# How does a DC generator vary power output to suit load

I am working on an project where I intend to install a bicycle DC generator to power a set of LED strip lights. The specific product I will be using is the Pedal-A-Watt permanent magnet generator which is capable of output around 400 W of power. For the sake of my question let's assume that I will have 100 W of lighting load with additional an 50 W lighting load being switch on by a relay if a certain condition is met.

My setup will be as follows: DC Generator -> 12 VDC Voltage Regulator -> 12 VDC /120VAC Inverter -> LED Driver/Power Supply -> LED Strip Lights.

My concern is that the the motor can output significantly more current than the lights will require. I realize the load dictates the current being drawn from the generator, but if someone is pedaling hard and determined to max out the generator, will they be able to overcome to counter torque cause by the magnetic fields inside the motor and fry the lights, inverter, and/or power supply? If no, why not?

I should add that my field of expertise is not electronics and thus I would like to use off the shelf products as much as possible.

• Welcome to EE.SE. You also need to consider the fact that a healthy human can only generate about 1/4 HP-at best. That is 186 watts. For sustained power consider 100 watts maximum.
– user105652
Feb 7, 2018 at 4:04
• The output voltage is set by the speed of motor rotation. If the person pedals faster, the voltage will increase. The voltage increase is linear with speed. The resistance felt by the person depends on the electrical load at the output. You should design your system so that it produces 12V at a reasonable cadence for pedaling. Feb 7, 2018 at 4:07
• @mkeith. Ha-ha. If the bike was stationary, why would you need lights?
– user105652
Feb 7, 2018 at 6:11
• Thanks for the comments! The bicycle will indeed be stationary and used to power the lights which form part of an artistic display of sorts. It will be accessible by the public and hence the reason I'm trying to make sure someone can't fry the electronics by over-pedaling. @AlmostDone - I will be purchasing the voltage regulator from the manufacturer which outputs a constant 14.1 V at variable current depending on pedaling speed. Feb 7, 2018 at 14:39
• There are going to be problems. I believe going through the inverter is a mistake. I strongly recommend you use LED's which can be run directly off of DC. Inverters need to maintain a minimum input voltage or they will shut down. Trying to put out 100W pedaling is hard work. People will hesitate or pause, and during that pause, the inverter will cut out, and this will generally be a bad experience. Also, the inverter adds one more conversion step which reduces efficiency somewhat. If you connect LED's directly to the regulator output, no cutout. Just brightness varying with speed. Feb 7, 2018 at 15:18

My concern is that the the motor can output significantly more current than the lights will require. I realize the load dictates the current being drawn from the generator, but if someone is pedaling hard and determined to max out the generator, will they be able to overcome to counter torque cause by the magnetic fields inside the motor and fry the lights, inverter, and/or power supply? If no, why not?

Rather than speaking in riddles I'll give you the answer black on white. A DC acting as a generator is not a current source. It is a voltage source. Here's the difference:

See what happens when I increase the resistance of the load. For the current source it goes bad (imagine if there was no resistor => you'd see sparks across the terminals of a DC motor), perhaps in a way you think. With a voltage source the current is lowered. To, as you were saying, dictating the current being drawn.

The DC generator can't force current into the load because it is a voltage source. Like a battery (with a very noisy output). If you can buy that your load will work with batteries, then it will work with a DC generator.

Here's some extra information for you, if you take a DC motor and short its terminals and try to rotate its shaft, then you will feel that it's harder to rotate than if the terminals wouldn't have been connected at all.

Let's take the pedaling in different angles for an example with the same gear. If you are pedaling up-hill, you are putting in much energy to push yourself forward. If you however are pedaling on a straight ground or even tilted down-hill then you will feel that you won't have to put as much energy into the pedaling. It gets much easier to pedal right? The same thing happens for the DC motor if you put a load across it vs no load at all.

Same thing happens in a car, there's a generator there as well. Let's say you're using a custom 10 W amplifier for listening on radio non-stop. Then the generator will drain 10 W mechanical power + losses.

To quote you which is the correct answer, the load dictates the current being drawn from the generator.

• Thank you for the explanation and the simulations, much appreciated! Feb 7, 2018 at 18:12