Charging a 400v capacitor with 1.5v batteries.

[al360ex]

Hello,
I'm currently designing schematics for a personal EMP Generator (for my own entertainment).

Basically, I have a power source that will charge a capacitor bank. That energy will be dumped into a 10AWG enameled copper coil wrapped around a soft iron core.

Power source: 8x 1.5v Duracell Procell "D" in parallel.
Capacitor bank: 4x 400v/8800uF wired in parallel.

I have a some of questions:
#1 Will the discharge speed of the capacitors be affected since they are wired in parallel?

#2 What would be the most simple and/or efficient way of charging the capacitors? Using an inverter transformer? An oscillator with a transformer and a full wave rectifier bridge? A ZVS Flyback Driver?
An other method I haven't thought of?

#3 Taking into account that I want something relatively portable (fits in a briefcase) would it be more efficient and/or simple to use a power source with a higher voltage (9v, 12v, 24v or even 48v)?

Thanks!
al360ex

 

[tcmtech]

A common photo flash circuit from a disposable camera does the 1.5 - 3 volt up to 350 - 400 volt DC conversion rather well. However given your size of capacitor bank it would take it a few tens of minutes to charge up all the way.

Then on top of that as far as generating any level of usable EMP you will need a capacitor bank about a million times larger with a ESR down in the micro ohms equivalent.

 

[DerStrom8]

What range are you expecting to get with this EMP gun?

 

[al360ex]

I don't expect any effect beyond one feet top.
I'm trying to adapt designs I have found on the Internet.
There are a lot of schematics for a coil gun (same basic principle) but schematics for generating an EMP are rare.

Here's what I've looked at:
https://www.coilgun.info/mark4/schematic.htm
https://electronics.howstuffworks.com/camera-flash3.htm
https://webspace.webring.com/people/gl/lemagicien/elecpage/maxflash/maxflash.html

The 2 PDFs I've uploaded are for much more powerful (and dangerous!) EMP Generators than what I want to build.

 

[tcmtech]

There are a lot of schematics for a coil gun (same basic principle) but schematics for generating an EMP are rare.

Unfortunately making a real EMP generator takes insanely huge amounts of power. For practical comparison consider that a typical lighting strike releases around 500 million to 2+ billion joules of energy yet unless it hits something directly the EMP from that doesn't have the power negatively affect anything more than a few tens of feet way.

The big circuit you references is 500 - 600 Joules. It might blow your little fingernail part way off and stop your heart but the odds are it wouldn't even mess up your digital watch on your wrist when it did it.

That's why there are no real good EMP schematics on line. The realistic parts lists for a electrically based one require items that most mad scientist billionaires couldn't afford to build!

 

[Menticol]

I suppose you need a power source like this one, not to mention the other millions of dollars in equipment.

https://en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator

 

[alec_t]

The 2 PDFs I've uploaded are for much more powerful (and dangerous!) EMP Generators than what I want to build.

400V on even a fairly low value cap can KILL YOU ! Your proposed circuit is dangerous too.

 

[al360ex]

I know that it is dangerous if handled incorrectly.
What I want to build is something very similar to this: https://www.youtube.com/watch?feature=player_detailpage&v=I0lSRRWkN9w#t=333
However, instead of having an empty cylindrical core (that contains a metal projectile), I will use a soft iron core, that way I'll be able to have a somewhat directional pulse, instead of having the EM wave go out in all directions.

I'm currently at the stage of designing the schematics, and I simply want to know what would be the best way to charge my capacitors using batteries (for portability).
If it isn't possible (or too unstable/hot/dangerous/etc), then I'll simply use a AC current, wire that to a transformer and then a full wave rectifier bridge to convert it to DC.

 

[al360ex]

If you have about 16'000$ to spend, you can buy a working EMP generator here: https://www.amazing1.com/emp.html
It uses about 400 Joules with capacitors of 40kv and an amperage of 45ka.

 

[MrAl]

Hi,

Generating a magnetic field is as simple as connecting a coil to a power source such as a battery or in this case charged capacitors. However, generating a very intense field with a limited power source is quite a challenge because it requires building up that field very quickly before all the energy runs out. If the field does not build up enough by the time the energy is depleted, that's the end of it and the field starts to collapse.

So the real game is how to get the field to build up fast, and since the field strength is stronger with higher current the goal is to get the current to rise as fast as possible. This in turn would require very low coil resistance and/or very high voltage because the higher the voltage and lower the coil resistance the faster the current rises, and the faster the current rises the faster the field rises before the energy runs out.

That's not all unfortunately. Another problem that comes into play is the coil inductance. When we apply a voltage to a coil the coil's self inductance does not allow any current to flow right away. Instead, it takes some time to get that current to flow (back EMF is one way of explaining this). This means that in addition to the other requirements, we need to keep the inductance as low as possible.

In addition to all that, since we want the field to build up fast that means we are dealing with high harmonics, and high harmonics get attenuated in wire through another physical phenomenon known as skin effect. The higher the harmonic, the more the skin effect makes the wire look like it has higher resistance, so we are stuck with this problem in addition to all the others.

To list these requirements we have:
1. Low DC coil resistance
2. Higher applied voltage
3. Low AC coil resistance (skin effect)
4. Low coil inductance

We want to solve all these problems somehow.

Number 1 is not too hard, we use a heavy gauge wire and that reduces the DC coil resistance. But then the skin effect problem is not that well solved (number 3). So perhaps instead of using one heavy gauge wire we use several strands of insulated wire in an attempt to lower both the inductance and the skin effect.
So we might have solved #1 and #3 here.

Number 2 is a little harder because we have to use a higher voltage. This means that the initial current surge will be higher and thus that means we get a faster rising field. Unfortunately this requires careful construction using high voltage caps which may have to be wired in series which means care in charging.

Now we come to number 4, the coil inductance. The effect of adding a magnetic core such as soft iron is such that the field strength becomes higher with a given amount of coil drive current. Unfortunately this also means that the self inductance rises, and it rises up quite high. This means it takes longer to get the current up to the max level before all the energy runs out of the caps. But there is another more problematic restriction here when using any type of magnetic core, and that is one of core saturation.
Core saturation occurs when the core is driven with a given coil to a given level of current where after that level of current there is no more appreciable rise in field with increasing current. This means once we get to a certain level of current it will be very hard to get a higher field level even with more applied current. This of course means that with a core we will be limiting the rate of rise of the field even though we might see some increase after saturation. We also loose some energy through the core itself as it takes some energy to magnetize.
There is a chance however that we might see a fast enough rise in field to the required level using a core than not using a core, but it depends on the level of field that we need. For a magnetic pulse it is more likely that a higher voltage will do more for the intensity than anything else.

Higher voltage:
High voltage means caps in series, and so they have to be charged more carefully. But with low coil resistance and low coil inductance this means when we connect them to the coil there will be a huge surge of current through these caps, and it is very hard to say if they will survive or not. Caps have surge ratings but it is usually for line operation where the surge will be high but it wont be super high, and it is possible for these caps to actually blow up if the surge is too high. This of course means the caps should be mounted inside a vented protection box.

So you see there are a number of problems making it very hard to build one of these things.

Also, depending on the level of current reached and the time it takes, the coil may have to be replaced after each use due to overheating.

As an alternative that could be used for testing objects for EMP susceptibility you could perhaps build a much lower energy device and just shorten the distance.

Please let us know what you come up with.

 

[audioguru]

If you have about 16'000$ to spend, .....

Bonjour (G'day mate?) to you too.
Here in Canada $16,000 is $16,000, not 16'000$.

 

[NorthGuy]

Bonjour (G'day mate?) to you too.
Here in Canada $16,000 is $16,000, not 16'000$.

Unfortunately, now in Canada $16,000 is only $14,600 and looks like it's getting a lot less

 

[audioguru]

I am retired so when the cost of living goes up then my pension also goes up.

 

[chemelec]

Unfortunately, now in Canada $16,000 is only $14,600 and looks like it's getting a lot less

Yes, But a 1KG bar of gold in Canada Today is worth $44,200.
In US Dollars it is Only worth $40,200.
So I don't lose.

 

[audioguru]

A few decades ago I had some gold. I can't find it today. It disappeared.