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I've bought some capacitors because I needed to generate a current pulse into a coil. The idea was to charge a capacitor separately and then connect it to the coil to have a strong and brief magnetic field, but they don't discharge properly.

I've tried to charge them using batteries (9V), phone chargers (5V, 2A) and also a car charger (12V, 5A) and despite the capacitor's voltage being the same as the source's, it doesn't give any measurable power output, and even though there's no particular power output, the capacitor's voltage after the process is always 0.

I tried with a 16V, 47μF capacitor and it went as I described, after the fifth time it worked, but only once, and all the other tries were pretty useless. I also tried to change capacitors (I tried with a 16V, 220 μF, a 16V, 470μF and a 16V, 1000uF capacitor) and nothing changed. I was expecting a surge in the current and therefore in the magnetic field, but it doesn't happen.

I didn't use any particular circuit to charge the capacitor; I've simply connected it to the power source (battery or charger) using wires, without any switch. Sometimes I included the amperometer to see when the current stopped flowing. The same goes for the discharge, I've simply connected the capacitor and the coil using wires.

To know when the capacitor is charged I put an amperometer in series, when the current reaches zero I disconnect the capacitor from the charger and then I measure the voltage and verify that is roughly the same as the source's voltage. After the "discharge" I measure the voltage of the capacitor again and verify that is zero.

To minimize the resistance of the circuit and also to know if there is a current flowing in it, I avoid using the multimeter and instead measure the magnetic field generated by the coil (but it remains practically zero, so I assume that there's something wrong). To measure the currents and the voltages I use a multimeter (which has a resolution of 10mV and an accuracy of +/-0.8% for the voltage and a resolution of 10mA and an accuracy of +/-2.0% for the currents.

I charged the capacitors connecting the negative wire of the source to the negative pole of the capacitor and the positive wire of the source to the positive pole of the capacitor.

Here is my setup

My wires are taken from a scientific toy kit. I know they aren't the best, but I have only those.

If you have any idea about what could be the cause of the problem I'd be very happy to hear it. Thanks in advance.

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  • \$\begingroup\$ It's far too weak and too brief for you to notice. Cap is far, far, far too small. How much force and time were you expecting to see? Your voltage could also be too low. You never said coil resistance so I can't say for sure. Also, does your coil have a core? \$\endgroup\$
    – DKNguyen
    Commented Nov 25, 2021 at 16:17
  • \$\begingroup\$ Probably you're just not storing enough energy in a cap, or you're damaging your caps, or both. What's your end goal (i.e, what are you really trying to do)? I see the coil -- what do you want it to do? Have you experimented with what the coil does when you power it from a battery? \$\endgroup\$
    – TimWescott
    Commented Nov 25, 2021 at 16:38
  • \$\begingroup\$ Also -- do you have an estimate of the amount of energy you need to do whatever it is you're going to do? \$\endgroup\$
    – TimWescott
    Commented Nov 25, 2021 at 18:28
  • \$\begingroup\$ @TimWescott I want to use the coil to accelerate metal objects, so I need the a strong field that lasts only for an instant (because otherwise the object would only oscillate back and forth). I tried this with about 10A (connecting it directly to the car charger and this broke the fuse which maximum current was 10A) it worked well, but I hoped I could make it stronger , so I'd like to have more current. \$\endgroup\$
    – Phys
    Commented Nov 25, 2021 at 18:53
  • \$\begingroup\$ First, Stackexchange wants you to edit your question so that it stands alone without requiring people to dredge through the comments, and second, you're building a coil gun. I suggest you follow the grand engineering tradition: find someone else's design and shamelessly copy it. Once you've duplicated something that works, you can start understanding what's really going on, and work on improving it. \$\endgroup\$
    – TimWescott
    Commented Nov 25, 2021 at 20:43

3 Answers 3

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There is energy being stored in the capacitor and it is being discharged into the coil. I suspect if you used a more sensitive instrument on your coil you would see the effect. Try a compass and you will likely see the needle "wiggle".

The problem here is that you're failing to take into account how much energy must be dumped into this coil that you have built in order to get a large magnetic transient generated. Large magnetic transient requires a large electric current which means you have to have a large capacitor. None of the capacitors you mentioned qualify as "large".

I'm going to guess based on what I see in your photo that a capacitance of several thousand microFarads or more will be needed. You might look into one of those large capacitors that are used in automotive sound system installations. It's not uncommon to find those in values of 0.5 to 1.0 Farads or even more.

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  • \$\begingroup\$ Do you think I can solder many of them together to get more capacity or is it necessary to have also a higher voltage? I don't have the safety equipment to handle high voltages, so it would be better if I can have the same result by soldering less powerful capacitors. \$\endgroup\$
    – Phys
    Commented Nov 25, 2021 at 17:09
  • \$\begingroup\$ What is the result you are looking to achieve? \$\endgroup\$
    – jwh20
    Commented Nov 25, 2021 at 17:24
  • \$\begingroup\$ I'm trying to generate a really strong magnetic field, but only for an instant. That's because I wanted to use the coil to accelerate metal objects, therefore I need a strong variation of the magnetic field flux over a really short time period (almost instantaneously). As you said I need a strong current, and I can't use high voltages capacitor or generators, so if this result could be achieved by soldering many capacitors together, it would be perfect. \$\endgroup\$
    – Phys
    Commented Nov 25, 2021 at 18:39
  • \$\begingroup\$ The usual technique is to charge a bank of capacitors to the required energy level and then "dump" that stored energy into your coil. \$\endgroup\$
    – jwh20
    Commented Nov 25, 2021 at 18:53
  • \$\begingroup\$ So how should I connect them? Parallel or series? I thought parallel could be the best choice \$\endgroup\$
    – Phys
    Commented Nov 25, 2021 at 18:55
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Learn the equations that predict your outcome and then define what you want in peak current, Ipk, risetime, resonant freq. Q and PW50 of current.

e.g. \$Ipk=V/DCR+ESR)\$
\$\omega^2=1/(LC)\$
When does \$E=1/2 CV^2=1/2 LI^2=I^2(DCR+ESR)\$?

If \$X_C=X_L\$ What is X/R? etc.

Also in small e-caps have a low ESR <10 us which increase with size due to Q of the parts Xc/ESR and same of choosing Q for L limited by wire resistance and size.

If anyone can produce 3 T at f=400 Hz with Q=2, I will send a large reward.

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In addition to the answer by jwh20, I doubt that your coil is going to be good enough. The more turns of wire you put into a magnet, the more magnetic field you get out of it, for any given current.

Your coil looks like it's made of thick copper wire with thick insulation. You are likely to achieve more with many hundreds of turns of thinner wire. The wire used for magnets is often called "magnet wire", or enamelled copper wire.

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  • \$\begingroup\$ It is a multi-strand wire, so I don't know if is better or worse than a normal thin wire. (There are about 35 single wires inside it) \$\endgroup\$
    – Phys
    Commented Nov 25, 2021 at 22:14
  • \$\begingroup\$ @Phys Unfortunately, a few turns of multi-strand wire doesn't count the same as lots of turns of single strand wire. \$\endgroup\$
    – Simon B
    Commented Nov 25, 2021 at 23:17

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