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Ring launcher problems/questions

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benbmw

Sat Dec 05 2015, 07:59PM

Registered Member #55052 Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 12

I did some calculations and used Barry's simulators to figure that if I made a coil with 10awg wire(about 19uH and 21 turns) and put 8 capacitors in parallel I would get about 8ka peak with a 2ms pulse. I have a 500a 1600v puck scr that has a 6.4ka peak current rating for 10ms. I would like to use this instead of the rat trap switch. Do you think it will handle the current? I know that I will also have to make a clamp for the scr. I was thinking some pieces of 1" square aluminum stock bolted to sandwich it with a few layers of circuit board(garolite) for insulation between the clamp and aluminum bus bars being clamped to the scr. I can use a torque wrench to get the proper clamping force. Here is the datasheet:

I am also looking at this diode on ebay:

It doesnt have a datasheet but it looks similar to these diodes on mouser and about $10 cheaper. datasheet for mouser diodes:

In the RLC simulator there was just over 4ka reverse current and these diodes have a 6850a peak current rating so they should be fine.

klugesmith

Sat Dec 05 2015, 08:37PM

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

Now you're talking, Ben.

You don't need power resistors if this is just for reforming, not charging. For example, sticking with 15 kÎ© (I type capital omega by holding alt while typing 234 on numeric keypad of Windows PC),

Something is wrong if a capacitor sustains a current of many milliamps while reforming. if you limit DC source voltage to 75 volts above the present capacitor voltage that's 5 mA, 375 mW. 10 mA would be 150 volt average drop (!) and 1.5 watts in current-limiting resistor. Actual RMS current and power will be higher if you're using unfiltered rectified AC.

Why did I say neon lamps? LED's are usually specified at 20 mA. I can send you some surface mount LED's that are OK with 20 mA, and plainly ON in ordinary room light at 0.1 mA (like pretty much any LED with a visible die).
[edit] LED might want protection against reverse voltage in this application, for example with an antiparallel regular diode.

benbmw

Tue Dec 08 2015, 06:31AM

Registered Member #55052 Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 12

Once i get the capacitors reformed ill give an update. Ill try using LEDs as indicators as well. I wasn't thinking right about the reforming resistors. I had it in my head that there would be 400v through them, but it will really only be the difference between the cap charge and the power source.

Signification

Mon Dec 21 2015, 04:19PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

benbmw wrote ...

I recently finished construction of a ring launcher and I am having some issues. I think most if the problems come from the capacitors I am using but I wanted your opinion so ill describe my setup. I am using 8 400v 3600uf electrolytic caps in a 2s4p configuration.
.
.
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So after the first shot at only 100v the aluminum disk barely jumped no more than an inch. I tried a 300v shot and just before firing I heard some metallic "pinging" or wrinkling sounds coming from my capacitors, but I fired anyways with no better results than last time. I checked my bank because it wouldn't hold any voltage after that.

After benbmw mentioned the ** metallic "pinging" or wrinkling sounds coming from my capacitors** I was surprised that no one addressed this further...sounds like this likely came close to killing one or more of the caps!
Also in the aforementioned "reforming of 8 capacitors" were you speaking of this same bunch of 8?

One more thing: NEVER forget the --voltage-- rating of the resistors! I did---ONLY ONCE!

Signification

Sun Dec 27 2015, 12:53PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

DerAlbi wrote ...

Hoooly F. You are working with quite a lot of energy while you are missing fundamental principals of electronics. I suggest you massively scale down your setup to learn whats going on without damaging expensive-to-replace parts. That you startet at 100V was actually very good... but you can learn and experience your circuit flaws also with 2x 100uF @ 12V in series, smaller diodes and a smaller coil.

This statement has made me think about actually setting up guidelines stating what components can be replaced with -safer/smaller- ones to actually make high power experiments without effecting desired actions due to the replacements--thanks Albi.
============================================ =====

OK: OAN, The some responses to the statement by benbmw:
""Would a diode in series with the work coil to completely block reverse voltage be better?""

DerAlbi responds:
..."A diode in series will not block reverse voltage."...

Which I believe is WRONG since this implies the voltage "reverses polarity"

He (DerAlbi) continues...
..."Pease understand that a negative voltage can not be blocked by a series diode.
A diode will be conductive as long as the current flows in the right direction."...

Again, here, it is my opinion that the first sentence is, again, wrong. However the next line is accurate.

Then klugesmith responds 100% correct, clearing up that it is CURRENT DIRECTION (NOT unsigned di/dt) with:
"No. The diode would conduct until the current reaches zero. Then your capacitor bank would be sitting with the maximum possible reverse bias (assuming an under-damped RLC network)."
================================================= ======

So, I think that it should be understood that it is REVERSAL of current / Voltage polarity that counts--NOT RELATIVE RATES. The diode continues to conduct as long as voltage is at a forward polarity. The diode will still conduct even if the voltage decreases (but does not change polarity). For example: in a forward biased diode with a sine wave signal with +peak=+300VDc and -peak +200VDC, the forward biased diode ALWAYS conducts this entire sine, even when it goes from RISING to FALLING!

AM I CORRECT??? IF NOT PLEASE HELP!!! (I may be wrong...I did feel a brief moment confusion here)

DerAlbi

Sun Dec 27 2015, 01:29PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I understand your confusion about the diode stuff.
I think my point is the same as klugesmith just with different words.

"Reverse polarity" is really dependend on where you measure. You are completely right, that as long as the voltage drops a certain way across the diode the diode will conduct. However that will not imply that the whole circuit series string of components will have the same voltage drop direction.

Just imagine a "capacitor - diode - inductor" series arrangement. The Capacitor starts at +100V and will end at -100V even with diode. Because the diode did not block reverse voltage across the cap since it allowed the current to flow independent of capacitor voltage polarity.
The key here is to understand that in a series string not the voltage across the whole string is important but the current direction.

Thats why my senstence is (imho) right:
"Pease understand that a negative voltage can not be blocked by a series diode. A diode will be conductive as long as the current flows in the right direction.".

If the first sentence was wrong then it would say:
"Pease understand that a negative voltage can not be blocked by a PARALLEL diode."

i hope that helps.

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