Forty wrote ...

Hello. I'll try to shed some light on all of your questions (under the assumption that you're trying to make a strong, pulsed magnetic field via discharge of capacitors into a coil of wire)

1. An iron core will increase the inductance of the coil and thus cause the magnetic field to change less suddenly. If you're using the pulse to induce currents into something else (such as an induction launcher) then this would be a bad thing, because induced currents depend on the rate of change of the magnetic field that's creating it.

2 and 3. The magnetic field created is dependent on the number of turns and the current. To obtain a given field strength, you can increase either one. The current is limited (aside from the source) by the resistance of the wire. The resistance of the wire decreases with more cross sectional area and, of course, less length. So to increase the current in a coil, you'd want thicker wire. With more turns comes more inductance, which again causes the current (and magnetic field) to change more slowly. So for a pulsed coil you want thicker wire, less turns, and lots of current, and for an electromagnet you'd want thinner wire, lots of turns, and less current. The voltage and the resistance of the wire will determine the peak current (roughly) by ohms law.

4. I don't think putting that many electrolytic capacitors in series is a good idea. But for a given coil, increasing the voltage (by series-ing the capacitors) will increase the current and thus increase the field. Since the capacitors will be discharging must faster because of the increased current, the pulse will be stronger.

5. Yes. The time constant for charging and discharging a capacitor through some resistance is equal to RC. So for a given load (the resistance), increasing the capacitance will increase the time constant and thus increase the time it takes for the capacitor to discharge. Another way to look at it: more capacitance=more energy stored, and if being discharged under the same circumstances, =more time to discharge.

6. Using more wires in parallel will act like a larger wire and reduce the resistance.

7. For an emp generator you can use an antenna (I've heard of all sorts of shapes) to focus and project the em signal. I'm not much of a signal or radio guy though so I don't know how you'd go about designing one. For a strictly magnetic field, using a long core extending from the coil might be a way to bring the flux out of it, but like I said in 1. that's more for an electromagnet.

What is it that you're trying to do exactly with the pulse?

Forty wrote ...

The large wire suggestion was really more for the rapidly pulsed coil. Less turns would give you less inductance, but to still achieve a strong field you'd need lots of current, and thus thick wire.

For an electromagnet that is on for longer periods, the inductance won't really matter, so you can use many turns of thinner wire, and power it with less current to achieve a strong magnetic field.

The field created (in Teslas) is approximately B=4*pi*10^(-7)*N*I
where N is the number of turns, I is the current, and the 4*pi*10^-7 is the magnetic permeability of free space. The product of N*I gives you the amp-turns that Steve discusses

Here's a project you might find interesting that could involve the electromagnet and your permanent magnets:
The IR emitter and detector pair can be salvaged from many things (like computer mice)

Forty wrote ...

If you're making an electromagnet, and not a pulsed coil, then I don't see why you need the capacitors. Just power it with a 12v battery.

Charging that many electrolytics in series probably wouldn't work too well. They have a series resistance to them.

The camera charger makes 330 volts. so it can't charge anything higher than 330 volts, but it could charge all the capacitors in parallel.

The energy stored in a capacitor is 0.5*C*V^2
and time constant (seconds to charge to 63% or discharge by 37%) is equal to RC. With that you can answer your second question.

Transformers can be used for lots of things, but since I have no idea what it is you're trying to do, I can't tell if you might have a use for them.

Are you making a pulsed electromagnetic coil or an electromagnet? You said electromagnet but then you went back to talking about capacitors so I don't know.

wrote ...
Can you take a look at this for me?;

My calculations say that this would output 33,750J, I don't think that can be correct.
What am I getting wrong here?

Mattski wrote ...


Your calculation is fine, the problem is that the specs are wrong. No way is that capacitor 300uF, it's probably 300pF. So instead of 34 thousand joules it's 34 thousandths of a joule.

Harry wrote ...

Correct, 1 joule is nothing. A kWh is 3.6 million joules, for perspective.
But it is the rate of energy transfer that is important with EMP's, and in general, capacitor experiments. A 1000J could be used in 1 second in a heater at 1000W, but in a 100 microsecond capacitor experiment it will reach (1000j/0.0001) 10 000 000W or 10 million watts, realistically this figure is lower because of resistance and inductance in circuits.