High School project - electrical power generator

[splinter]

Dear readers,

i'm a high school Science teacher in Belgium and i've entered my students in a Science and Technology fair.
They have a good idea but my knowledge about electronics has its limits...
I've got 2 general questions first:
1) i've started with Arduino projects but do you guys know good books, tutorials, courses, etc so that i can make my own "simple" projects? I've been following the manual untill now.
2) What equipement is essential when starting with project?

and now about the project.
They would like to hook up a dynamo to a hometrainer to do 3 things:
1) Charge a phone
2) Power a small fan (12V)
3) Charge a battery pack

Can it be done? I've been reading alot about it but i find alot of contradictions.
What i think so far (please correct me if i'm wrong)
A normal bike dynamo delivers a current of 6V AC. I need to convert it to 6V DC to charge the phone, charge the battery and power the fan.
If i use 2 dynamo's in serie i can ad up the current to get around 12V AC converted to DC. I need and Elco or a condensator to do this (don't know the correct name in english to convert AC to DC) If i try to power the fan and charge the battery pack it won't work really well so i'll need a switch to chose between the 3 functions. I should check the output current to see how many mAh it delivers. When i multiply the capacity by 1.4 (why 1,4? loss of current?) and devide it by the output current i know the time it takes to charge the battery pack.

Hope you guys can help me so i can start asap.

I'll post some pics if we are making progress.

Thank you in advance!

greeting from Belgium
Sander Hungenaert

 

[audioguru]

You do not know enough about electronics to do what you want to do. You will teach wrong things and the students will be confused.

If the battery pack is Lithium (used in cell phones and laptop computers) then you MUST charge it correctly to prevent it from exploding and catching on fire. It needs a charging circuit designed for it. A cell phone has a charging circuit inside it that needs an input of 5VDC.

You must not connect an "elco" or capacitor to the output of an AC dynamo. The AC from the dynamo is converted to pulsing DC with a rectifier and the capacitor filters the pulsing into smooth DC.

Show how the dynamo can light a light bulb, run an AC fan or ring an AC bell instead.

 

[splinter]

First of all thank you for your quick response.
I'm fully aware that my knowledge at this point isn't near good enough but we have an electronics and electromechanics department in our school so if i get stuck i can always get help.
Before i get help in school i would like to understand it myself by doing some research and consulting some experts (forum).
I would never teach my students wrong things but a project like this is a good way to get them curious. And if something doesn't work sometimes that's even better. I would drive them to find the mistake, to do more research,...
If i just let them turn on a light bulb it will never get them exited enough... By the way all good inventions started out as failures (but i will always keep student safety a priority!)

For the battery pack i was thinking about a 4-cell NiMh pack because i read that it is less sensitive to power spikes and shouldn't overload that fast. (pls correct me again if i'm wrong)
Do i need a specialised circuit for the NiMh pack as well?
And about the dynamo should i use both the rectifier and the capacitor filters? Will there be a power loss or will i keep around 6V?

Thanks again 4 responding.

greetings
Sander

 

[spec]

POST Issue 03 of 2016_11_26

Hi splinter,

Greetings from England to Belgium,

One of the things about engineers is that we all have different ways of looking at things. So the normal form on ETO is that we all post different views and different solutions. It will be up to you to choose the posts that suit. You say that you have seen many contradictions on the net- you are dead right there. What confused me when I was a student is that you would get two experts arguing exactly 180 degrees out of phase.

So with that in mind, this is what I think.

As audioguru says this is quite a big task to take on if you have no experience, but my impression is that you have a positive outlook and are keen to see this project through. So if you are prepared for a steep learning curve here goes about the project first:

[The students] would like to hook up a [bicycle] dynamo to a hometrainer to do 3 things:
1) Charge a phone
2) Power a small fan (12V)
3) Charge a battery pack

Bicycle dynamos, come in two common types, hub and bottle. I will assume that you are considering a bottle dynamo, which are available in two common powers: 3W and 6W. I will assume a 6W version.

As you say, bicycle dynamos produce an alternating voltage and current (so strictly speaking, they should be called alternators rather than dynamos). Also, as you say, you will need to convert the alternating voltage and current into direct voltage and current. To do this is quite simple and requires a bridge rectifier and a reservoir capacitor.

You mention that dynamos produce six volts. This is true for a 6W load, say a 6 Volt, 1 Amp lamp. But without a load, the dynamo could produce around 40V or even higher. Because of this characteristic of bicycle dynamos, you would need a power shunt to limit the voltage. Once again, a power shunt is simple and can be built with about four standard components.

(1) Charge a phone.
To charge a mobile (cell) phone you would need a constant 5.1V, ideally at 1A. A simple, low-cost converter board will do this, and that's all there is to it. One mobile phone, while charging, will take 5.1V * 1A = 5.1W, so there would be no power left over for other devices.

(2) Run a 12V DC fan
A single switch mode power supply (SMPS) board will convert the varying voltage across the reservoir capacitor into a constant 12V to power a fan. But if you chose a 5V fan you could use the same power supply board that is used for charging the mobile phone.

(3) Charge a small battery pack.
You mention a nickel metal hydride (NMH) battery pack, but my advice would be to go for a single lithium Ion (LiIon) 18650 battery with a capacity of 3,4000 miliamp/ hours (3.4 A/H). A lLiIon battery cell has a nominal terminal voltage of 3.6V. Once again, all you would need to charge the battery would be a standard charger board.

I have only scratched the surface so far, but just to reassure you that your project is fairly straight forward and achievable. All the techniques involved are pretty standard and, in the main, off-the-shelf, low-cost, boards will do the job.

In general, it is best with any project to break it down into manageable chunks and have clearly defined milestones. I would suggest that the first move should be to get the mechanics of the dynamo sorted so that you can reliably light a lamp by peddling the exercise bike.

Then get the rectifier and power shunt working, and so on.

If you wish to discuss this project further, jut say, and I, and no doubt others, will fill you in with some more details. In the meantime, below are some links to the types of parts that have been mention above. Take these as a guide, rather than the boards and components optimized for your project, although the battery is the real thing.

spec

LINKS
(1) Rectifiers (10A, 100V Schottky): https://www.onsemi.com/pub_link/Collateral/MBR1080-D.PDF
(2) Reservoir capacitor (4,700uF): TBD
(3) NMOSFET transistor, power (for power shunt): https://www.ti.com/lit/ds/symlink/csd18532kcs.pdf
(3) 5V SMPS: TBD
(4) LiIon Battery charger: TBD
(5) LiIon battery cell (only buy batteries from mainline manufacturers and a reliable source- many batteries are a rip off): https://batteriesplus.co.uk/acatalog/copy_of_Lithium_Ion__Li-Ion___3.7V_Batteries-1.html

 

[spec]

POST Issue 2 of 2016_11_26

Hi splinter,

Below is an outline circuit for the dynamo power supply unit. The output voltage will vary according to the speed of the dynamo and the current drain from the PSU, but the output voltage will be limited to around 23V. The four diodes in the bridge rectifier are high-current Schottky types, for high efficiency.

spec

​

LINKS
(1) Rectifier (10A, 100V Schottky): https://www.onsemi.com/pub_link/Collateral/MBR1080-D.PDF
(2) Reservoir capacitor (4,700uF): TBD
(3) NMOSFET transistor, power (for power shunt): https://www.ti.com/lit/ds/symlink/csd18532kcs.pdf
(4) Zener diode:
Zener diode: BZX79xxx (500mW, 2%) [0.17$] **broken link removed**

 

[splinter]

Ok so i'll start out slow and look for all the needed equipment. We'll go for a 5V fan so we can use 1 circuit thanks for the advice.
If i get another rectifier with for example 150V will it influence the circuit in a negative way?
And we'll try a light first and then we'll move on. Do i need a power regulator 5V at the end of the circuit.


1 question abou the circuit: Why do i need the Zener diode?

greetz
Sander

 

[audioguru]

A bicycle dynamo (alternator) does not have a voltage regulator. When the dynamo is spinning fast then its voltage is much too high. A car alternator has a voltage regulator that prevents its output voltage from exceeding 14.5V but the alternator can still produce almost its rated voltage when spinning fairly slowly (a bicycle dynamo cannot).

The zener diode in the circuit shown turns on the Mosfet when the voltage becomes too high and the Mosfet and its resistor overloads the dynamo to prevent high voltage. I suspect that the light on a bicycle overloads the dynamo to prevent its voltage from going too high. You must find a fan that uses the same current as a bicycle light, but if the light and fan are both turned on the dynamo will be overloaded and its voltage might be too low.

The circuit shown and a voltage regulator IC will do nothing to boost the voltage when it is too low. A circuit called a "boost/buck converter" is what is needed to boost or buck the voltage from the dynamo. More complications.

 

[spec]

A bicycle dynamo (alternator) does not have a voltage regulator. When the dynamo is spinning fast then its voltage is much too high. A car alternator has a voltage regulator that prevents its output voltage from exceeding 14.5V but the alternator can still produce almost its rated voltage when spinning fairly slowly (a bicycle dynamo cannot).

The zener diode in the circuit shown turns on the Mosfet when the voltage becomes too high and the Mosfet and its resistor overloads the dynamo to prevent high voltage. I suspect that the light on a bicycle overloads the dynamo to prevent its voltage from going too high. You must find a fan that uses the same current as a bicycle light, but if the light and fan are both turned on the dynamo will be overloaded and its voltage might be too low.

The circuit shown and a voltage regulator IC will do nothing to boost the voltage when it is too low. A circuit called a "boost/buck converter" is what is needed to boost or buck the voltage from the dynamo. More complications.

Gently gently AG. One step at a time. I have already made all the points, that for some reason you feel the need to restate. I also know how the circuit posted works- I should do I designed it.

Please do not snipe from the side lines and fill this thread with negative statements. You have made your contribution and said that it is all to difficult and also used your usual scare tactics about exploding lithium Ion batteries.

spec

 

[spec]

Ok so i'll start out slow and look for all the needed equipment.

Good move; that is the way to succeed- divide and conquer.

We'll go for a 5V fan so we can use 1 circuit thanks for the advice.

No probs and a good approach. 5V fans are freely available.

If I get another rectifier with for example 150V will it influence the circuit in a negative way?

The peak inverse voltage (PIV) of the diodes will affect nothing, providing that the PIV is higher by say 20% than the voltage that the diode will be subject to in the circuit. The two other important characteristics are the diode speed (unlike your mains supply which has a fixed frequency of 50Hz, a dynamo frequency will go much lower and higher) and the voltage drop when current is passing through the diode in the forward direction. Schottky diodes are particularly good in both of these respects. The diodes that I have specified are widely used and are dirt cheap, so I would advise going for those.

And we'll try a light first and then we'll move on.

Excellent

Do i need a power regulator 5V at the end of the circuit.

Yes, a step-up/step-down (a type of converter that does both) DC to DC converter will be best, but you could probably get away with a step down converter. This will all be thrashed out in the detailed design phase.

Why do i need the Zener diode

The bridge rectifier and reservoir capacitor in the circuit of post #5 convert the AC from the dynamo (alternator) into DC. By the way, 6V RMS rectified will give 1.414 (root 2) * 6V = 8.5V. You need to subtract the two forward voltage drops caused by the diodes = 2* 0.4V = 0.8V to give a final DC voltage across the reservoir capacitor of 7.7V.

The Zener diode, two resistors, and NMOSFET prevent, the DC voltage from rising above around 23V when there is no load current on the dynamo. This not only protects the circuits connected to the reservoir capacitor, but also protects the coil in the dynamo which can flash over without a load. The dynamo is designed so that, in normal use on a bicycle with a 6W load (bulb), the dynamo RMS output voltage will be 6V.

spec

 

[spec]

POST Issue 2 of 2016_11_26

Hi splinter,

Below is a functional block diagram of a suggested system for your project. This should clarify my presently suggested approach.

As you know, no system can use more power than the input power (in this case 6W).

Here is the power budget for your system:
(1) Mobile phone charging 5V @ 1A = 5W,
(2) Fan, say 5V @ 0.1A = 0.5W
(3) LiIon battery charging (10 hour rate) 3.6V @ 340mA = 1.2W

Total power= 5W + 0.5W + 1.2W = 6.7W.

But the various conversions in the circuit will not be 100% efficient. As a rough estimate, an efficiency of 50% would be about right. This means that you would need an input power of, 2 * 6.7W = 13.4W.

So you have some options:
(1) Run the fan and charge the mobile phone and Liion battery at different times (you mentioned this as an option by using a selector switch).
(2) Use a 6W bottle dynamo and accept a reduced charging current
(3) Use more than one bottle dynamo (you mention this, but not to increase power but to increase voltage).
(4) Use a more powerful generator

All four options are feasible, but I would recommend considering option (4). I don't like to be a killjoy, but you could get more power and efficiency from a different type of dynamo, as bicycle dynamos are specifically designed to limit their power output so that they do not blow the bulb- they still tend to though!

spec

 

[spec]

Hi again splinter,

as you say that you have little experience of electronics, perhaps you would like to have a look at this ETO article, to get a feel for typical electronic components: https://www.electro-tech-online.com/articles/core-components.782/

spec

 

[splinter]

I'm going to start out by reading the article and getting the basic supplies that I don't have yet.
After that I'll have a go later this week at finding a stonger power source.
The price will be higher I guess but can I test out the circuit with a low power bike dynamo without connecting a device?
I can just measure the current and voltage to see if I didn't make a mistake. What should the output voltage be? around 5V?

Does anyone know a better power source? I found some 12V dynamo's but they're pretty expensive for an experiment.

greetz
Sander

 

[spec]

1) I've started with Arduino projects but do you guys know good books, tutorials, courses, etc so that i can make my own "simple" projects? I've been following the manual untill now.
2) What equipment is essential when starting with [the Arduino] project?

The Arduino is an excellent microcontroller board, and is used widely, especially by ETO members. But may I suggest that a much better platform to use for teaching students the principles of computing is the Raspberry Pi (Pi).

On the technical side here is a rough and ready comparison between the Arduino and Pi:
(1) Low-level electronics control- Arduino has the edge but now the Pi has caught up.
(2) Assembly language: same.
(3) Elementary high level programing: same.
(4) High level programming: Pi.
(5) General computing: Pi.

The Pi is a single board computer as opposed to the Arduino which is essentially a single board microcontroller. The Pi also has vastly greater processing power and memory.

But, the real cruncher is that the Pi has been specifically designed for teaching, especially young students, and there is a wealth of tutorials, magazines, and books available, many being free.

In view of the above, please let us know which way you would like to go; Arduino or Pi. Either way, I and, no doubt others, will be able to answer your two questions above.

spec

 

[splinter]

Thank you for the information I'll also be looking into the raspberry Pi this weekend.
The shields and equipement (sensors, capacitors...) will work for both I presume? Because i've ordered a sim900 shield last week.

greetz Sander

 

[spec]

I'm going to start out by reading the article and getting the basic supplies that I don't have yet.

The article just lists core components that I recommend for starting out in electronics. For your project you will not need all the components unless you plan on setting up a general electronics facility at your school. I have also, just posted a draft article, describing 'core electronic equipment' to get started in electronics @ https://www.electro-tech-online.com/articles/core-equipment.787/

After that I'll have a go later this week at finding a stronger power source.
The price will be higher I guess but can I test out the circuit with a low power bike dynamo without connecting a device?
I can just measure the current and voltage to see if I didn't make a mistake. What should the output voltage be? around 5V?

Yes, it will be fine to test the principle with a low-power bicycle dynamo. The normal power is 3W, rather than 6W.
When the dynamo is spinning at operating speed it will generate 3W AC, so you can say that at 5V AC output voltage the dynamo will produce 5V AC at 3W/5V Amps = 0.6A AC. But if you rectify and connect a 5V output converter board to the rectified output from the dynamo you would probably get around 5V at 300mA when you take efficiency into consideration.

Does anyone know a better power source? I found some 12V dynamo's but they're pretty expensive for an experiment.

Apart from bicycle dynamos, dynamos are rare and expensive. But having said that, there is absolutely no reason why you could not use a permanent magnet motor as a generator. A 50W or greater 12V fan motor would be a good choice. Fans are available either free, from scrap equipment, or at low cost, new.

The subject of using a motor as a generator is a common topic on ETO and has attracted much comment and debate.

spec

 

[spec]

Thanks for the information. I'll also be looking into the raspberry Pi this weekend.

No probs. The Pi is well worth investigating. You may read that the Beagle Bone single board computer is better than the Pi, and so it is, technically. But the Pi will be more than adequate for your and your students needs and is much better supported. In the meantime, here is a link to the Pi Organization home page. You can download the free Pi magazine and tutorials. https://www.raspberrypi.org/

The shields and equipment (sensors, capacitors...) will work for both I presume? Because I've ordered a sim900 shield last week.

The electronic components will be identical and, in general yes, the shields will be compatible, but you will need to make different connections to the Pi input/output pins. I am not sure what a sim900 shield is

spec

 

[JonSea]

Despite what spec says, there may be some better choices than bicycle dynamos for your project. eBay has some astonishingly well constructed mini-hydro generators for possibly less money than what you're looking at. Of course you don't want a water-driven generator, but inside these devices, there's a permanent magnet generator waiting to be used for something different.

This type actually has a voltage regulator built in, and they are available with 5 and 12 volt options. Want to what's inside? Big Clive's video as he "takes it to bits" will be helpful.

This is another option. This type doesn't include a voltage regulator but is beefier and might be a more usable form. Big Clive helps us out again with another video. I believe these units are available in different range voltage outputs.

You'll probably need to use a belt drive or gearing to drive these generators at the speed they need to turn. Good luck.

 

[camerart]

Hi S,
Apologies if I'm repeating anything.
A small point as you are teaching children. The 4 diodes are used as a rectifier, and you can show how the alternating current flows forwards through the diode, The drawing in #5 explains the rectification better than using a rectifier.
C.

 

[splinter]

Hey Camerart thank you for the advice. The drawing that spec made is indeed really clear and can help me explain how the AC turns into DC.
But just to be sure is the only function of a diode to let the current pass trough in one way and block it from going the other way?
JonSea: do you have experience with these water turbines? Do you know the output voltage and current?
spec: I'll also be on the lookout for old fans so I can let my students test the different options (for voltage and current) and let them decide what they think will be the best option.

greetings Sander

 

[camerart]

Hey Camerart thank you for the advice. The drawing that spec made is indeed really clear and can help me explain how the AC turns into DC.
But just to be sure is the only function of a diode to let the current pass trough in one way and block it from going the other way?
JonSea: do you have experience with these water turbines? Do you know the output voltage and current?
spec: I'll also be on the lookout for old fans so I can let my students test the different options (for voltage and current) and let them decide what they think will be the best option.

greetings Sander

Hi S,
Yes, if you treat them in a similar way to valves, you will see that as the sign wave goes high and low on the AC side how the current flows into DC.
C