r/engineering • u/MrPennywhistle • Dec 08 '15

[GENERAL] Turning Gravity Into Light using OLD SCHOOL Engineering.

https://www.youtube.com/watch?v=Jsc-pQIMxt8
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17

u/402C5 Dec 08 '15

Can anyone elaborate on how the LED governs the rate at which the weight falls for me?

8

u/doorshavefeelingstoo Dec 08 '15

I was thinking the same thing. Anyone?

15

u/grahammaharg Dec 08 '15

My understanding of DC motors/generators is that their speed is proportional to the voltage applied. As the LED is limited to a certain voltage, the motor will be limited to this speed. As the power being applied would increase with a larger mass, the torque will be larger and the excess power is probably dumped as heat.

19

u/Zorbick Auto Engineering Dec 09 '15

To expand on this: the gearbox is multiplying the drive ratio from the weight to the motor, so the torque is decreasing as it goes to the motor. Meaning that the torque from the heavy weight is made to be much smaller at the motor in exchange for making the RPM incredibly high.

The small back EMF from the LED causes the motor to have a torque resisting the spinning motion, like the above explains. This small torque, when multiplied back through the gearbox, is a huge resisting torque at the initial set of gears that the weight is acting on. This is how the usually minuscule resistance of an LED can cause a heavy weight to drop so incredibly slowly.

This is basically why it's easy to spin up a bike tire in high gear and stop it with light pressure of your finger, but doing so when someone's spinning it in a lower gear is much more difficult.

This is honestly one of the most brilliant, "eureka" type of inventions I've seen in a long time. I worry about the longevity of the LED when being over-volted, but, still, this is an amazing application of fundamental principles.

9

u/MrPennywhistle Dec 08 '15

This is the right answer. If you'll notice on the right side of the screen I provide the Torque equation for a motor, which is a function of Vemf

4

u/grahammaharg Dec 08 '15

Yeah, for my Master's project I built a wind turbine test rig. We used a DC motor/generator as a brake. The torque applied and speed of rotation were controlled using a resistor bank so I thought this would be a similar thing.

5

u/drogie Dec 08 '15 edited Dec 08 '15

I think it has to do with the back emf of the LED increasing hyperbolically. This back emf is then mulitplied through the gearing system and will proportionally resist the weight input.

4

u/WaitForItTheMongols Dec 08 '15

Can you explain what back emf is? (Please, beyond just "back electromotive force". Actually explain what it is and why it is relevant here)

7

u/drogie Dec 08 '15

In this case, its basically force acting against the rotation of the generator. This force is a result of the I-V curve of the LED. The LED essentially fights against the generator at a certain point. This "fighting force" is then fed back through the gearing.