

I had a bad capacitor in a string once, and things went a bit wrong,
so perhaps I am being a bit over cautious now.

Thanks for your input.

Regards Steve


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 Posted: Mon Mar 29th, 2021 04:01 am
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Si.
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I can see 2 of Freerails top technical brains ^^ operating at FULL POWER !!  :java::brill: :java::brill:
Cognitive cog wheels grinding away in the ol' grey matter.  :dope: :slow:


So I thought I'd throw a spanner into the works !  :P


And propose a  'Bi-Polar Super-Capacitor'  :shocked:  for regular 12 Volt D.C. trains.  L:

An electrical 'flywheel' that can operate whilst reversing polarity & train direction.  :mex:


Just in case of a dead rail ... WHOOPS ! ... I mean dirty rail (or railS) !  :old dude:

( Is 'dead rail' just 1 un-powered rail ? ... Or should that really be 'dead railS' ? ...  ;)

... boB need not reply, regarding the singular rail question ! )  :pimp:


:java::moose: :dt:


Si.




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 Posted: Mon Mar 29th, 2021 06:48 pm
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Si.
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:old dude:


Si.




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 Posted: Tue Mar 30th, 2021 01:29 pm
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Helmut
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Well, for that purpose may use these:

https://www.monacor.com/products/components/speaker-technology/crossover-networks-and-components-/lsc-2200np/

Needs less space.







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 Posted: Tue Mar 30th, 2021 05:20 pm
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John Durbetaki
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The capacitors you are suggesting are pretty small,
compared to the power requirements.


A super-capacitor of 2.7F can also be referred to as 2,700,000uF
The 220uF capacitor is 1/12,000 the size of the super-capacitor.

If a 220uF capacitor is charged to 15v,
that is about 0.025 joules, which is just 1/40th Watt/second.

So if the motor is using 0.2A at 15v (3.0 Watts),
the motor will run for less than 8 milliseconds (0.008 seconds).


A 220uF capacitor powering a 3.1v LED at 5mA (15.5 milli-Watts),
means the LED will stay lit for 1.6 seconds.


Also, putting polarized-capacitors 'back to back',
almost always ends really badly.

Want to try it?  Go ahead.

Just make sure you do it in a safe place (outdoors),
with a shield between you and the capacitors.

Hearing protection is advised...




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 Posted: Tue Mar 30th, 2021 10:02 pm
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bobquincy
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My single plastic rail, with any metal on both sides,

becomes a capacitor, non-super type.


boB




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