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Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
At 05:28 AM 5/29/2001 +0000, you wrote:
>The Group is (within the next few
>weeks) going to guarentee it's caps. This will be a retroactive guarentee to
>everyone who has already bought a Group MMC cap and also for future sales.
>It will basically be something along the lines of "If you buy your MMC caps
>from the Group, hook them up in a properly designed manner and run them
>within reasonable limits (their design specs) and they fail, we'll send ya
>new ones in exchange for the old ones (we autopsy the old ones to see how
>you killed it).
>What do you guys think of this? Ideas? Thoughts?
Since the caps have such high voltage and current ratings, failures should
be very rare. Examining the ones that do fail is very valuable! They are
easy to autopsy and a great way to study how they work in our very rough
service. It also gives the cap manufacturers a special look into how poly
caps behave that their competitors don't have ;-)
Some caps are going to die from "user error". I found that sending along a
sheet with the following tips really helped stop most common mistakes.
Well worth the $0.05 to send it along with all orders.
I would especially watch #s 2, 3, and 4.1
Here are the essential tips in building MMCs:
1. Always make the strings individually selectable so you can get many
different values of capacitance.
2. Leave a small (~1/8")air space in between the caps in a string for
air flow and to leave room if one fails.
3. Be sure to leave a good distance (~1" or more) between the strings
so if some are not used, that active strings will not arc to inactive strings.
4. Put 1/2W 10Meg ohm (or some close value) resistors across each cap for
safety and balancing.*
4.1 Do not place the resistors directly against the capacitors! The thin
capacitor shell may not provide good insulation. The capacitors need a good
space between them and other objects to insure they will not arc over. I
would recommend about 1/4 inch clearance between the capacitor cases and
5. Be sure the resister leads are actually wrapped around the cap leads
so they will stay in place if the solder gets hot or the solder connection
was not sound.*
6. Remember that the whole cap is at very high voltage so it needs to be
mounted away from metal and in a safe location.
7. Perfboard, like DigiKey# V1011-ND, makes mounting the caps really easy.
7.1 Be careful when bending the leads not to damage the connections inside
the capacitor. Tubular style caps can especially have fragile lead connections.
8. Do not connect the caps between strings together even though they are
at the same potential. This defeats the cap's self healing and value
9. Even though the cap leads seem small, use heavy primary wiring and try
to use brass or copper connections as with any Tesla cap. Bringing the ends
of each string to 1/4" brass bolts works well.
10. Remember that even a small EMMC cap can be just as dangerous as many
big oil filled caps... The resistors go a very long way in making them safer
but don't get careless.
* People have gotten shocked by not doing these.