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I am completely new to electrical engineering.

For a school project I am making a crank-electricity generator. My idea was to attach a crank to a stepper motor and when I crank the shaft, use the electrical output to power an LED.

I want a smooth flow, so I was going to attach a capacitor to the motor so the LED wouldn't flash because of the inconsistent turning motion. However, when I did so the capacitor didn't work. Moreover, the shaft became hard to turn. I tried many different types and capacities of capacitors, but that didnt' solve the problem, so I tried switching to a DC motor but it still didn't work.

When I hook my capacitor and LED to a battery it functions properly.

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    \$\begingroup\$ It's because you don't understand how the motor works. You need to show us the schematic you used. Also, get some diodes. You are going to need them either to rectify the voltage from the stepper motor or to simply stop the capacitor from discharging back through motor winding during your erratic turning when the shaft is stopped or not moving fast enough. An additional issue is that you probably aren't turning the DC motor fast enough. A DC motor that spins really fast on 1V is going to need to be spun just as fast to produce 1V. \$\endgroup\$
    – DKNguyen
    Mar 18, 2021 at 5:32
  • \$\begingroup\$ @DKNguyen generally it has to be spun even faster because the losses are backward. Some motor types require current injected into field windings in order to generate power. Your motor that becomes hard to turn when you have a capacitor attached is likely working properly and you have shorted the windings somehow so a lot of current is flowing. Most motors output AC, so you need a rectifier built for the number of phases the motor has. If you attach a cap to the output of the rectifier it should charge easily. This will protect the LED from AC voltage. \$\endgroup\$
    – K H
    Mar 18, 2021 at 6:12
  • \$\begingroup\$ Unless the LED is high power you need a resistor for the LED at the least. LEDs generally require current regulation but for short tests you can get away without it if you have a sufficient size bank of LEDs (I used a grid of 20mA LEDs gutted from a dollar store light fixture) \$\endgroup\$
    – K H
    Mar 18, 2021 at 6:17
  • \$\begingroup\$ You should start by following a guide for how to do this, and branch out from there: google.com/search?q=hand+crank+light+project+circuit \$\endgroup\$
    – Drew
    Mar 18, 2021 at 7:05
  • \$\begingroup\$ If you use a brush motor instead of a stepper you would get a better result. Hand crank generators however usually use an AC generator which is quite different \$\endgroup\$
    – Lorenzo Marcantonio
    Mar 18, 2021 at 7:15

1 Answer 1

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When you crank a stepper motor, it generates AC. Your capacitors look like a short circuit to AC, so all the current flows through the capacitor and none through the LED. This is also the reason it gets hard to crank. The short circuit allows a lot of current to flow. You have to crank hard to generate the high current.

At a closer look, both of your linked motors are stepper motors. Both will generate AC when cranked.

You have three options:

  1. Leave out the capacitor, and use two LEDs in anti-parallel. The LEDs will flash, but when one is on the other will be off. This protects the LEDs (somewhat) from the extremes of reverse voltage from the AC.
  2. Put a rectifier diode in series with the LED, and the capacitor in parallel with the LED only. That gets DC for the LED, which the capacitor can smooth.
  3. Get a DC motor, and use your LED with the capacitor in parallel. You'll have try cranking the motor in both directions to see which will make the LED light up.

Regardless of which solution you choose, keep in mind that you have to crank the motor fast to reach its rated voltage. If the motor is rated 5V at 3000 RPM, then you will have to "crank it" at 3000 RPM to get 5V out of it.

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  • \$\begingroup\$ I am going to try to build a bridge rectifier with 4 diodes then to turn the output into a dc current, thanks! \$\endgroup\$
    – Elijah Ferrin
    Mar 18, 2021 at 23:49

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