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I'm building a project which basically consists of the electronics inside a lucky cat: a coil that generates a magnetic field to make a permanent magnet move, which is attached to a swing. Check this link, which shows you the circuitry and the principles on which it is built. The project would run from a 5V battery.

For this project however, the magnetic field to 'push' the permanent magnet is not strong enough so that it swings out far enough. I believe I have these options to increase the 'power' from the magnetic field:

  1. Reduce the coil length,
  2. Increase the number of turns on the coil,
  3. Increase the current through the coil.

Is that correct, or am I missing another option?

To increase the current running through the coil, would it be enough to increase the capacitance of C1 (from this article), to double it for example? What voltage rating should the capacitor be?

Are there other things to consider when this current increases? Also, should I worry about the resistance of the magnet wire (I see that there are different types of magnet wire)? For example, does the resistance through that coil also increase, or would it get hot over time? This circuitry is meant to run for a long time, so if it gets hot, we'd need another solution. Or would the flashing LED burn? Could I replace it with another diode (not an LED)?

I'm sorry if I use terminology completely wrong, or if this is a very noob question here. It's been a long time since I studied this in high school and I want to learn it again now. Would be great if you people here could help me :)

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    \$\begingroup\$ What does "reduce coil length" mean? Shorten the wire? Remove turns? Space the turns closer together? \$\endgroup\$
    – DKNguyen
    Commented Jan 24, 2022 at 20:49
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    \$\begingroup\$ I fear for the not so lucky cat. Should you really be experimenting like this with animals? \$\endgroup\$
    – Andy aka
    Commented Jan 24, 2022 at 20:52
  • \$\begingroup\$ Increasing the voltage and adding an iron core to the coil are your biggest bang for the buck. Increasing C1 would have minimal impact. \$\endgroup\$
    – Aaron
    Commented Jan 24, 2022 at 23:33
  • \$\begingroup\$ Interesting, why wouldn't increasing C1 have not so much impact? Wouldn't the current increase in this case? \$\endgroup\$
    – toto
    Commented Jan 25, 2022 at 8:33
  • \$\begingroup\$ @toto No. You're new so you probably don't get it but it doesn't. It won't take much to understand either. C1 is a charge reservoir to stabilize the voltage against fast changes in demand (if they exist because power supplies need time to react) and is like a temporary voltage source charged from the supply. Because of that it has same voltage. Same voltage = same current. \$\endgroup\$
    – DKNguyen
    Commented Jan 25, 2022 at 14:43

2 Answers 2

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Add a magnet core to the coil. An enclosed core if possible so the magnetic flux lines can take the easy path through the iron to loop around rather than having to pass through the air to loop around.

Other than the core the two things that influence magnetic strength is the current and the number of coils (which is basically you just recirculating the current back again so it's own magnetic field stacks up on itself). Those are your two variables. More turns or more current increases strength.

But if you do something that affects these variables simultaneously like only increasing the number of turns, this means more wire which means more resistance which means reduced current if you do not increase your voltage. So if you only increase the number of turns without doing anything else you could very well end up with a weaker magnet because you've actually messed around with both variables and not kept all other things equal.

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  • \$\begingroup\$ Okay, so an iron core, enclosed if possible, is the best option as I learn from your answer. I understand why only increasing the number of turns could weaken the magnet, but would it be the case too if I could increase the current? And how do you increase that? (I learn from the comments above that this is not something you can increase with C1.) \$\endgroup\$
    – toto
    Commented Jan 25, 2022 at 8:33
  • \$\begingroup\$ @toto Because as I explained in my answer already, only increasing the turns doesn't actually only increase the turns. It also affects the current. To truly only increase the number of turns without decreasing the current requires you to take extra steps to compensate. \$\endgroup\$
    – DKNguyen
    Commented Jan 25, 2022 at 14:23
  • \$\begingroup\$ That sounds pretty obvious now you explained it. As @vu2nan mentions in his answer, increasing the cross section of the wire could compensate for the loss of current (because less resistance in the wire) without having to raise the voltage, is that correct? \$\endgroup\$
    – toto
    Commented Jan 25, 2022 at 20:29
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    \$\begingroup\$ Yes because increasing the wire cross section decreases the resistance. It will also length the entire coil a bit because more space. Whatever you changes do, just make sure that increasing one of the two important variables mentioned above doesn't inadvertently decrease the other and offset everything. \$\endgroup\$
    – DKNguyen
    Commented Jan 25, 2022 at 20:31
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The magnetic field strength, \$H\$, of an air-core coil is given by \$H = NI / L\$ where \$N\$ is the number of coil turns, \$I\$ the electric current through the coil, and \$L\$ the length of the coil.

The magnetic field strength could be increased by:

  1. reducing the coil length \$L\$
  2. increasing the ampere-turns \$NI\$

Increasing \$I\$ would entail an increase in the cross-sectional area of the wire, in order to reduce the resistance of the coil.

Increasing \$N\$ would increase the coil resistance. Again, to counter that, an increase in the wire cross-section would be necessary.

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