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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Ring launcher problems/questions

Author Post
benbmw
Fri Dec 04 2015, 11:49PM Print View
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Posts: 10
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. I sourced the caps from a junk yard and im not sure how old they are but the most likely went through numerous 110 degree summers. I am using a mot controlled by a variac to charge them through a 750ohm resistor. I am switching using a rat trap to slam wedges of copper together which discharges into a 4 turn coil of 2/0 copper cable. I have also made a 10 turn coil of 8 gauge wire. My projectile is a 1/4" thick by 4.5" diameter aluminum disk. To protect my cap bank I had a Chinese 1600v 100a stud diode in anti-parallel to hopefully conduct and dissipate negative voltage spikes.

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. I checked each side of the series config and one was at -5v while the other was at +5v. I also had removed and checked my fly back diode to find that it was short circuit in both orientations. There doesnt appear to be any physical damage to the caps or diode. How could the diode have blown with that low of voltage? Any suggestions? Would a diode in series with the work coil to completely block reverse voltage be better?

Thanks for any help,
Ben
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klugesmith
Sat Dec 05 2015, 01:10AM
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Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1478
Welcome, Ben.

benbmw wrote ...
Would a diode in series with the work coil to completely block reverse voltage be better?
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 underdamped RLC network).

As for destruction of a diode, how much current did you send through it, and for how long?
How did you figure those values?
Barry has an online calculator for RLC pulse discharges here:
and an online calculator for inductance of air-core coils here:

You can easily figure the resistance of your coil and interconnections, and explore some wild-ass guess values for capacitor ESR. More tricky is the magnetically coupled projectile, which will reduce the effective inductance and increase the peak discharge current.

Did you make any effort to reform your long-idle electrolytic capacitors before suddenly giving them hundreds of volts?


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DerAlbi
Sat Dec 05 2015, 01:27AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
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.

First problem: Series capacitors
Series capacitors will differ in capacitance and ESR and leakage. Therefore they will be charged differently and whats even worse: they discharge differently. That explains your -5V on one side of the bank after you test. This is actually a damaging condition for an electrolytic capacitor and you should only apply voltage slowly and over a high resistance. This gives the electrolyte a chance to recover the dielectricum.
Solution: put an antiparalel diode across EVERY capacitor. You need to avoid the chance that a cap can discharge below 0V.

Second: 100A-Diode.
When the Capacitors are maximal discharged, the current is at its peak (most likely not - because you are most likely ESR-bound, but accept that for now). This peak current will then flow through the diode which is verry likely to be way undersized with 100A. (datasheet?)

Third: A diode in series will not block reverse voltage.
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.
Think about it this way: imagine a LC-Oscillating circuit. Now put a Diode in. The cap is initially charged then its discharging. The diode conducts, the indurctor builds up its magnetic field. Will the diode provent the capacitors to be so much discharged until its negatively charged? No! because all the time the current flow was in one direction.

Fourth: A diodes death...
is caused by over current and over heating. That there is low voltage after the Diode is shorted is only an implication of the shorted diode. I mean: how can any voltage be across a shorted component? The -5V and +5V add up to 0V. If you mean with "low voltage" the 100 or 300V, then still: it does not matter.
The "low voltage" will still be enough to cause a significant current, and the diode simply couldnt handle it.


What is actually wrong in your design and why your disc does not move as powerfull as you expected i cant say. Maybe others know more of what is going on. I am not into ring or disc launchers. It could simply be that your voltage is too low and you current does not rise fast enough.
But i dont want to encourage you to crank the voltage up... your current circuit and understanding likely cant handle that safely.

Maybe you could tell us more about what possibilies you have to measure whats going on?
Do you have an oscilloscope?
Are you able to work with LT-Spice?
Maybe a photo of your setup?
How old are you actually?
What education did you receive if you are older than 18?

Edit: damn. Klugesmith. you were fast. and you have links to usefull sites. grrr. you win! But we adress similar points.
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benbmw
Sat Dec 05 2015, 01:34AM
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Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 10
klugesmith wrote ...

Welcome, Ben.

benbmw wrote ...
Would a diode in series with the work coil to completely block reverse voltage be better?
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 underdamped RLC network).

As for destruction of a diode, how much current did you send through it, and for how long?
How did you figure those values?
Barry has an online calculator for RLC pulse discharges here:
and an online calculator for inductance of air-core coils here:

You can easily figure the resistance of your coil and interconnections, and explore some wild-ass guess values for capacitor ESR. More tricky is the magnetically coupled projectile, which will reduce the effective inductance and increase the peak discharge current.

Did you make any effort to reform your long-idle electrolytic capacitors before suddenly giving them hundreds of volts?





Thanks for the info. I think my mistake was trying to reform them in the 2s4p configuration. I just finished reforming 4 more capacitors all in parallel and tried them(keeping them in the parallel only configuration) and I got better results. It will slap a tinfoil disk against the ceiling with some force. the disk is made of tin foil folded for 16 layers and weighs about 10 grams. unfortunately the 150 gram 1/4" aluminum disk will only launch about a meter. this is much less than 1 percent efficiency. I am using the 8 gauge coil for these tests and it is sandwiched between some 3/16 plywood and held together with rubber bands temporarily. The disk starts about 1/4-3/8 away from the actual coil. Im not sure if this would affect it.

Another thing I noticed when reforming these capacitors is that when I first reformed them to 400 volts my variac was at about 17 or 18 on the dial. Now, to get it to 400 volts I need to go to 20 or 21 on the dial.

I have not made the calculation to estimate the current pulse yet.

EDIT

Thank you for all the information DerAlbi. I will upload some pictures of my setup in a bit. I think I was very unwise about how I used(and reformed) the old capacitors in series. I dont have an oscilloscope but I charged them all up individually and then measured how much they would self discharge over 30 seconds with a multimeter. This ranged from 6.5 volts to 30 volts leaked over 30 seconds. Im sure the load and charge was very unequally shared. I just turned 19 and I am currently at a community college but I am planning to transfer to a university and major in electrical engineering. I have some knowledge of electricity and circuits but obviously not that much. Enough to make a spark gap Tesla coil two summers ago. Also i have never worked with LTspice

here is the link to the album showing my setup:
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DerAlbi
Sat Dec 05 2015, 02:20AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Hmmmh. Well i would like to encourage you to try yourself in LT-Spice. Its a skill you definitively must have if you want to go in this direction. (i am a university teacher in analog circuit design) LTSpice is really easy to use and basically every question you could have is easily answered by asking google that exact question Its cheaper to experiment in LTSpice and actually safer. It just takes a little time to learn. And with will payback 10-fold.

The different leakage on your caps could vanish if you reform the caps over night. It will still not help with the different capacity... discharging series capacitors to 0V is a flawed concept in that manner.

The variation in your dial can simply be explained by voltage differences in your local power grid.

And now try to estimate your current pulse! Current is what propells your disc.. so you should have actually started there right from the beginning.
I mean.. how else you could know if your diode will be ok? Or your switch.. or your wire or... whatever you have in the circuit
Every situation you encounter in electronics will be described by numbers and can be checked against specifications. Thats the proper way to do it.
If the numbers are hard to calculate (it becomes easier over time), you use a simulation program like LTSpice. Simulation is however a science on its own and it also provided many sources of error. But better an error there than.. you know..
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benbmw
Sat Dec 05 2015, 02:37AM
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Joined: Thu May 14 2015, 06:47PM
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Posts: 10
Thanks again Albi. I will definitely check out LT-spice and learn to use it. I will also try reforming all my capacitors overnight. I assume the way to do this is leave them charging at their rated voltage overnight. Is this correct? Or should I bring them up slowly in increments and then, once I reach the rated voltage, leave them overnight?
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benbmw
Sat Dec 05 2015, 07:53AM
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Joined: Thu May 14 2015, 06:47PM
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Posts: 10
I started reforming 8 capacitors in parallel and I brought it up to 400 volts with the variac and let it sit for a few hours. After that time it was down to about 370v with no change to the variac. I measured the voltage across the 750 ohm charging resistor and it was about 20v. Also when the voltage on the variac was turned to zero, or when the charger was completely disconnected, the voltage in the capacitors dropped at a rate of about 1 volt per second. It seems that the leakage is getting worse. could this be caused by one faulty capacitor that might be beyond repair? Im not sure if I should leave them to reform overnight.
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DerAlbi
Sat Dec 05 2015, 11:41AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Hmmmmmmmmh. Dont leave them over night.
First of all reforming means a SLOW increase of voltage. So basicall you start at 50V, wait until the leakage is within specifications (Datasheet), then you dial the voltage up to 100V, wait again.. and so on.
Applying 400V directly is not reforming, its "stressing the hell out of the suckers".
An electrolytic cap is basically a aluminium foil covered in aluminium oxide. This oxide is constantly damaged by stress and reformed by a chemical reaction thats happening if there is current flow from the electrolyte to the aluminium. The holes in the dielectric oxide layer close by them selfes usually. However they can be beyond repair... why?
Imagine: a tiiiiiny little hole. (plase stay focused on dielectric layers! ) Though this hole there is 400V dropping with about 26mA. That means Power disipation! And its a LOT. its around 10W. This "W" does not only mean Watt, its means WOOOW in this case!
Tiny structures wont handle 10W of power. What happens is that the capacitor heats up locally around the dielectric failure and the electrolyte is destroyed. So the electolyte has no chance to repair the failure.

I suggest you test their leakage one by one and sort out the really bad one(s). If you end up with half the capacitors - no problem. Now your project has just become more interesting, because you need to double the efficiency to get the same results
Next time you reform some caps, use a way bigger resistor. 10K+
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klugesmith
Sat Dec 05 2015, 05:43PM
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Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1478
Ben, you are right about not wanting to test or reform old capacitors in parallel. You can't tell which way the leakage current is going.

You can reform them all at the same time, using a separate resistor for each capacitor. As Der Albi said, 10 kΩ is at the low end of recommended resistance. I think it would be fun to put a neon lamp in series with each resistor, for visual indication of capacitor currents & reminder to be conservative with the variac knob. Be sure to post a picture for us.
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benbmw
Sat Dec 05 2015, 06:50PM
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Joined: Thu May 14 2015, 06:47PM
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Posts: 10
Thank you for the suggestions klugesmith and DerAlbi. I will order some 20w 15kohm resistors and wait for those before continuing to reform. I dont have any high wattage resistors besides 4 200w 750ohm resistors from the original cap banks that these capacitors were in. The visual indicator is a good idea. The majority of small neon bulbs i see online are rated for less than a millliamp at 120v. Its seems that I would have to find a lamp that will handle up to 20 ma but I would still want it to be visible when the current drops as it charges.
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benbmw
Sat Dec 05 2015, 07:59PM
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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.
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klugesmith
Sat Dec 05 2015, 08:37PM
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Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1478
Now you're talking, Ben.

You don't need power resistors if this is just for reforming, not charging. For example, sticking with 15 kΩ (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.

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benbmw
Tue Dec 08 2015, 06:31AM
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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.
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Signification
Mon Dec 21 2015, 04:19PM
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Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 285
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.
.
.
.
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!


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Signification
Sun Dec 27 2015, 12:53PM
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Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 285
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)

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DerAlbi
Sun Dec 27 2015, 01:29PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
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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