<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>A big echo for that comment! I would not only say it is "also
important", but it actually is essential. And one reason for
saying this is that, while explosions are often spectacular and
hazardous, and sometimes even fun (type 3 fun usually), most
electrolytic capacitors do not actually fail in an explosive way
in real life. The more common case is that the designer heeds at
least the maximum voltage rating with a more or less conservative
rating, and possibly also the ripple current rating. But even so,
electrolytic capacitors are one of the things in life that show a
very continuous range of characteristics in a number of ways, and
one of them is how they age. With a liquid electrolyte inside, and
the can plugged with a rubber bung, there is that possibility that
some solvent molecules escape as the inside pressure increases.
There are also some score marks on the top of the aluminium can to
allow for spontaneous decompression when the pressure accidentally
increases so sharply that it cannot be released past the rubber
bung. Many normally operating designs will eventually lead to
capacitor failure as the electrolyte dries out due to the loss of
solvent over the years.</p>
<p>Soooo... the loss of solvent is mainly driven by temperature, and
particularly high temperatures. The electrolyte temperature is the
sum of the ambient temperature and the differential achieved
through internal heating. A constant and perfectly smooth DC
voltage applied to an intact capacitor (one without DC shorts)
contributes nothing to internal heating (but an excessive DC
voltage can still lead to catastrophic failure by breaking through
the oxide layers on, and the spacer between, the electrodes). But
the ripple current obviously generates heat inside the capacitor.
And while it isn't hard to deliberately feed through a ripple
current high enough to explode a capacitor, even a moderate to
high current well within spec will contribute to the ageing of the
component.</p>
<p>So where does this lead? Well, it would be easy to over-spec
conservatively for both voltage and ripple current. But this has
numerous disadvantages: it costs, it adds size and weight, and it
may come at the expense of increased parasitic properties that are
counterproductive for other reasons. But electrolytic capacitors
also come in different temperature ratings, and that helps where
things may get warm... Everything else being equal, a 105C rated
component will live longer than an 85C rated one. It may cost
more, but depending on design life that may be well worth it.</p>
<p>Capacitor de-rating calculations should be easy to find somewhere
between component data sheets and application notes...</p>
<p>Kind regards,</p>
<p>Helmut.<br>
</p>
<div class="moz-cite-prefix">On 1/10/2020 15:22, Richard Jones
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAGQt8jz1nSAHraTmjgsNaf-GOWCia_-yniDg6xyrj2g3o4+niQ@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">In relation to the explosion part of the
question...
<div>It is also important to check that the ripple current
rating of capacitors is not exceeded.</div>
<div><br>
</div>
<div>Richard</div>
<div><br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Oct 1, 2020 at
11:19 AM Stephen Irons <<a
href="mailto:stephen@irons.nz" moz-do-not-send="true">stephen@irons.nz</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div id="gmail-m_-8799095192780379565geary-body"
dir="auto">At a previous company, when I did hardware
develoment, the policy was 200% maximum design voltage.
For example
<div>
<ul>
<li>Nominal 12 V, with 5% regulator -> maximum
design voltage is 12 V x 1.05 = 12.6 V. Minimum
capacitor voltage rating: 25.2 V, so you would
have to use a 35 V capacitor, not the 25 V one. </li>
</ul>
There was some flexibility. If space or cost were
tight, we could probably negotiate the smaller one in
this example, because it is all under our control. But
for external inputs, where things are less controlled,
we would go the other way.</div>
<div><br>
</div>
<div>No idea where you would find 800 V capacitors?</div>
<div><br>
</div>
<div>EV City (Waltham Street) have a Nissan Leaf cut in
half (diagonally, so it is still has 4 wheels and is
drivable) showing the internal components. I think
they also have an open drive unit and control unit.
You might be able to see what they use.</div>
<div><br>
</div>
<div>Stephen Irons</div>
</div>
<div id="gmail-m_-8799095192780379565geary-quote"
dir="auto"><br>
On Thu, Oct 1, 2020 at 09:16, Peter Harris <<a
href="mailto:petes.username@gmail.com" target="_blank"
moz-do-not-send="true">petes.username@gmail.com</a>>
wrote:<br>
<blockquote type="cite">
<div dir="ltr">General question: What is the rule of
thumb for selecting the voltage of electrolytic
capacitors?
<div><br>
</div>
<div>Specific question: I am working with a Nissan
Leaf battery, the absolute maximum pack voltage is
405V. Are 450V electrolytics likely to explode
used at 405V?<br>
</div>
<div><br>
</div>
<div> P<br>
</div>
</div>
</blockquote>
</div>
_______________________________________________<br>
Chchrobotics mailing list <a
href="mailto:Chchrobotics@lists.ourshack.com"
target="_blank" moz-do-not-send="true">Chchrobotics@lists.ourshack.com</a><br>
<a
href="https://lists.ourshack.com/mailman/listinfo/chchrobotics"
rel="noreferrer" target="_blank" moz-do-not-send="true">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a><br>
Mail Archives: <a
href="http://lists.ourshack.com/pipermail/chchrobotics/"
rel="noreferrer" target="_blank" moz-do-not-send="true">http://lists.ourshack.com/pipermail/chchrobotics/</a><br>
Meetings usually 3rd Monday each month. See <a
href="http://kiwibots.org" rel="noreferrer"
target="_blank" moz-do-not-send="true">http://kiwibots.org</a>
for venue, directions and dates.<br>
When replying, please edit your Subject line to reflect
new subjects.</blockquote>
</div>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
Chchrobotics mailing list <a class="moz-txt-link-abbreviated" href="mailto:Chchrobotics@lists.ourshack.com">Chchrobotics@lists.ourshack.com</a>
<a class="moz-txt-link-freetext" href="https://lists.ourshack.com/mailman/listinfo/chchrobotics">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a>
Mail Archives: <a class="moz-txt-link-freetext" href="http://lists.ourshack.com/pipermail/chchrobotics/">http://lists.ourshack.com/pipermail/chchrobotics/</a>
Meetings usually 3rd Monday each month. See <a class="moz-txt-link-freetext" href="http://kiwibots.org">http://kiwibots.org</a> for venue, directions and dates.
When replying, please edit your Subject line to reflect new subjects.</pre>
</blockquote>
</body>
</html>