I teach 6th grade physical science. My electronics knowledge is limited. I have a bike powered generator from windstream power, and I have it hooked up to a board with light sockets so students can feel the difference between different kinds of bulbs. The students easily generate 120 volts, and can sustain .5 amps to power 60 watts of bulbs.

Problem one: I'd like to be able to have LED bulbs in the mix (Philips 12.5 watt, 110 volt edison base), but the voltage is too unpredictable. My understanding is that fluctuating voltage will damage them. The generator is capable of producing 150+ volts. Am I looking for a voltage regulator? Does it need to regulate minimum and maximum?

I also have a battery pack with an inverter from the same company, but it's giving me problems and I'd rather dispense with the battery altogether. I don't want to deal with the maintenance issues, and the battery seems to make it harder for the students to understand the whole system.

Problem two: What's the best way to build an array of LED lights that could be run continuously off of the generator. I'd like to have enough storage (in a capacitor maybe?) that the lights would stay on for a few seconds while one student gets off the bike and another gets on. I'd rather not have a battery. The purpose of this is to "light" the room for a day while teaching about other subjects. I put that in quotes because 60 watts of even the most efficient LED's is a little dim for my classroom. The lesson for the kids is to gain an awareness of how much power we're consuming regularly. I can't use incandescents for this purpose because my room has no windows and it would be too dark. I'm looking for bright white LED's as the most efficient lighting source. The reason I'd like to have the lights stay on while the students switch on and off the bike is because otherwise it will be too dark in the room.

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    \$\begingroup\$ I think you need to specify the type of LED bulb you're trying to drive. Is it an off-the-shelf light bulb-looking thing that just replaces a standard 120VAC light bulb, or some low voltage DC module? \$\endgroup\$ Commented Oct 24, 2012 at 16:06
  • \$\begingroup\$ I don't understand the final goal. What's wrong with incandescent bulbs if you only want to illustrate the point? You meantion a wind stream, but then talk about students cycling to provide power. What is the point of having the lights stay on for a few seconds? What are you trying to teach with that? 60 W for a few seconds is significant energy, around 200 J at least. 200 J at 120 V requires 28 mF. That's a large and expensive capacitor bank. \$\endgroup\$ Commented Oct 24, 2012 at 16:07
  • \$\begingroup\$ 28 mF at 120 V is large, expensive, and dangerous. \$\endgroup\$
    – The Photon
    Commented Oct 24, 2012 at 17:38
  • \$\begingroup\$ @Olin Lathrop Windstream Power Generators, FWI \$\endgroup\$
    – rdivilbiss
    Commented Oct 25, 2012 at 23:39
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    \$\begingroup\$ @rdivil: Argh. That means he should have written Windstream Power, not windstream power. There is a big difference between the two. -1 for extreme sloppiness, and this is from a teacher no less who should be used to looking for this kind of mistake from his students. \$\endgroup\$ Commented Oct 26, 2012 at 12:52

3 Answers 3


For Problem 1:
A string of LEDs hooked up in series with a current regulator like SuperTex CL220 would do the trick: The component is a simple a 2-terminal device (like a diode) and needs no additional components or configuration. It allows 20 mA (+/-10%) current to pass through as long as there is sufficient voltage headroom: 5 volts above the total forward voltage of the LEDs is sufficient. This current regulation is stable up to 160 volts, enough for your purposes.

Note that LEDs are current-dependent rather than voltage dependent devices. They glow at essentially the same brightness as long as the current is constant.

For your application, if the LEDs need to start glowing from around 60 Volts, half the 120 Volts your students generate, then a string of 25-30 standard 5-mm red LEDs would be optimal. They would glow without intensity change till your maximum voltage.

Too many LEDs = they won't glow till a higher voltage.
Too few LEDs = the CL220 device would overheat in dissipating the surplus voltage

For Problem 2: The extent of energy storage required to keep a string of around 25 LEDs (from above section) glowing for even half a minute, is pretty high. Capacitors would not be the way to go, unless you have access to big power-line capacitors through surplus channels.

  • Your capacitor bank would need to provide 20 mA at a minimum of 60 volts (again from above section), for "a few" seconds.
  • Capacitor needs to be rated for a voltage higher than the highest the generator could conceivably generate.
  • Though "supercapacitor" is a popular term these days, typical supercaps are rated for 5.5 Volts or 12 Volts, not hundreds of Volts.
  • Adding in buck/boost generator trickery to make this work would result in complexity far beyond using a battery and off-the-shelf charger.

I hope this helped.

  • \$\begingroup\$ Thanks, that helped a lot. Maybe I need to just allow the lights to go off when we switch riders. I would want to use white LED's because I'm trying to provide enough light to teach/learn by, and my room has no windows. \$\endgroup\$ Commented Oct 24, 2012 at 18:26
  • \$\begingroup\$ For white LEDs, you could use a string of around 17 LEDs (3.2 Volts per LED) instead. Red LEDs require about 1.7 to 2.0 Volts per LED, hence a longer string in that case. \$\endgroup\$ Commented Oct 24, 2012 at 18:53
  • \$\begingroup\$ What about DC LED fixtures, like the kind that are meant to be mounted under kitchen cabinets? \$\endgroup\$ Commented Oct 24, 2012 at 20:29
  • \$\begingroup\$ DC LED fixtures might work, depending on their rating and configuration: Some such fixtures already have current regulation built in, and most have current requirements higher than the 20 mA of the simplistic solution proposed above. This begins to crawl into the realm of complexity. \$\endgroup\$ Commented Oct 25, 2012 at 3:31
  • \$\begingroup\$ @DanRatliff It might even merit a separate question, to evaluate LED lighting options for AC/DC at varying and various voltages and currents! \$\endgroup\$ Commented Oct 25, 2012 at 3:33

We need to know exact model of bike to be sure. Regulating the light level and voltage level and the quality of light are all variables. Is your generator AC or DC or either? or an alternator Automotive type?

It takes less time to charge a 12V battery with low amp hours but lasts for a shorter time. A 60W power level converted to 14V wet cell would require 4Amps to discharge and charging efficiency may be as poor as 50% depending on electronics. I would think to demonstrate power vs brightness you disable the battery but never turn off the loads and to provide light all day, someone may need to peddle all night or use a battery charger, so storage management options needs further discussion.

For storage through-out class, a marine cell battery is still the best solution and easier to maintain than a gyro motor generator or other forms of energy storage.

For illumination, there are many options, used by "Specifiers" such as Luminous Efficacy in Lumens/ Watt, Color Rendering Index or CRI 50~100%, Brightness & Viewing Angle for indicators in Candella and degrees etc etc.

A dimmable economic solution is Philips warm A19 12.5W LED similar to 60W rated for 120Vac. Not sure about over tolerance but I suspect 10% is doable but 20% may get too hot for extended duration. I will try to find exact specs.

The nice solution here is it is dimmable with excellent CRI in a warm light using different phosphors coated inside the bulb over Blue LEDs inside. It is also called remote phosphor conversion LEDs, somewhat like Fluorescent lights except those start with UV light converted to visible light but some think far superior color quality. enter image description here

Can you use an motorcycle alternator instead with a constant voltage out and switches to see how many lightbulbs they can run? or do you want to measure power by brightness of the light. You will need at least 4 of 12W LED lights and may need spares. You will want some battery management meter to show charge meter to fill the batteries with charge to run your Lights otherwise class may end soon if someone is not peddling.

Marine cell Lead acid can handle deep discharges, whereas regular car batteries cannot.

There are certainly many more options such as 12V dc strings of SMT LEDs or use Xmas LED strings which may be transformer based or direct 120Vac connect.


I am sure budget is an important factor considering your time, storage costs, Electrician rates, wiring, bulbs, luminaires etc.

Look at going rates for consumers in reasonable volume or discount stores today but expect these to drop in future. - DC LEDs on substrates that need heatsinks, luminaires etc $0.5~2/Watt depending on quality, qty - AC LED bulbs >$1~$3/Watt, higher for warm white, higher for dimmable, higher for quality (CRI,MTBF), lowest for otherwise.


Above shows the best 1st from Philips at $2/Watt (and cheapest at $1.2/Watt)

If your generator runs 160Vdc, with a 60W bulb it will burn out, so you need an electronic load to limit RPM when the voltage exceeds reaches 130Vac but you can switch on more bulbs or charge the battery with either current limit and/or voltage limit or use a dummy light load such as 300W Halogen light controlled by a relay and over voltage sensor or use AC with a transformer and a suitable relay to regulate the voltage.

  • \$\begingroup\$ DC Generator. 20 amp, 12 volt. The bulb you reference (Philips 12.5 watt) is the exact one I'm trying to power from the battery pack. I have 5 of them. Battery is deep cycle 12 volt gel. \$\endgroup\$ Commented Oct 25, 2012 at 3:30
  • \$\begingroup\$ 12Vdc to 120Vac Inverter driving a dimmable stepdown converter LED can result in many incompatibilities with resonance, high impedance AC and may need chokes. I would design around 12Vdc LED's with 4 per string and use a current limiter with minimal voltage drop. <1V. requiring 13~14V. with a matrix of Nx4 1W LED or an existing similar array. Any good results yet>? \$\endgroup\$ Commented Oct 25, 2012 at 21:28
  • \$\begingroup\$ With a PTC protection, the voltage drop in minimal using string of 3 or 4 to require 9 or 12V to turn on. then 2x holding current to shutdown 50cents each... R Min/Max 0.830 ~ 1.240 Ohm has Ihold = 330mA, Itrip=740mA belfuse.com/pdfs/0ZRE.pdf Unlike headlights, these will increase to brightness voltage from 12 to 14.2V and demonstrate power of generator and battery without driving it flat. LED's need big aluminum strip like tile edging to spread heat and support LEDs. \$\endgroup\$ Commented Oct 25, 2012 at 21:43
  • \$\begingroup\$ This could be the most efficient way to drive them and has undervoltage protection for battery and overcurrent protection level you choose and brightness is proportional to peddling power or battery charge level, rather than well regulated and not useful for educating. \$\endgroup\$ Commented Oct 25, 2012 at 21:51

That's easy. Just a buy a LED bulb which is rated for voltages of 110V AC and 240V AC. For example, the search words "LED 110V 240V" will yield a few relevant (and huge bunch of irrelevant) results on amazon.com. That's not meant as an endorsement of Amazon, it's just the one of two internet shops I tried which gave me relevant results.

This will be an LED bulb which will, most probably, not dim when the voltage gets too low. It might simply turn off at some voltage below 110V or start flashing. Some LED bulbs produce bright flashes of rated brightness at intervals of many seconds or a few minutes when driven with very low voltage.


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