Re: SOLENOID QUENCH
2Spoons, Mon Mar 04 2019, 09:01PM

Applying a large reverse voltage is the fastest way to stop the current.
If you drive your solenoids with a partial bridge ( two transistors, two diodes) you can return the solenoid energy to your storage capacitor - which will improve the overall efficiency of a multi coil system.

Re: SOLENOID QUENCH
Signification, Tue Mar 05 2019, 03:52AM

Thanks,
I was also thinking of a center-tap on the solenoid, and when the projectile had traveled to the cut-off point, then just shorting the latter half via the center tap and far end...sound feasible?

Re: SOLENOID QUENCH
DerAlbi, Tue Mar 05 2019, 08:11AM

Oh Signification.. you and your creationist mind...

If i get your drift correctly, you want to build a transformer and compensate the magnetic field by short circuiting the secondary. A center tap is the wrong way to go in that case. In a normal transformer you have a core, so separated windings are coupled very close to 1. The leakage inductance will always be there and prevent a true instantaneous magnetic field shut down. The leakage inductance is therefore something you want to minimize... bifilar winding would do that way better than a center tap.

Now please, sit down and ask yourself why this idea is bad.
1) You want to add turns to your coil that will not contribute to accelration. What does this to your power density?
2) If you care so much about turning off the coil... do you equally care about turning on the coil? If not, why not?
3) How do you think, a magnetized projectile will react to a short circuited coil?
4) What is your overall goal? Reducing suck-back? Boosting efficiency? Why ask specifically about turning off? Can your actual goal maybe reached differently?

Re: SOLENOID QUENCH
Signification, Tue Mar 05 2019, 10:12AM

@ Albi:
How would YOU handle suckback and reverse electrolytic voltage?

Re: SOLENOID QUENCH
DerAlbi, Tue Mar 05 2019, 12:20PM

..by avoiding or even tolerating it.
Depends on your topology. Mostly its a timing isue.

Is it your only priority, or do you also want to maximize energy transfer and/or effiiency?

You have to understands that your single sentence is representing one key issue when building a coilgun. You can not isolate the problem nor solve it in isolation. Any solution comes with trade-offs and you have to evaluate them to find an optimum.
A suck-back-less coilgun with never negative capacitor voltage is easily buildable by means of an asym. half bridge - basically what Spoon said.
A SCR-Design with over-damping or beefy diodes can do the job too. Having suck-back may not be bad, if your overall energy transfer is better. This is why i question your motive behind the generalized-beyond-usefulness formulated topic.
Without boundary conditions, the optimal (concerning efficiency) halfbridge waveform is an infinitly short, infinitly high (dirac) current pulse that requires infinitisimal small inductance. Congratulations..

Re: SOLENOID QUENCH
Signification, Wed Mar 06 2019, 10:00AM

I have only seen electrolytic caps used in coilguns...what is a pulse capacitor were used? I am thinking perhaps most of the energy would be discharged, accelerating the projectile, and perhaps would be mostly discharged by the time the projectile is centered in the coil. With calculations and trial-and-error, of course.

@Albi: I do agree with your number (3), in this case the projectile would undergo braking.

Re: SOLENOID QUENCH
DerAlbi, Wed Mar 06 2019, 11:39AM

Yandersen (inactive forum member now) had a breakthrough with foil capacitors used in coilguns here and afaik hit an efficiency record for the time. He basically used the LC circuit with a SCR and totally tolerated the reverse polarity on his bipolar capacitors. This also shaped the current pulse to the typical sinus-halfwave you would expect, and mathmatically leaves him with a partially charged (altough negative voltage) capacitor which is an equivalent to the halfbridge recouperation achieved with the simplicity of the SCR circuit. That was real out of the box thinking.
The energy density of foil capacitors sucks of course, but i was a great idea. Trde-offs as usual.

Re: SOLENOID QUENCH
Signification, Wed Mar 06 2019, 01:26PM

I found on ebay some plastic energy discharge caps that were to be used in drones--I think. They are 'dry', rated at 500uF @ 1440v (acid test shows they hold over 3kv before failing begins). They weigh between 15oz. and 17 oz. about 11 square inches. At full charge, that's about 520J (47J/sq in.).
I also have some oil-filled metal energy caps: about 200uF @ 2300v (~530J) about the same mass. I know this is not great energy density comparatively. But there may be some advantage to the faster discharge and the fact that the energy is proportional to the SQUARE of the voltage. Compared to 450v, the energy ratio is about 26:1 in favor of pulsars. Capacitance ratio~10:1 in favor of electrolytics (I hope I did this right--didn't check). Setup may require differently wound coils among other things (like triggering circuits).

Re: SOLENOID QUENCH
2Spoons, Wed Mar 06 2019, 09:17PM

If you run through the math, for a given quantity of any particular dielectric it doesn't matter if you go low voltage/high C or high voltage /low C the energy storage is the same in J/kg. The speed with which you can extract that energy will be different, however.

Re: SOLENOID QUENCH
DerAlbi, Wed Mar 06 2019, 10:59PM

If you come anywhere close to the capacitors internal RC time-constant with your LC-circuit period time, you by definition, dont have much L at all. Therefore the coil of the "coil"gun is missing. Therefore you dont have a coilgun.
Energy extraction speed matters in induction launchers, not reluctance motors.

Re: SOLENOID QUENCH
Signification, Fri Mar 08 2019, 04:20PM


I think I get what you are saying before going through the actual relationships pf parameters such as E, C, Q, k, q, etc.
OK.....We use a given 'volume' of a particular dielectric. I am assuming that the energy of the charged capacitor is stored in the Electric field. Also, it would be convenient to have a rectangular (including square) flat dielectric of which the plates (identical) can fit close-contact, fully, and squarely. The dielectric can be made thicker for a low C / high V OR thinner for a high C / low V.
I am mainly thinking in terms of the equation C=(A/d)e*k. Where A is one plate area (same for both), and d is the plate separation. e is epsilon naught (the permittivity) and k is 'kappa' (the dielectric constant). Since 'e' and 'k' are constants, the ratio of A/d determines the 'hi /lo' C, V parameters (I will later check to see if this ratio acts as I suspect).
Since capacitance is, by definition, Q/V (charge per potential 'difference'), I will say (WRT your last sentence), that the capacitor with the low C / high V will be the one that tends to discharge (in general) the fastest.

Re: SOLENOID QUENCH
DerAlbi, Fri Mar 08 2019, 07:19PM

Why is it important that the capacitor discharges fast. Looking at other design, you know already with certainty that electrolytics are very much sufficient devices. Lowering the ESR of the capacitors is only interesting if you can capitalize on that somehow - so why is it your priority?
All your other considerations are... without a clear destination. Capacitor technology performance is characterized, among other stuff, by energy density. There are 2 forms that are important: volumetric energy density (energy per capacitor size) and mass related energy density. (energy per kg of capacitor).
In both aspects electrolytic capacitors are superior over foils. (because AlO3 has higher EpsilonR).
Foils come into play when you have high dV/dt requirements (that is how you specify the current rating/capability of a capacitor technology independent of capacitance). However in a coilgun, if you really need foil capacitors, you have done something wrong in your design and will invest into badly utilized resources for little effect and end up with a bigger an heavier build than actually necessary..

Re: SOLENOID QUENCH
Signification, Fri Mar 08 2019, 09:23PM

@Albi:
MAYBE NOT!!

Re: SOLENOID QUENCH
DerAlbi, Sat Mar 09 2019, 12:55PM

Is this your best rational argument you can do? I mean.. could you elaborate?

Re: SOLENOID QUENCH
Signification, Sat Mar 09 2019, 01:45PM

@Albi: Whenever I refer to a video or a new idea on the subject, even if it works like I wanted, you seem to have already shot it down and criticized me for even referring to it. One example if that YAK coil gun that fires at 256fps. And, YES, I am mainly interested in velocity first, then the efficiency mods (you will say that I am doing this backward). In this case an energy cap charged to a high voltage seems to fire the projectile a good velocities. One improvement was to use large AWG wire so that the high voltage pulse can get a lot of high current through the coil fast! This type coil is larger wire and fewer layers than 'normal'. I have found that using the first stage as a 'standard' electrolytic-powered coil (maybe the first two) to start the acceleration for the fast coils. If I get something worthwhile, I will post it. But it does seem that the slower coil is necessary for the fast coils to function and add high acceleration.

Re: SOLENOID QUENCH
Sulaiman, Sat Mar 09 2019, 03:04PM

just for an idea of orders of magnitude,
if you hope for Mach1 projectile speed, with say 66mm long projectiles,
coil (charge + discharge) time = 100 us
so the coil ON/charge time will be around 90 us and discharge time around 10 us.

I consider electrolytic capacitors suitable for 100 us pulses and marginal with 10 us pulses.

so for projectile velocities below the speed of sound, electrolytic capacitors should suffice,
... in my opinion.

Re: SOLENOID QUENCH
DerAlbi, Sat Mar 09 2019, 07:42PM

For a multistage approach i am with you; fast pulse currents can do their job if the projectile is pre-magnetized preferably up to saturation.
The thread title did not really imply that you dont want efficiency first. If you want to maximize energy transfer no matter what, then also tolerate suckback. Any current waveform that maximizes energy transfer will have some suckback. It has to do with the force vs position curve and with the nature of current rise- and fall times in an acceleration coil and the velocity that is gained.

However the way you put it, i seriously ask, why you dont just build. If nothing matters except kinetic output, then throw money, capacitor mass and build volume at the problem and you will get "velocity first" with no problem. This russian guy who spammed his gun everywhere ("V2006") had the same approach and he build the strongest hobby builds i know. Why not. I personally see it as a massive waste of resources, but thats everyone's own business i guess.

What is often missing is a goal that you want to chase. Some kind of figure of merit that tells you objectively what is good and what is bad. A coilgun, seen as an electric motor, does provide engineering challenges. Problems that require the balancing of a lot of problems and benefits vs loss considerations.
If you only present ideas without context and without goal or figure of merit, any idea on its own may work, but you have to agree that it is always important to point out which trade-offs you will make. You can, of course, close your eyes and ears if you like.
In the end you are a grown man. If you want to implement your ideas, go ahead.

On my side, your sudden idea to use pulse capacitors sounds much more motivated in misleading marketing headlines since the word "pulse capacitor" fits well with the "current pulse" idea in a coilgun.
You have presented no data, no calculation no goal, not even an order of magnitude why you would need such special capacitors and no argument why electrolytics will not suffice.

Re: SOLENOID QUENCH
Signification, Thu Mar 21 2019, 09:58PM

One reason for the pulse energy caps is to allow a faster more 'critical-to-underdamped' pulse which relaxes the problem of capacitor reverse polarity, while allowing a higher current via higher voltage fed coil. The underdamped pulse allows about three (3) times more: e=2.72 (NOT PI = 3.14) than the damped / overshooting / capacitor damaging pulse.

Re: SOLENOID QUENCH
DerAlbi, Mon Mar 25 2019, 01:25AM

Correct me if i am wrong, but arent pulse capacitors mainly foil capacitors? Why fo foils care about polarity?

Re: SOLENOID QUENCH
Signification, Wed Mar 27 2019, 11:50AM

In fact, DO foils care about polarity--especially a brief voltage reversal. Another thing came to mind which should apply to electrolytics as well. If an underdamped pulse is applied through a firing SCR, then why, when the current is approaching the minimum "holding current" don't they just shut off before going significantly below zero (or sooner)?? I thought this was one of the normal ways SCR's turn off!!!!

Re: SOLENOID QUENCH
DerAlbi, Wed Mar 27 2019, 03:14PM

Because you are looking at the wrong waveform: SRCs turn off if the current goes towards zero, not if the voltage is small. In an undamped oscillation (triggered by an SCR) the SCR conducts for the first current half wave - leaving the capacitor at maximum negative voltage due to the phase shift.
What you want is to only conduct for the first quater voltage waveform which is a no go, since current is at its peak if voltage is 0, so the SCR is happily turned on.
In a damped circuit the phase shift is less dramatic of course. But damping also means less efficiency - in the end you shape the current waveform by dissipating heat at a vertain rate so that no (or little) oscillation occurs.
Using foil capacitors and accepting the reverse voltage, utilizing the first current half wave in its undamped shape (sinus) is an incredible efficient and easy design. The issue is how to start the second shot from a reverse polarity (and how to recharge it - needs some kind of a bipolar charger).

Re: SOLENOID QUENCH
Signification, Thu Mar 28 2019, 10:30AM

@Albi:
Um...please excuse me. I was visiting another planet where Planck's constant is a TINY bit different. Of course (assuming R~0) when switched
on, the -LAGGING- SINUSODIAL CURRENT through the SELF-INDUCTOR (L) is going to be zero at the
turn-on instant (t=0+)...by it's very nature. The CAPACITOR (initial VOLTAGE -LEADING-
CO-SINUSOIDAL energy source) has V0=Vmax, Then a Vc(t) form follows.
and will start an exponential envelope cos decrease after t=T0+.

How about an add-on for making high-power SCR's with programmable holding current/inflection polarity cutoff?

Re: SOLENOID QUENCH
DerAlbi, Thu Mar 28 2019, 12:02PM

Aww i lost context. Sry. Did you just make fun of me because i explained something, that you already know and didnt ask (because i did get the problem wrong) or did you just had a blackout and remembered or... i am lost.
If you wonder why SCRs turn on at all since the start current is 0, then the answer is that they are forcefully conducting as long as you power the gate. If the gate-pulse is too short, they actually dont stay on. (i ran into such issue in my coilgun)

Concerning the SCR turn of: why do you think this is necessary.
First, there are GTO-SCRs already which, with some circuitry, can achieve what you want. However, consider what happens, if you turn off a switch as long as you have inductive current flow. You quickly end up in a topology like a half-bridge or with a freewheeling diode - just with an SCR instead of an IGBT or Mosfet.
You cant just switch off current in an inductor. What you wish for makes only sense for resistive loads.

Maybe you should shift your thinking away from the suck-back issue and the whole current-waveform forming idea.. there are limits to the achievable waveform given a certain drive circuit topology. It might be worth a thought that fighting the inevitable physics is a waste of time and instead engineering a solution with the givens could be more productive.

You still didnt state a goal btw.. do you want to maximize energy transfer or efficiency or weight-power density or volumetric power-density or what? If you just want something moving, there is no need for such discussions, it will work

Re: SOLENOID QUENCH
Signification, Tue Apr 02 2019, 11:56AM

What if I wasn't looking at the wrong waveform? In particular, at the capacitor VOLTAGE waveform (cosine). It would be ideal to kill the current (sine) through the SCR right here, since it would be large. What about killing the current flow through the SCR by applying a series capacitor, actually a couple of small supercaps (~2.7VDC) in series which are in turn placed in series with an air core coil (~10AWG wire, two or three layers of 20-30 turns/layer)...two series caps since over 3v is safer. I think I heard this referred to as a class D type SCR commutation.
The whole thing will be triggered at the zero-crossing of the capacitor's zero voltage cosine waveform.

Does anyone see why this series LC circuit will not cut off (via briefly shorting) the SCR at peak (or near peak) current?? It will be a temporary short to the SCR current, bringing it below the holding current.

Re: SOLENOID QUENCH
Sulaiman, Tue Apr 02 2019, 12:46PM

Assuming that a coil does not saturate; current (A) = (integral of applied voltage)/inductance (V.s/H)
(ignoring energy lost to heat and the projectile)
So for a given coil, for the initial and final currents to be zero, the reverse/discharge volt.seconds must equal the forward/charging volt.seconds.

The half-bridge topology reduces the coil current at approximately the same rate as it increases it,
as the voltage across the coil is similar but opposite,
a benefit of the 1/2-bridge is that unused energy in the coil is returned to the power supply.

A coil operated in 'flyback' mode will have the most rapid coil current decay as it has the largest reverse voltage.

One untried (AFAIK) option would be a lower than normal inductance allowing very rapid increase in current over a very short period,
then a diode across the coil to maintain the current when the driving transistor is switched off.
The current waveform would be rapid rise with slow decay, the opposite to a flyback
This would allow a simple topology with high electrical efficiency and low suckback,
but probably only suited for lower velocity accelerators.

Re: SOLENOID QUENCH
DerAlbi, Tue Apr 02 2019, 04:23PM

Signification, HOW do you turn off current in an inductor? You talk about turning off the SCR at peak current... guess what.. at that time, also peak energy is stored inside the magnetic field of the inductor. This energy must dissipate somewhere over time in terms of electrical power, that means, current must flow while a voltage over the coil is measurable.
I mean, congratulations, if you make a circuit that arbitrarily switches off an SCR, but for the moment, dont think about the SCR, but about a general purpose switch, (on system level, not schematic level)
A switch that you just want to open during inductive current flow.. It will arc and destroy it self. This is true for Mosfets that will avalanche, that is true from SCR that will re-trigger due to overvoltage spilling into the gate, that is true to a bipolar transistor's first breakdown - it is true to every current controlling element in series with an inductor: switching off current is impossible. By physics law. You cant violate it. Not even if you start a 1000 more sentences with "What if".

Sulamain: the approach is not untried, but rather the normal operation of some mosfet switched coilguns. If the energy stored in the coil, during turn-off is more than the maximum single pulse avalanche energy stated in the datasheet, the mosfet will die in a very short time and many people have tried to solve this issue by using a free-wheeling diode as you describe. Yet they found that it was not necessarily optimal to choose an extremely low inductance but rather preferred a fast current decay by adding a series resistor to the diode.

Re: SOLENOID QUENCH
Signification, Wed Apr 03 2019, 06:50AM

So, shorting the SCR A-K terminals briefly with a series pair of supercapacitors (which act as a temporary short as they start to absorb energy-forget the wirewound resistor), won't turn off the SCR by bringing it's current below 'holding current'?
I have tried this on a somewhat smaller scale--A discharged high-capacitance capacitor does indeed turn off the SCR. And in this application it's OK for the SCR's main series cap to retain a high charge after turning off. However, IIRC it worked by far best with low C, large V. I think the C made the difference.

Re: SOLENOID QUENCH
DerAlbi, Wed Apr 03 2019, 01:28PM

Yes Yes Yes, you can of course turn off an SCR somehow. I myself would use a different circuit, but that is not the problem. The issue is, that you then have no path for the current to flow after you turned off the SCR. But current must flow! (Because it cant instantaneous change in an inductor.)
The resulting voltage spike will go so far that it will break through the SCR and turn it on again.

Please understand that i do not really comment on the "turn off the SCR" idea, but i try to tell you that such circuit is useless, since the underlying problem (that you cant turn off current in an inductor) is not solved by the circuit.

Again, you try to fight laws of physics. You will loose. I told you before that the best action is to work with what you can actually achieve.
I am not sure if i will comment further on this topic. Your willingness to deep dive into a topic without understanding the context of the problem and this unwillingness to analyze an issue on a system level will lead to nothing substantial but exotic implementations of some underlying topology.
E.g. any topology that you can implement with a turn-off-able SCR you can implement with an IGBT or Mosfet - just much easier. If the current rating, that an SCR gives you, is actually needed is unclear.

Re: SOLENOID QUENCH
Signification, Thu Apr 11 2019, 08:53AM

@ Albi: don't quit just yet...
This reference you sent me in another forum concerning 'fast quench of large capacitors'



was most helpful..I had even done (previously, over time) most of this and other related integrals which helped a lot...I think it may also help here:

@ all:
OK, lets forget quenching via the main SCR switch, and use the capacitor "C" in the series RLC circuit. Does anyone have any thoughts on simply shorting the capacitor "C" at the desired time by perhaps...

1) Simply shorting the cap with another SCR.
2) shorting via the SCR circuit with the capacitor in series (or parallel, or both) with the 2nd SCR?

Re: SOLENOID QUENCH
DerAlbi, Fri Apr 12 2019, 01:17AM

If you keep refusing to tell what goal you want to achieve, i cant really help. Now you want randomly short circuit your capacitor. I mean.. go on. Do you think this will impact your coil current or even shut if off or impact the magnetic field at the instant you remove the voltage?
No offense, but i really cant follow all your misconceptions without you giving context.

Re: SOLENOID QUENCH
Signification, Fri Apr 12 2019, 02:23AM

@Albi:
My primary goal is maximum velocity. I wanted to weaken the capacitor charge for minimum suck-back as a first step. As I move on, I will want to improve other aspects of the coil-gun, depending on what happens with 1st priority being muzzle velocity. I was thinking, first: max. velocity by MINIMUM suck-back (with a single coil), then a 2nd coil to ENERGIZE at the right moment, for a max. velocity 2-coil gun, etc. Perhaps a max. of four to six stages...don't know at this point.

Re: SOLENOID QUENCH
DerAlbi, Fri Apr 12 2019, 09:54AM

A one dimensional goal is always easy to optimize. You ficus on the wrong thing and utilize the wrong methods to accomplish your goals. Maximum velocity is equal to maximum energy transfer. That means you achieve your goal by scaling up. You need at least a multistage (n>=2) design in order to optimize for the zero velocity start and high velocity exit. Also you need to solve the problem at a concept level and not get lost in circuit implementation details.
You overestimate the effect of suckback. Really, just ignore it. Any currentwaveform that maximizes Energy transfer will have a suckback phase, because the high current peak is much more important than the low current tail that decelerates the projectile. It is just a limit that comes with the finite current rise/fall times in an inductor.
Removing energy input (by short circuit or whetever) is far less efficient than actually actively recuperating energy, since this actively removes energy from the coil instead of just wait for energy dissipation in the coil. This governs your circuit topology by compromising at least a second dimension of your optimization goal, namely build size:
a) You use a halfbridge which gives you the best current waveform control and will achieve the best performance in a multistage design, however it is the most complicated to build and the applied currents are limited by IGBT silicon limits.
b) You just scale up your capacitor storage capability and build a multistage SCR design. An optimal SCR design will have more suckback than a Halfbridge, however the current peak, and therefore the acceleration from a given coil is potentially much higher due to less critical silicon constraints. This is also the bigger compromise for the build size - the additional energy storage you will need for this less efficient approach will be very visible.

Again, i want to stress that suckback is really not a too bad thing. Do not focus on avoiding it.
Of course suckback has to be controlled by timing, but if you choose good timing with the natural current waveforms of a given topology, you wont get much benefit if you somehow could remove the suckback portion of the acceleration. And even if you experience gains, you have to weigh those against the additional circuit complexity you have to manage. At some point, it is easier to just add another stage if you are just after velocity.

I also want to mention that there is a slight possibility that you suffer from premature optimization syndrome. How will you ever know that your current waveform does not produce suckback? Measuring force on the projectile is really, really hard. Without measurements you actually have no idea how big the actual problem is nor if you solved the problem. So anything you will do to your gun in order to optimize the issue you are focusing on will be snake oil. At the end of every shot you only have the exit velocity and if you change timing and you get a better result you will never know if this was due to less suckback or a more optimal placement of the current peak which transferred more energy than the additional suckback removed.

To sumarize: live with the topology you are comfortable with. If you dont want to build a haldbridge, accept the behavior of an SCR design and put your work into adding another stage. If you want to add complexity to the SCR design, dont. Invest in a halfbridge instead.
That is the best advice i can give. Sorry.,

Re: SOLENOID QUENCH
Signification, Sun Apr 14 2019, 02:12AM

@Albi+
Perhaps I still haven't given enough information about optimization:
In school, I loved to apply differential calculus to finding the actual rates of change for applications in real devices. Where I worked in the 90's, I would plot (on very large paper sheets about the size of a large screen TV) "rotary encoders" (digital cameras messed that up--and sooo many people thing the digital screen has so much more resolution than film!). This saved the company a fortune. I would make the drawing and photograph it with old B&W film--the simple developed 24 x 36mm negative would be the encoder! Custom encoders cost $$$.
I can rig a linear encoder that can read encoding rings in the black projectile and determine it's position, velocity, and acceleration (and additional derivatives, I doubt I need). I can get 'lots' of information this way from the single firing of the first stage, and where things are not optimized, make adjustments. "This is with just the 1st stage in place" Then I can add the 2nd stage and try to maximize velocity by adjusting parameters with stages 1 and 2, etc. I haven't done it yet, but from previous experience, it can be VERY effective. I hope this is enough detail to show the kind of multi-testing that can be done. One more important thing..."What's a halfbrdige?"
EDIT:
Just one more thing...Suppose you could have a -perfect- (R=0) LC circuit (PURE UN-DAMPENED SINUSOIDAL RESPONSE), if you now add any resistance in the LC circuit (now a damped series LCR circuit), will the natural frequency of the of the LC circuit decrease (in addition to the amplitude decay)?
Thanks all...

Re: SOLENOID QUENCH
DerAlbi, Sun Apr 14 2019, 11:31AM

Using linear encoders to analyze a coilgun is an interesting thought but i doubt it will work. You would need a very rigid connection to the encoder. Suppose you have a lot of suckback and massive deceleration, the inertia of the connection (lets say you use a string dragging over an encoder wheel) will not follow the slow down. But give it a go, if it works its interesting.

What is a halfbridge? Google: "asymmetric half bridge" -> go to images. In the german version of google, the first find is this picture: which is very instructive.

RLC-Circuit: Bringing an R into the equation is a weird thing. the self resonant frequency will actually be lowered, yes. fr = 1/(2pi) * sqrt[ 1/(LC) - (R/L)^2 ] (with R being a series resistance)
However if you excite the RLC circuit with an external sinusoidal (from a signal generator maybe) you will still find the minimum/maximum impedance (=R) at the uncorrected LC-frequency.

Re: SOLENOID QUENCH
Signification, Sun Apr 14 2019, 12:10PM

Thanks Albi:
But it looks like I have have been a bit unclear on the encoder--this is linear with shiny shallow reflective circles spaced a bit closer than 1/4" along the black metal projectile: It is all ONE rigid piece. The detector is fast and accurate.

I was asking about the RLC circuit because a youtube playlist included a video with a series RLC circuit of R=100 ohm, L=0.5H, and C=8uF. The zero resistance (R=0) natural LC frequency is 500 Rad/s. and the oscillation frequency came out NEGATIVE. In particular -9,500 Rad/s (10,000 Rad/s - 500Rad/s). I worked it out one way (according to Barry's site-which I am yet to study in any detail) and got: 489.9 Rad/s a 10.1 reduction in Rad frequency. In the video, he changed the 100 ohm resistor to 20 ohms (only x5) and got 100 Rad/s. So I guess I was asking: if this is done properly with the 100 ohm resistor, could the answer possibly be this NEGATIVE frequency?

...Extra thanks for the reference: The schema's look very helpful. I am going to study them now.

Re: SOLENOID QUENCH
DerAlbi, Sun Apr 14 2019, 01:26PM

Frequencies in general can be negative. For a sinus, this is the same as the non-negative frequency with a phase shift of 180°. For a cosine, there is no difference at all since it is time symmetric. However negative frequencies do usually not occur outside signal processing math.
In context of a RLC circuit, negative frequencies do not make sense. You can basically answer this your self with the formula i have given you: fr = 1/(2pi) * sqrt[ 1/(LC) - (R/L)^2 ]. There is simply no way this formula will ever produce a negative value, because the square root is always positive (or 0).
[In actuality, this formula issnt even correct but is only applicable as a simplification for very small R
(A damped oscillation must have harmonics; it cannot be describes with one single frequency) ]
BULLSHIT. I corrected my self below.

Ideally you describe such a system in its differential equation form, solve it and discard the solutions that make no physical sense. I guess this is what happened.

Re: SOLENOID QUENCH
Signification, Sun Apr 14 2019, 08:25PM

Here is the video I was speaking of:

Also, when it is convenient for you, look at the comment next to the orange icon by Dale Nassar (me). Is this guy right??? You can see where he surprises himself at about the 6:00 timestamp.

Re: SOLENOID QUENCH
DerAlbi, Sun Apr 14 2019, 11:22PM

I just comment along:
- I dont like his circuit definition. There are no current arrows or voltage arrows. His Minuses are pure choice. Not obvious.
- Holy shit, he goes the q-route
- he just pulls a solution out of his ass. wtf.

- Ok, i was wrong above. The oscillation frequency formula is valid for all resistance values until the square-root becomes 0.

- Yes your comment is correct, even 100R is low enough to make this under-damped. He made a mistake. The circuit makes very visible 5 oscillations.

Re: SOLENOID QUENCH
Signification, Tue Apr 16 2019, 02:17AM

It's OK for the radical to be ZERO (0), in which case the circuit undergoes "CRITICAL-DAMPING"?
Radical = POSITIVE gives "OVER-DAMPING"
Radical = NEGATIVE gives "UNDER-DAMPING"

I would love to see what the curve looks like JUST as the radical:

1) crosses from "0" to "+" and:
2) crosses from "0" to "-"

I am still rebuilding my shop and have no easy way to graph this. Anybody know where such a plot can be seen? I'm sure it won't be long before I can plot them.
The way I see it is that all these variations MUST produce REAL results, and produce REAL plottable curves since they are actual circuits that can be built.

What is most amazing to me is that (for this application) you have to go through -complex- or -imaginary- (i) numbers to rigorously derive them at a most basic level. Nothing mathematically invalid was done in this derivation process which yields totally REAL (no complex) solutions!

Re: SOLENOID QUENCH
Signification, Tue Apr 23 2019, 03:54AM

Lately, I have been wondering about"
The quadratic formal in GENERAL vs the quadratic formula for coilguns:

In standard high school algebra texts the quadratic formula is used to solve for x with the following general equation: If

A(x^2) +Bx +C = 0, It's solution for the two x's is:

-B +- [sqrt(B^2 - 4AC)] /2A. The tern INSIDE the square root symbol is called the DISCRIMINANT.
(B^2)-(4AC), is thus the DISCRIMINATE of the x-solution equation. A lot of information can be obtained by just examining this discriminate. For example:

For (B^2)-(4AC):
1) If the discriminant is >0, then there are two solutions.
2) If the discriminant =0, then there is ONE solution.
3) If the discriminate <0, then the solutions are complex (no real solutions, but can be just as useful).

Now let's go to the quadratic solution for a coilgun (I guess this is a good way to describe it).

The 2nd order differential equation for a series (RLC) coilgun circuit can be approximated as:
Li"(t) + Ri'(t) + (1/C) = 0
Where (i) is the current, i'(t) is the 1st derivative of current (i) with respect to t, and i(t) is the function of current (i) with respect to time.

For the coilgun form of the quadratic formula the discriminant is: (R^2)-(4L/C):
From Barry's page, a lot of useful information lies in the "coilgun" discriminant also: So for the discriminant

(R^2)-(4L/C): (Let's call the coilgun solutions s1 and s2 (in places of the general x's above. (Ls"+Rs'+1/C=0)

1) If the discriminant is larger than zero, the two solutions (s1 and s2) are real numbers and the circuit is OVER-DAMPED.
2) If the discriminant is =0, each solution is 0 (s1=0 and s2=0), here, the circuit is CRITICALLY-DAMPED.
3) If the discriminant is <0, s1 and s2 are complex numbers and the circuit is UNDER-DAMPED (decaying oscillation).

HERE IS WHAT I AM WONDERING:
In the general algebraic quadratic formula the constants: A, B, and, C, can be positive or negative. However, in the coilgun discriminant the circuit constants R, L, and, C can ONLY be positive since they represent actual circuit components. SO, are there any rules or laws that simplify or modify what the coilgun discriminant values can signify?

Re: SOLENOID QUENCH
Signification, Sat May 04 2019, 03:03AM

...just noticed this: With the particular high voltage metal Aerovox energy pulse caps that I fixed and plan to use in in a couple of projects, there is likely to be slight negative undershoot--apps like high power laser tubes and special coilguns. Does anyone know if this is bad for the caps? I just noticed that -some- are polarity-marked--barely noticeable...can't be sensitive as 'lytics???