[RE-wrenches] Electrolytic Caps vs. Thin Film Caps

Sun Jan 15 05:32:58 PST 2012

Dan,

Any worries with inverters located out in 30 below zero temps with either
Electrolytic Caps or Film Capacitors? It's very cold here this AM (minus
twenty something F) and it makes me wonder about all the inverters, both
micro's and string, outside in cold weather areas. We've been told in the
past by SMA, Fronius and Solectria that we should be good down to 40 below,
although this is typically not found in their technical documents in print.

David Palumbo
Independent Power LLC 
462 Solar Way Drive
Hyde Park, VT 05655
www.independentpowerllc.com 
NABCEP Certified PV Installer
Vermont Solar Partner
24 Years Experience, (802) 888-7194 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Exeltech
Sent: Sunday, January 15, 2012 12:48 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Electrolytic Caps vs. Thin Film Caps

Hello Ryan,

I can only speculate as to why the "electrolytic
capacitors are bad" rumor persists.  There are
any number of possibilities, such as those I'm
about to list below.  It likely is combinations
of or even all of these...

Possibility #1:
40 years ago, the best electrolytic caps didn't
last more than 5 to 10 years.  Today, a quality
electrolytic cap used within its specifications
will live 60-70 years, yet this rumor won't die.

If YOU were making film capacitors to compete with
electrolytic caps, wouldn't YOU exploit this
misconception about electrolytic caps to your
advantage?  That's what is happening here.

Part 'B' to this answer, film caps cost more than
electrolytic capacitors of the same value.  Based
on 40 years design experience, I"m guessing there's
more profit per part as well.  Capacitor firms
making both electrolytic caps and film caps may
seem to be in a win-win situation, but that's
not entirely correct.

Their motivation to push film cap is obvious:
Sell the products where you make more money.

Possibility #2:
Because there ARE very low-quality electrolytic caps
that DO have a short lifespan we commonly find used
in cheap equipment by manufacturers whose goal is to
sell you a 1,000 watt inverter for $29.  We all know
about those.

Possibility #3:
Because film caps will <theoretically> last longer
than electrolytic caps.  However .. as a point of
information, film caps can be ruined in a short
time by using them beyond their specs in a poor
circuit design, just like any other part.  Also,
film caps are larger and more expensive than
quality electrolytic caps.

If as a design engineer you could choose between
a quality electrolytic cap that will live 60-70
years, or a film cap that may last 80 years, but
is larger and more expensive than the electrolytic,
which would YOU choose?

Possibility #4:
Why do film cap folks continue this rumor?

Because they can .. and it's to their advantage
to do so.

---

Constructional Differences:

First, let's define the basic construction of a
capacitor.  It's nothing more than two electrical
conductors separated by an insulator.  Two wires
side-by-side in a power cord form a capacitor,
but the capacitance value tends to be so low that
we can ignore it in a 50 Hz or 60 Hz application.

Capacitance value is fundamentally determined by:
a) the surface area involved between the two
conductors; b) the distance from one conductor
to the other; c) the insulating material between
the conductors.

The type of insulator between the conductors helps
to establish two aspects of the capacitor's specs:

a) Capacitance value, and;
b) breakdown voltage rating.

Electrolytic capacitors achieve their capacitance
in part by using an "electrolyte" that resembles
a very light oil.  The electrolyte is located
between the two conductors, and is kept there
in a paper-like material.  The conductors in
this case are two very thin layers of special
metal (usually aluminum) wrapped in a circle,
much as if it were a paper towel roll.

Film capacitors are manufactured by applying a
metallic spray to each side of a very thin layer
of special plastic that resembles the plastic
wrap we use to protect food in our refrigerator.

The plastic in this case is the insulator, but
it doesn't have nearly as high as what I'll call
"capacitance multiplication factor" as does the
oil-type electrolyte.  Thus, to achieve a similar
capacitance value, a film capacitor must be larger
than its electrolytic cousin.  This in some cases
can be *much* larger.

Some of the weaknesses in film capacitors have
defined above .. a larger physical size for any
given value capacitance, and higher cost than
comparable high-quality electrolytic caps, for
openers.

When size and costs are important in our solar
energy market today, either and both of these
shortcomings are show-stoppers to me.

Why make a product larger and more costly than
it needs to be if the alternative, when used
correctly, will outlive all of us anyway?

Ryan...

You asked some excellent questions in a very
short post.  My apologies that it took such
a lengthy reply to answer all aspects of your
query.

Dan Lepinski
Senior Engineer,
Exeltech

--- On Fri, 1/13/12, Ryan J LeBlanc wrote:

> From: Ryan J LeBlanc <ryan at naturalenergyworks.com>
> Subject: [RE-wrenches] Electrolytic Caps vs. Thin Film Caps
> To: re-wrenches at lists.re-wrenches.org
> Date: Friday, January 13, 2012, 5:20 PM
> 
> Dan,
> 
> Thanks for the E-caps post.
> 
> Could you briefly describe why the Thin-Film Caps
> guys continue to spread this rumor?
> 
> Perhaps give a brief description of the differences?
> 
> Perhaps some weaknesses of the thin-film caps? 
> 
> I'd love to get your take on this.
> 
> Thanks,
> 
> Ryan 
> 
> 
> ------------------------------
> 
> Date: Thu, 12 Jan 2012 15:59:04 -0800 (PST)
> From: Exeltech <exeltech at yahoo.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] SolarEdge v Enphase
> 
> Wrenches,
> 
> I wish this urban myth about electrolytic capacitors 
> would die, but it won't. It's been a wonderful
> marketing ploy for some time now.
> 
> Simply stated, existence of electrolytic capacitors
> in inverters does NOT mean the inverters are less
> reliable. Satellites have been using electrolytic
> caps for decades, and look how long THEY last! (I know,
> because I've helped to design power supplies for aerospace
> applications.)
> 
> What it boils down to is this: if a quality electrolytic
> capacitor is used within its specifications, you can
> expect 60+ years of life from it in an inverter application.
> 
> Conversely, if a circuit is poorly designed, you can ruin
> any electronic part in a very short time.
> 
> What defines capacitor "life"? It's defined as the
> doubling of the internal effective series resistance
> of the capacitor from the value it had when it (the cap)
> was new.
> 
> This is energy lost. It also causes the capacitor to
> operate at a slightly  higher internal temperature
> than when it was new (all other factors being the same).
> 
> Result? It all depends on how the circuit is designed.
> In our case, the inverter efficiency decreases
> approximately a quarter to one half of a percent.  The
> waveform distortion also increases, but it takes lab
> grade equipment to detect the change.  If you're worried
> about a 1/2 percent drop in the inverter efficiency, your
> concern is misplaced. After all, PV degrades over time
> much more than that!
> 
> So why the bad rap on capacitors?
> 
> Like tires, they DO wear out. 40 years ago, "premium"
> tires were rated for 30,000 miles. Could you wear them
> out in less? Absolutely!  Drive like a 16 year old kid
> with a muscle car, and it's easy.  On the other hand,
> I just bought a new set of tires.  The old ones still had
> tread, but at 88,000 miles, they'd seen their better days.
> This is a far cry from 30k mile tires.
> 
> Capacitors have progressed in the same manner. A long
> time ago, capacitors wore out in 5 to 10 years. Today,
> it's an entirely different story.
> 
> As long as the circuits in which the electrolytic
> capacitors are used are well designed, you have nothing to
> worry about.  Electrolytic capacitors in a well-designed
> application will outlive all of us.
> 
> 
> Dan Lepinski
> Senior Engineer
> Exeltech

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