View single post by Helmut
 Posted: Sun Nov 27th, 2016 02:40 pm
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Helmut



Joined: Sun Feb 17th, 2013
Location: Friedberg, Germany
Posts: 1129
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http://www.zeva.com.au/Projects/SpotWelderV2/
is a nice device for Si
to show that CDUs produce arcs. Of course they do, as will every other AC-transformer, too, if you make it to deliver enough amperage. The superiority of CDUs over other ways of operating solenoids is due to their inner resistnace being ~zero and ability to deliver an instantenuous current. So of course you risk to fry a contact if there is a solid short in the switch machine circuit. Toy transformers in the market have to meet certain requirements, the most important one being that in a case of a continuous short, the energy dissipated on spot must not be sufficient to cause a fire. Therefore these transformers are built in such a way that their short-circuit current is limited, e.g. a 16VAC transformer rated 32W  can only deliver ~4A @0V.
Any capacitor charged to say, 20V and being shorted by 1 Ohm, which is the common value of the sum of lead resistances, delivers 20A if shorted. As the duration of that spike depends on the capacitance, no harm is done when it is small enough. Only delicate IC structures can even be damaged then, however. When you go up to 2200µF, you can produce a short arc ( some 10msecs ) hot enough to melt the contact.
Take a transformer rated 100VA@16V intended for industry purposes -their characteristics are completely different from those for the toy market - and you'll produce very nice arc, too.
So it's all about inner resistance and short circuit duration whether there will be damage or not. No reason to for any advantage of AC over DC. Edison even fried an elephant to defend his point in the 'AC/DC war' and lost nevertheless.

Last edited on Sun Nov 27th, 2016 02:42 pm by Helmut



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