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Topic: What size capacitor needed to power solenoid? (Read 4 times) previous topic - next topic

Terry King

Redtank, take another think about this...

You only need 35 to 40 amps for 50Ms.  Your power supply only supplies 3.8 amps.  But the duty cycle is only .05/5 or .01  (1%).   So the AVERAGE current is less than .5 Amps.

So you need some storage.  Unless this whole thing has to be real portable, why not just use a small 12V GelCel battery, maybe 25 aH or so.  It should not have any problem with 45 amps for short duration.  Your 12V supply can charge it, and not overcharge it.  No reason this has to be a capacitor.

So you have a fairly 'stiff' (well-regulated) 12 volt supply good for short term currents of probably 50 to 100 amps. 

Just switch the solenoid circuit on for the 50 mS and then off, through some power FETs.  Make sure you have a good spike suppressing diode across that solenoid inductor, probably one rated for use in switch-mode power supplies or something.

It would be "good" if you could characterize the time it takes for current to rise in the solenoid (It's an inductor so the current will start off low and then rise and then level off at the resistance-defined value after a while.)  Defining the "While" would be good. Do you have an oscilloscope?? 

Lefty, Grumpy ?? Sanity check?? 
Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

retrolefty

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Lefty, Grumpy ?? Sanity check?? 


I agree that a battery would have a better chance of success then a 'hang a hugh cap on it' solution. At that coil current level and induction I'm not sure you could afford enough caps with the needed ESR to pull in such a hefty solenoid.

Lefty


MarkT

[ I won't respond to messages, use the forum please ]

Grumpy_Mike

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Lefty, Grumpy ?? Sanity check??


I was once sat a very short distance away from a coil being fed DC current with a large capacitor across it. The capacitor exploded, the whole room was filled with silver paper fluttering down like snow. It was only after this did the engineer in charge of it think of ripple current.

When current flows a mechanical force is generated in the conductors. In normal low currents this is insignificant and well withing the limits of a device be it battery or capacitor. However as the current increases so does the force.

Without the correct and sufficient calculations this could be a dangerous thing just to try.

However, the gung-ho attitude is very prevalent in some areas of western culture, and if you survive it is very funny.

Terry King


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Lefty, Grumpy ?? Sanity check??

...
However, the gung-ho attitude is very prevalent in some areas of western culture, and if you survive it is very funny.


Grumpy, I assume you mean areas starting some 1000 + miles West of Manchester, UK  :smiley-roll:

I'll concede that the number of capacitors intentionally exploded in school laboratories in the USA probably exceeds that of all other countries combined.

However as we mature we graduate (IF we graduate) to resistor values that simply smoke and burn in 2 seconds of less, with a satisfying "pop-whoosh".  (Values supplied upon adult request).  This is a cultural tradition and I am happy to say that my 40+ year old son has kept it up in teaching "Electronics 101" to employees of his company in Vermont. 
Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

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