Cap Bank – Rogowski Coil

So before I really get started with my capacitor bank, I decided I would design on paper all the instrumentation so I could characterize the capacitor bank discharges. In order to measure current over time, either current shunts (DC), current transformers (AC), or rogowski coils (AC, pulse) can be used. The advantage of a rogowski coil is that it is equivalent to a air-core current transformer allowing for much faster response to changes in current flow. Also, due to construction, a rogowski coil can be opened and closed for placement in temporary positions.

Yesterday I finished my constructing my rogowski coil, however it still requires an active integrator to make voltage proportional to current. I used RG6 coax cable, stripped the outer sheet, braid, and foil, then wound 30AWG magnet wire evenly around the dielectric. The magnet wire was attached to braid and core at either end, heat-shrinked, and luer-lock syringe fittings were placed on the ends to allow for the coil to be placed around an object.

A major key to being able to construct a accurate rogowski coil is that the windings must be absolutely even, and remain even as the coil is bent and closed. It is easier to wind a coil on a straight segment of dielectric, heatshrink, then bend, instead of attempting to precision-wind around a torroidal coilform.

The luer-lock fittings prior to attachment.

The completed rogowski coil, note the heatshrink around the coil and BNC connector on the end.

Closeup of the joint where the coil closes. I didn’t have small enough heat-shrink so I had to wedge some tiny pieces of balsa wood in to keep everything snug and secure.

Once I had the coil built I gave it a test, first using a small motor, and a second test using my old coilgun.

I used a induction motor fan wired up to 120VAC for the first test, it didn’t seem to be affected by an un-centered conductor, however I may still build a plexiglass support so that under higher-voltages there is no risk of arcing into the coil (the heatshrink + enamel can only stand so much).

The test setup and waveform (sine = 120VAC, other = current).

The second test was performed using my old 430J coilgun, I looped the coil around the heavy cable from the stud-SCR and tested a few voltages. I captured the waveforms using a soundcard oscilloscope program and got some data, however the higher powers generated voltages exceeding the max rating of the soundcard, causing clipping.

The 3 test waveforms captured at increasing capacitor voltages, note the last test at 430V caused significant clipping of the current waveform.

And also, 2 videos (the last 2 tests)

You can never have enough capacitors

So one of my next projects is going to be a (slightly insane?) 3kJ electrolytic capacitor discharge bank using a mishmash of 450V caps. My current collection is as stands:

4x Aero M Inverter Grade 3100uF 450V caps

4x Nippon Chemi-Con Computer Grade 3300uF 450V caps

6x Cornell Dublier 1500uF 450V caps

I plan to purchase another 4-6 of the Aero M capacitors as cash becomes available, inverter grade capacitors stand a better chance at surviving extreme current discharges.  I will likely replace the 4 Chemi-Con caps with 4 more inverter grade caps, and eliminate the CDE caps completely (save them for another project, perhaps a multi-stage CG).

The capacitor bank will be switched by a Powerex C397 Hockey-puck SCR rated for 5500A non-repeating surge current, and I will likely invest in some beefy stud diodes to place in-series on the opposite leg of the capacitor bank to minimize the ringing that will occur due to voltage reversal in inductive loads.

The reason for the series diode is that for any inductive load, there will be significant ringing resulting in voltage reversal. Electrolytic capacitors do not tolerate reverse voltages, especially when swinging from -300 to +300 in rapid succession. A example LCR simulation based on a can-crushing coil results in some… interesting figures.

All in all this should be an interesting project, having a 3kJ capacitor bank will allow for some experiments in coilguns, railguns, magnetic launching, magnaforming, can crushing, and wire exploding.

Monday Rant!

Ok, so 9am classes on Monday, not the best thing – especially when fighting the tail end of a cold.

First and foremost – people need to watch where they are walking. This is a long ongoing rant of mine, but as a greater aspect, people no longer seem to care about other people by basic consideration at all. When you walk down a hallway people now expect you to move out of their way, its rediculous. When the hallway is narrow to 2 people and you can’t move, they ram through anyway and give you dirty looks as a result. I don’t want to seem like a jerk, but I think that this general lack of respect for other human beings is due either to generational and/or racial change, *or* that corporate media has brainwashed people to care about nothing other than consumerism.

Moral of the story: Watch where you are walking, especially when you are smaller than the person infront, and they honestly don’t care if you get knocked over cause you didn’t move…

Edit: Also, girls with TNA bags: no one cares, you look silly and your bag takes up on average 1-1.5 body widths

Science Songs!

So I stumbled across this gem of a find – a song that speaks about why we love NMR.

Hooray for NMR Spectroscopy and   Lyrics

Source: http://www.science-groove.org/

So it turns out there are a whole collection of songs on biology and chem… a select choice for any chem students (or mol. bio). A is for Alanine – Good for any student stuck memorizing amino acid residues

Links:

http://faculty.washington.edu/crowther/Misc/Songs/lyrics.shtml

http://www.science-groove.org/Now/

Bloggers Beware – USAF issues counter-blogging response flow-chart

So, while this blog is mostly tech/science based, I occasionally touch on a subject that I feel needs to be addressed, and in this case it impacts something I feel very strongly about. Today Wired magazine wrote an article describing the US Air Force’s counter-blogging program and response chart regarding how to handle online activities for enlisted airmen/women.

Now the chart appears to be a well designed and thought out document to prevent negative image or PR regarding the USAF, the current war or politics in general, it also creates a potential problem. For anyone that writes any non-pro war statement on their blog, this may attract the attention of their counter-blogging program, particularly if your blog attracts a good amount of traffic per day. With the chart as a general ‘marching order’ for any USAF personnel that blog or read blogs, this creates an (un?)intentional counter-intelligence program that may lead to USAF personnel explicitly attacking those online that voice opinions contrary to that of the current government.

The best propaganda, is that which you don’t believe to be propaganda; someone who leaves a nasty comment at the bottom of a blog post can’t be hired now could they? If you get enough people spouting the same view against someone elses, they become the loudest voice and can hold more influence over others.

Moral of the story: Take everything online with a grain of salt, especially since no responses are credentialed typically. Just because a certain view is the most common does not make it correct by default.

Remember, the citizens media is powerful, just look at all these examples of bad cops being filmed, and posted on Youtube so that they can’t cover up whatever they did. And don’t take my word for it, but historically government agencies have been known for counter-intelligence programs, specifically COINTELPRO. What better way to influence public view than to become it?