Are my capacitors working properly or not?

Question

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.

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.