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Registered Member #2563
Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280
Hello, i will soon be buying one or a few of these capacitors, they are DC 500 volt 5600uf, Which would be about 700 joules each, and i was wondering what i could use as a switch on the buisness end of them. for use in a can crusher.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Look at ny bank for inspiration, a spring loaded trigger would save your lytics from some ringing that will damage them.
EDIT: My spring trigger works like this, the line in the middle is a spring with a piece of string and connected to the circuit, see schematics on my site.:
charging terminals --> | <-- discharge terminals
Pulling string for charging: -->\ <-- Letting go of string for discharging: --> /<-- Spring returns fast to upright position: --> | <--
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
I have lots of experience with can-crushing, but not doing it with electrolytic caps.
A mechanical switch or mechanically-triggered spark gap (if you have well over 500 volts) will do fine. As will a screwdriver, or touching a flexible wire to a terminal (please use eye protection and electric shock protection). Of course there are semiconductor switch and electrically triggered spark gap solutions.
I don't see how any of the switch options address the problem of voltage reversal in electrolytic caps, since can-crusher circuits tend to be underdamped (more so when the work coil is empty). Clamp diodes are one possible remedy. Another is to design-in enough resistance for critical damping, and start with more stored energy (by a factor of 4 to 8, as a SWAG).
Some data points from what I believe to be an especially efficient design (52 uF HV pulse cap, different 4 turn coils): 26 joules makes a barely detectable indentation on can. 1000 J cuts cans in half.
If you have more microfarads and fewer volts, I think the optimum coil will have fewer turns, but the required currents will be higher.
[edit] 1. A few successful electrolytic-based can crushers are easy to find by Google or searches of this forum. 2. I urge you to play with Barry's RLC simulator at to explore current levels and voltage reversal. A 4-turn can-sized coil is about 2 uH empty, and inductance varies as the square of number of turns. I have no idea how electrolytic capacitors handle pulse currents of tens of kA. The way to reduce current (for a given amount of crushing) is to use more turns & higher voltage. 3. Amateur crushers often use ridiculously thick wires, but wires too thin will explode. If the work coil contains most of the circuit resistance, and has the thinnest wire, make sure it has enough copper to absorb the full capacitor energy. For example, 2800 J will heat an 80 gram coil by a tolerable 100 degrees C.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I would bet they are not good because they tend to have high ESR and ESL and do not survive reverse voltage well. They also are only made up to 500v or so, so they limit the maximum voltage you can use in your system. Not to say they won't work though, they will probably work just fine for a while in a properly designed system.
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Capacitor switching,
