I am making a door bell as a school assignment. It works by having a solenoid produce a magnetic field which attracts a pice of iron attached on a conductor. when the iron is attracted towards the solenoid the current is broken so it falls back, inducing the magnetic field again. This will produce a frequency. The investigation is based on the influence the current has on the frequency.

I have tried to power the circuit by ten 4.5 volt battery packs connected in parallel as well as in series. But however I connect the batteries to the circuit Current will stay constant, even when I disconect battery packs. In series I get a constant current of 3 amps. In parallel I get a current of 7 amps maximum.

What should I do to vary the current?

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  • $\begingroup$ Sounds like you've already found a way to vary the current: Change the number of batteries in parallel. Apparently, your circuit is drawing such a heavy current that it the current is limited by the internal resistance of your batteries. That would explain why the current drawn by the circuit goes up when you increase the number of batteries in parallel. BTW, 7 Amps is a lot. It might make sense to re-wind your solenoid with more turns so that it doesn't require so much current to operate. Also, I assume that the currents you're stating are some sort of time-averaged values. $\endgroup$ – Samuel Weir Dec 12 '16 at 3:26
  • $\begingroup$ @Samuel Weir is right! Your solenoid draws too much current which also exhausts the batteries very quickly. You should rewind your solenoid with more turns as he suggested. $\endgroup$ – freecharly Dec 12 '16 at 8:08
  • $\begingroup$ I ended up coiling adding many many more coils, as well as 3 ohms of resistance. The coiling had and internal resistance of 0.9 ohm so the final resistance was 3.9 ohms (in series). with 5 batteries i had 3.5 amps, 4 batteries gave 3.0 amps, 3 gave 2.4, 2 gave 1.7 and 1 gave 1.1 amps. How come i get more amps available when the batteries are connected in parallel compared to in series. V=R*I, where I=V/R, Wouldnt this allow for more current being generated when the batteries are connected in series, since they would give out a higher voltage when the resistance is constant? $\endgroup$ – Axel Dec 28 '16 at 12:09
  • $\begingroup$ When my batteries are connected in parallel they give out a constant Voltage of 4.5 volts. My resisitance was constant at 3.9 ohms ( altough it could have varied slightly due to higher temperatures at the end of my experiement). According to I=V/R , where I=4.5/3.9 i would have gottan a constant current of 1.3 amps no matter how many batteries i add in parralel. at least according to I=V/R. I have looked in our physics book but cant find an explanation, online sources just stat that parralel increases current and series increas volts, but dont explain why. anyone who knows why? $\endgroup$ – Axel Dec 28 '16 at 12:15

You can use potentiometer in series with the battery to vary the current.


I had a similar problem and the best thing to use would be an Arduino. This is because firstly, Arduino is small - to fit in the project. Secondly, you can use resistors and program them to release certain volts of current. You can set the parameters. This worked for me!

  • 1
    $\begingroup$ Quote: "Secondly, you can use resistors and program them to release certain volts of current." Resistors cannot be programmed and currents are measured in Amperes nor Volts! Please learn at least the most basic high-school physics before venturing to give such answers. $\endgroup$ – freecharly Dec 12 '16 at 3:27

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