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LAN_403

picnic_bear

Thu Mar 11 2010, 05:57AM

Registered Member #2731 Joined: Thu Mar 11 2010, 05:48AM
Location:
Posts: 4

Hi,

I'd like to make a coilgun but before I have to understand how it works because I don't want to destroy material. I'm reading this page:

It states that:

electrolytic capacitors should not be placed in series for pulse applications because the ESR of the bank can very quickly become larger than the impedance of the load, and if that happens most of the energy of the discharge will be dissipated internally, damaging the capacitors.

I'm not sure to understand what it means. Can you help me please?

Thank you,

HazzWold 1993

Thu Mar 11 2010, 07:00AM

Registered Member #2563 Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280

It means if you use electrolytic capacitors you must have them in a parallel configuration, having them in series will cause the issues stated ^ and damage the capacitors.

Mattski

Thu Mar 11 2010, 07:01AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Every capacitor has some amount of resistance that is in series with it, which is called the equivalent series resistance (ESR):
o-----||----\/\/\/-----o
Where:
-----||-----
is the capacitor and
--\/\/\/---
is the resistor. If you put a bunch of capacitors in series then the resistors add up. In contrast if you put capacitors in parallel then your total ESR decreases. Your power dissipation in your capacitors is I*I*R where I is the current in your pulse, R is the cap resistance. So to minimize power dissipation in your capacitors, which can damage them, it can be better to put caps in parallel.

Of course if you start of with very low ESR capacitors maybe you can put them in series with no problem. Or you do some in series, some in parallel.

When putting caps in series you also have to worry about voltage equalization across the string.

teravolt

Thu Mar 11 2010, 08:30PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

If you put the elecrolytics in parallel you can reduce there over all ESR. Just remember to keep the polarities the same when you connect them together in parallel. when you do this make shure you use copper strip or aluminuim bussbar tro connecthem together because all materials also have resistance. when your coil gun is triggered ther can be on upward of 1000 to 4000 amps with the example given by powerlabs.

klugesmith

Thu Mar 11 2010, 08:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Aside from the minor issue of voltage equalization...
A change from parallel capacitors to series should make no difference in a coilgun application,
IF you re-wind the coil accordingly, and use a higher voltage charger and switch.
[edit]It's a basic exercise in tuning the load (voltage vs current tradeoff) to match the power source.

Suppose the reference design has 4 identical capacitors in parallel.
If you change them to a series configuration, the new bank has 1/16 as much capacitance at 4x the voltage, and stores the same energy. It also has 16x more ESR and ESL than the original bank configuration.
[edit] This will result in very different discharge dynamics, if the load is unchanged.

So wind a different coil, with 4x as many turns, using wire with 1/4 of the cross sectional area.
Coil size and weight are the same, but inductance and resistance are 16x greater than original.
The new pulse discharge will have the same time constant and damping ratio. Coil current is 4x less than original,
but there are 4x as many turns so the magnetic field strength is the same as original. Discharge current in each
capacitor is same as original, as are the ESR losses.

radiotech

Sun Mar 14 2010, 01:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

from the original quote "the ESR of the bank can very quickly become larger than the impedance of the load, and if that happens most of the energy of the discharge will be dissipated internally, "

If you consider the capacitor as a battery, the maximum energy will transfered when the internal resistance (esr) is equal to the resistance of the load. The total energy dissipated is I^2(esr +load resistance) The load reasistance in a coilgun is copper losses plus
losses (translated into heat) of the energy transferred to the projectile. Block the muzzle of a gun, and the barrel splits sometimes. Finding that point of maximum power transfer is the name of the game. The difference between two electrolytics in series and a single one with the same theoretical (joule output) of the sum of the two is that four metalic structures,and two dialectric to electrolyte interfaces are involved.

Capacitors have esr ratings so you would have to know what these are before comparing two in series to a single larger one.

Comparing impedance to resistance matching is difficult unless you know what the resistance component of the impedance is. Maximum energy is transferred when the source impedance is equal to the conjugate of the load impedance.

picnic_bear

Wed Mar 17 2010, 11:39PM

Registered Member #2731 Joined: Thu Mar 11 2010, 05:48AM
Location:
Posts: 4

Thank you!

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