# Kid's museum project

Discussion in 'The Projects Forum' started by Avadoggie, Jun 18, 2010.

1. ### Avadoggie Thread Starter New Member

Jun 18, 2010
4
0
As a volunteer I build exhibits for the local Children's museum. I am working on a bike power demo. A DC generator puts out up to about 35 VDC at no load. I think the kids can generate up to about 150 watts so I have an arrangement of 50 watt lamps to show how much power they can generate.

Problem: At low loads they can run up to 35VDC. Voltage drops with higher loads because speed decreases. How can I limit voltage at low speeds so as not to burn out the 12 volt lamp loads?

Thanks

2. ### bertus Administrator

Apr 5, 2008
15,806
2,389
Hello,

Can you make a drawing of the plan?

A 12 Volts bulb will burn at the higher voltage.
Putting 3 equal (for instance 21 Watt carlights) 12 volts bulbs in series will overcome the problem.

Bertus

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
When loaded voltage may go down too, and another factor is the back EMF produced at full load in the dynamo will make it slow down if not compensated.

{ed}
depending on the dynamo, you can build a regulator to stabilize the voltage

4. ### russpatterson Member

Feb 1, 2010
351
16
I know this is not what you asked but first off I'd use some energy efficient lighting system and add the "green" spin to the exhibit. How about a big board with bright white LED's (not the super bright blinding 1 watt ones) say a grid with one every half inch. You'd need a ballast resistor on each one but that's no big deal. The kids would see a much brighter light show for the power they generate. Better yet do both with a switch on the handle bars for ancient vs. modern lighting technology!-

As for your original question about the varying voltage levels due to how fast the kids pedal and the associated load:

You get a voltage drop across each LED. Let me know if you need help figuring that out. Lots of great articles on this sight on the topic. So you simply add up your voltage drops until you've used up your maximum voltage. If you get lucky then without any other parts the amount of pedal power applied will nicely dim/brighten your LED array. Pedal fast for super bright mode, make it pulse, etc.

The other, way more geeky way to do it that would take 20X the time to complete, would be to make a DC/DC voltage "buck" converter using a microcontroller (or just buy some IC's that do it). throw some super capacitors in there to smooth out the power. Use multicolor, RGB LED's and do light shows based on patterns, music, etc.

Let me know if you have any questions about how to do any of this or where to get any of the parts for cheap. The museum exhibit sounds great!

5. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
Why interject political correctness into a science exhibit? Bertus has the right idea. 150 W is a lot for a child, and that lesson alone, as they see how hard it is to light all three bulbs brightly, could be used to drive home the message that we should all conserve energy, regardless of its color.

John

6. ### someonesdad Senior Member

Jul 7, 2009
1,585
141
This is probably a little bit off-topic in the sense that it won't be of much use to the OP, but it brings back a fond memory. About 23 years ago my stepson was about 10 years old. For one of his classes, I made a demo from an Electrocraft DC servo motor (run as a generator), an HP computer running Rocky Mountain Basic, an A to D setup that measured the output voltage and current, and an HP 7550 plotter. The person was asked to crank the motor and the objective was to see who could make the best power curve over a set time. I don't remember what I used for the load, but it was no doubt a suitable resistor. The person was told to go and crank like crazy for some period (probably around 10 seconds). Then their performance was plotted on the 7550 and they got to keep the plot.

This was a pretty popular device and I later put it in the city's science fair. Most people loved to watch the plotter work, as it would hold a bunch of paper, feed in a sheet, then quickly make the plot and spit it out. The boys were very competitive...

7. ### Avadoggie Thread Starter New Member

Jun 18, 2010
4
0
Thank you all for your good advice.

8. ### Avadoggie Thread Starter New Member

Jun 18, 2010
4
0
Thank you all for your good advice. Problem is, on start up with low loads kids can generate 35 vdc. As the load is increased, by switching in 50 watt lamps, the voltage will droop. So at the first 50 w load I might have an output of 35vdc and burn out the lamp. On the other hand I can't have the system start with the full 150w loads and produce lower voltages because most kids will not be able to make that power.

9. ### bertus Administrator

Apr 5, 2008
15,806
2,389
Hello,

What happens when you start the generator with 3 X 5 Watt 12 Volts lamps in series (this is a load of about 15 Watts).
This can be the small signal lights from a car.
The 3 X 21 Watt 12 Volts lamps in series can be switched to it to create a total load of about 15 + 63 Watts = 78 Watts.

An other idea is the use of all 5 watt lamps.
In that way you can increase the load in steps of about 15 watt upto the max of 150 Watts AND see what the kids are able to.

Bertus

Last edited: Jun 19, 2010
10. ### Avadoggie Thread Starter New Member

Jun 18, 2010
4
0
I want the kids to feel the extra effort it takes to light more lamps but, I also do not want to burn out the 12 volt 50 watt lamps which are conveniently available. So I need an arrangement that will switch in the loads in a timed sequence. 12 volts will have to be maintained for the lamps to show full brightness. When the kid's power tops out the system will reset to no load. Some of the kids are really energetic so I think the 150 watt total can hold them.

11. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
The 35V that you measured was open-circuit voltage. What was it under a minimal load? It is quite possible that even a single, 50W/12V bulb will put sufficient load on the generator to drop the voltage enough that the bulb will not burn out.

Are you planning to connect the additional bulbs in series or in parallel with the original bulb?

My advice is to just try it. If you don't burn out a bulb, then you have saved yourself a lot of worry and needless complexity. If you do burn one out, then it is a cheap experiment, and we will have to come up with another solution. One such solution has already been suggested.

John

12. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
An incandescent bulb has very LOW resistivity while cold and increasingly higher as the temperature increases.

I dont think you will have a problem with the 35V Open Circuit voltage affecting or popping your bulb. Considering they will start peddling from a stop and 0v point, that gives the bulb AMPLE time to reach a happy resistance level to load the dynamo well before you reach the 35V.