Subject:
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Re: lego cap, polarity, etc
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 31 Mar 2001 06:34:51 GMT
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Viewed:
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980 times
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> No. DO NOT UNDER ANY CONDITION CONNECT AN ELECTROLYTIC (ie one with +/-
> on the terminals) IN REVERSE.
This is generally true, but not as dire as you make it sound.
> The electrolyte is a high resistant in one direction only. The other
> direction is nearly a short.
Actually, electrolyte is a good conductor of electricity (in any
direction). It comprises one of the terminals in an electrolytic
capacitor. The other terminal is aluminum, usually in the form of a
rolled-up sheet. As for the directional conductive properties you are
attributing to the dielectric, that actually describes the properties
of a diode - a different animal all together.
> You may end up hurt as the capacitor blows up
> in your hand/face and potentialy catches fire. If you're lucky it will
> only go 'pop' and spray yucky electrolyte over your work area.
A couple of calming words for those who might have been alarmed by the
above statement:
First, I've never actually seen one on fire. They can, however, get
extremely hot, but this highlights the point where the original
message is a bit alarmist. Electrolytics consist of three things:
1) one terminal which is immersed in an electrolyte - a conductive
liquid or gel
2) one terminal that is connected to a metal conductor - usually
aluminum.
3) an oxide of the metal that coats the metal's surface and has a
reasonably high (but not great) resistance.
A capacitors value is directly proportional the surface area of the
plate(s) and inversely proportional to the distance between the
plates. The advantage of the oxide layer is that it is only a few
atoms thick and thus allows the plates to be closer together than
would be possible with other types of capacitors (mylar, ceramic or
glass to name a few) This gives a very high capacitance but at a
price: the oxide layer is easily ripped off. This is accomplished by
running current the wrong way through the capacitor. When this
happens, you end up with the aluminum plate (a conductor) connected
directly to the electrolyte (also a conductor) with no oxide layer (a
medium resistance) with no oxide layer to keep them apart. Current
flows - quickly. The dielectric has a fairly high resistance (for a
conductive material). High current through a resistance yields a high
power dissipation (IxIxR) within the sealed can of the capacitor. that
generates heat. Heat boils off the electrolyte turning it into a
vapour. Like all gasses, the vapour expands to many times it's
original volume (or it tries to) and so the pressure within the
capacitor grows until either the safety rubber seal pops open to
release the electrolyte or, if it doesn't vent fast enough, the can
explodes. A few of my projects over the last 20 years have met this
fate.
So why was I concerned that the original message was alarmist?
Because:
A) The energy required to make a capacitor explode is very high. In my
experiments a while back, I found that even up to 12V (the power
supply I had at the time) the current through an electrolytic stayed
under 1/2 A. The capacitor did get warm to the touch, but I still have
all my fingers, and my fire insurance was never called upon. (1)
B) Lego has added a circuit board to the capacitor which prevents it
from being connected backward.
It should be pointed out that Lego - our beloved toy company - is not
in the business of putting mini-bombs into the hands of children. I
was concerned that the dire, all capitals warning of the original
message might lead some people who may be less familiar with
electronics to believe that the capacitor they bought for their child
to play with was dangerous. Lego is more responsible than that.
> Also, NEVER use an electrolytic that is leaking or has the top flat
> surface 'bubbled' up. Both of these indicate internal damage and the cap
> should be replaced.
Completely true.
> This asymmetry in resistance is also what allows an electrolytic to store
> charge from simply sitting. The brownian motion of the charges get trapped
A couple of technical points here...
a) The resistance *is* symmetrical. If you try measuring the
resistance of an electrolytic in either direction (*** with a low
measurement voltage ***), it will show a reasonably high resistance
either way (but it will be lower than other types of capacitors
because of the electrolytic's notorious internal leakage problems). If
you use too high a measurement voltage, you begin to peel off the
oxide layer and the capacitor begins to conduct. Technically, you are
changing the internal structure of the capacitor when you do this and
that's where the apparent asymmetry shows up. This is as opposed to a
diode which is truly asymmetric since reversing it's polarity changes
the resistance but not the internal structure (just the distribution
of majority carriers)
b) Electrons will repel one another. In order to develop a charge, the
electrons must be grouped in one place. This requires the application
of energy. A capacitor left sitting will *not* develop a charge on
it's own. The effect you are thinking of is called dielectric
absorption. A portion of the energy stored in the capacitor is trapped
within the dielectric and appears as a residual terminal voltage. This
results in some capacitors having to be repeatedly discharged before
the voltage finally reaches 0.
c) Brownian Movement describes the motion of microscopic particles
within a solution (such as coffee in water) and not electric charge
(wherein the electrons (only) move and not the parent atoms). It is
not applicable to capacitors.
Matthias Jetleb
VA3-MWJ
(1) So if a capacitor isn't as dangerous as it was made to sound, how
did some of my projects blow up? The secret is in the power
dissipation within the capacitor. Using Lego, which puts out a maximum
of 9V at say 500ma (4.5W at best) you won't get enough power
dissipated in the capacitor to vaporize the electrolyte. Hence no
bomb. My projects involved power supplies that were plugged directly
into the wall (maximum 120V, 15A - 2545W peak). There's a huge
difference. Under these conditions, things start flying. We were
literally in the habit of standing textbooks and briefcases between
ourselves and our power supplies when powering them up for the first
time. May I also say, from first hand experience (pun intended) that
they remain instant-second-degree-burn hot for a surprisingly long
time after they've landed at the far end of the classroom.
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Message is in Reply To:
 | | Re: lego cap, polarity, etc
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| No. DO NOT UNDER ANY CONDITION CONNECT AN ELECTROLYTIC (ie one with +/- on the terminals) IN REVERSE. The electrolyte is a high resistant in one direction only. The other direction is nearly a short. You may end up hurt as the capacitor blows up in (...) (24 years ago, 28-Mar-01, to lugnet.robotics)
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