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Hello, A few months ago I started a project using capacitors, needless to say I quickly found myself giving up as what I was trying to achieve didn’t seem to work correctly. I was playing with my neodymium magnets today and realised maybe I could make an electro magnet. I first wanted to use a car battery but then realised I had 24 330v capacitors, maybe I could make a strong magnetic pulse. I later found out that I would actually be creating an EMP. My lack of knowledge on the subject has slowed me down however and I have a few questions;
1. If I wanted to create a larger magnetic field would I want the iron rod to be long or wide? 2. People suggest that a thicker wire is better but is wrapping a thinner wire around to cover the same area just as effective? 3. What makes a stronger magnetic field; Larger Current or Voltage? 4. If I linked my capacitors in series what effective range would I have? (Considering the inverse square law and all that) 5. Regarding capacitors, does a higher capacitance mean that it will last for a longer period of time? 6. Would double layering the wire make the field stronger? 7. I know this is probably not possible but is there a way to direct or contain a magnetic field to limit the inverse square law’s affect?
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
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?
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?
I am actually only trying to make a strong magnet, I do not really want to make an EMP. Thank you for the really long post, any more tips to help me? Thank you!
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
Ah just an electromagnet then? well just wrap as many turns of whatever enameled wire you can find around a ferromagnetic core. For a lazy approach, I'd probably just cut the top off of a transformer and use it's core as the magnetic core, and the winding with the most turns as the electromagnet. (note: don't plug the transformer back into mains after you do that, use another supply or transformer to power it)
A question about the wire, You say the thicker it is the better but what is stopping you wrapping a huge wire with a Diameter of 5cm around your core. Are you saying 3 turns of that is better than 100 from a smaller wire? Could you possibly answer question one again now, I am not sure what the best size of core Is, long or wide I used to have a really crude charger like thing I made for my capacitors but I have realised this will no longer work, any other solutions to charging them if they are linked in series?
Thank you for all the help.
Edit; I have two neodymium magnets, N52 25mm Diameter, could these be used in this experiment somehow?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The magnetic strength ("magnetomotive force") of an electromagnet is measured in amp-turns. 1 amp in 100 turns gives the same strength of magnet as 100 amps in 1 turn.
For a given MMF and copper volume, the wire gauge doesn't affect the current density and I2R losses.
So, if you want to design a strong electromagnet, you just choose the wire gauge to draw the maximum power out of whatever power supply you have available. If it's a car battery, use thick wire. If it's hundreds of volts, use thin wire. In either case, stuff as much of it as possible onto the core and provide the best cooling you can to stop the insulation melting.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
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)
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)
Thanks for the suggestion!
- Do you guys know of any way I could charge these 24 capacitors in series? I'm not sure if I have to apply 7920v or just 330v (which would take a long time to charge). I have this disposable camera which charges a 330v capacitor and I am not sure if I can use that. - Which capacitors would be best to use? One with a high voltage rating or one with high capacitance? (For example; 3000 capacitors at 1kv and 100pF, would this be good even though it is 300nF overall?) - Can transformers be used in some way?
Thank you for putting up with my lack of knowledge on this subject, I really appreciate it.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
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.
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.
Sorry, I was sort of referring to both because they seemed similar. I think I am going to go the EMP route, the reason I wanted to use capacitors in a magnet was because I thought It would give me a very large instantaneous magnetic pull (Or push) I am guessing an increase in Voltage will produce more of an effect than an increase in capacitance from your equation? But what you are saying is that an increase in either will produce a more powerful effect? Thanks,
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Electromagnetism/Capacitor Help.
