Bicycle Dynamo

[Menticol]

Hello!
I just got an old Dynamo (alternator) and a light from the Bicycle Store, and I'm very happy with it. My bicycle is already equipped with a 12V 7,2 Ah battery to power a car horn and a small led lamp via LM317, but generating some electricity "on the road" is much more interesting.


I'm exploring two options (please see the attached picture).

Some facts:
Dynamo output: 12V 6W at moderately high RPM's
Battery: 12V 7,2 Ah SLA

Loads:
12V 6W incandescent bulb, switching to white LED as soon as budget improves
12V 4A car horn (rarely used)
LM317 regulator, feeding a 3 volt red LED lamp (that load is insignificant)


A) In your opinion, which one of the two options is better?

B) Dynamo output voltage easily exceeds 12 VAC. It may reach 20 or more downhill, burning everything. 7812 regulator has too poor performance (too high input requeriments, bad regulation if voltage drops), so what could be a better option for voltage limiting?


Thank you very much for your time

PS: Obviously I don't pretend to feed the car horn using the dynamo. Is connected to battery at all times.
PS2: I like the first approach, because battery could be recharged on the road. But I guess charging time may be insufficient.

 

[crutschow]

Do a modified option 1. Run everything off the battery and add a full-wave rectifier from the dynamo to charge the battery. You don't need the filter or voltage limiter. The battery will act as a filter and keep the voltage from going too high.

Note: A full-wave rectifier gives the most efficient use of the dynamo output but it requires either that the dynamo have an isolated output or all the lights and horn be isolated from the frame.

 

[Sceadwian]

If the dynamo is run directly in parallel with the battery it should eat any additional voltage created when going down hill charging the battery in the process, though it may produce some moderate breaking forces on the bike. You'd have to put a diode on the dynamo to prevent the battery from turning the Dynamo into a motor, however if you wire a good circuit up this might actually provide a little extra starting torque.

cruts, if this is a bike dynamo it's output is already DC, I don't think rectification is required, although filtering would be, in parallel with the battery it will provide the filtering. My only concern would be the amount of breaking force applied at higher speeds on the bicycle, however this would just add to the workout if that's what you're going for =) When the dynamo is producing less than the voltage of the battery it would effectively be electrically disconnected because the diode used to prevent the battery from discharging into the dynamo would be reverse biased, as the battery voltage drains substantially this will allow the dynamo to charge the battery and provide the rest of the electrical system with power as well. If a large enough capacitor bank was used it may even be possible to completely eliminate the battery, however then you'll have to deal with the possible higher voltages when the bike is traveling at high speeds as the battery won't be there to gobble up that extra voltage. Also if the capacitor bank was completely discharged like when you first start riding it would be very difficult to pedal the bike until it attained a moderate charge level.

 

[shokjok]

You may consider a switchable regulator and a tripler. The low voltage input feeds a tripler to raise the voltage to 12-13V, at faster speeds the tripler output exceeds 13V and feeds the regulator. A comparator could switch this input for you automatically. No diagram is posted, my scanner software is corrupt.

 

[crutschow]

cruts, if this is a bike dynamo it's output is already DC

The op stated the dynamo was an alternator with an output of greater than 12VAC when going downhill, so I based my answer on that.

 

[Sceadwian]

Crut, he didn't say AC anywhere in his post. I don't believe it's an alternator either, bicycle dynamos are usually simple brushed DC motors, with a filter cap you get straight DC out not AC. He just used the wrong terminology when he said alternator.

Again, I don't believe anything need be done about the higher voltage the Dynamo can produce, if connected in parallel with the battery the circuit should never see that voltage, the battery will act as a low impedance load when it goes over the batteries voltage and the current will flow from the Dynamo to the battery, the increased voltage shouldn't manifest in the circuit.

 

[Menticol]

Crutschow and Sceadwian, thank you very much by your time guys!! I just returned home, so I'll start working with your suggestions right now.


About the AC/DC discussion, I think two pole Alternator is the correct name for this device (check the pictures)


http://www.myra-simon.com/bike/images/dynamo-sm.jpg (Example of how it looks like)
http://nordicgroup.us/s78/images/dynamo1.gif (According to that, the thing must be brushless)


I don't know in the United States, but here "Dynamo" has been a commercial name for decades (like "Kleenex" for tissues). As we already know using this name is wrong, but no bicycle store knows anything about "alternators". Instead, they surely have "bicycle dynamos" on stock

Thank you again by your help, and to Shokjok too.

 

[Sceadwian]

Those images definitely looks like they output AC don't hook it up to a battery like that. Does it have a bridge rectifier built in or on an external circuit board? You said it outputs 12V at about 6watts, nothing is mentioned about the output polarity or if a diode bridge was required after the fact.

Dynamo might be a commercial name where you are, but it also happens to be the definition for a DC generator I'm not entirely sure you know what you have on your hands and you need to find out. If it's a true dynamo it'll output DC, if it's an alternator it'll output AC. I think you may be confused about it's internal construction unless you can provide the model and maker of the "Dynamo" that you have. As I stated, almost all bicycle electrical generator I have ever seen all ouput directly DC power. It would be highly unusual for it to be an actual alternator and outputting AC.

 

[Nigel Goodwin]

I'm going back a LONG time since I ever played with bicycle lights - but way back then some were AC and some were DC, it really makes no difference as far as incandescent bulbs are concerned.

However, bulbs never used to blow very much (I never had one fail), so presumably (like small motorcycle alternators) they are essentially self-limiting, as part of their basic design.

Considering power output is directly proportional to speed, full brightness is achieved at a fairly steady pace (15-20mph?), yet doing 60mph down a hill doesn't blow the bulbs - the same applies to small motorbikes - once you reach a certain speed the lights don't get any brighter.

As to what things are called, cars still use 'condensers' which changed to capacitors everywhere else many decades ago, and likewise all bycycle generators tend to be called dynamos by the cycling fraternity.

Thinking back, the last time I had any dealing with bicycle lights was with a work mate (about 15 years ago), he made a headlight for his bike using a 6V halogen spotlight (as used in shop displays - and I obtained some correct sized aluminium extruded tubing that just fitted it), he powered it from a 6V SLA battery - we used a bridge rectifier to provide charging for the battery. Obviously it didn't provide enough power to feed the bulb (50W!!) - but by running with the dynamo connected during the day, he was able to keep the battery charged enough for his purposes.

It was the brightest bicyle light I've ever seen

 

[crutschow]

Crut, he didn't say AC anywhere in his post.

You need to reread his post. In paragraph B) he states "Dynamo output voltage easily exceeds 12 VAC".

 

[Menticol]

don't hook it up to a battery like that. Does it have a bridge rectifier built in or on an external circuit board?

Thank you by you concern Sceadwian, I already know that voltage must be rectified before connecting the battery

It was the brightest bicyle light I've ever seen

I was going to post an awesome 2 x 50Watt halogen fixture for a bicycle, but the author has disappeared from Instructables it was an awesome overkill

You need to reread his post. In paragraph B) he states "Dynamo output voltage easily exceeds 12 VAC".

That's right Crutschow. Before mounting the alternator, I turned the rotor with a drill, then with a dremel. Output voltage with the drill stayed at 16 volts, but with the dremel it reached 48 volts VAC.

 

[crutschow]

Since the picture of the dynamo shows a two wire output it apparently is isolated. Thus you should be able to use a full-wave bridge rectifier connected directly to the battery. That should be all you need to charge the battery.

 

[Nigel Goodwin]

I was going to post an awesome 2 x 50Watt halogen fixture for a bicycle, but the author has disappeared from Instructables it was an awesome overkill

An added bonus is that the lamps are available in a number of different beamwidths, so you can pick the one you find best.

 

[Sceadwian]

Woops, guess I missed that sorry Crut.

 

[tcmtech]

Get a set of the halogen mini driving lights normaly used on vehical bumpers for $20 - $25 (35 - 50W) and you will have more than enough light to see where your going!
And get a good LED tail light or I will run you over!

 

[Menticol]

Here is a small video of the finished product

YouTube - Bicicleta de La Rotta

Crutschow, Sceadwian and Nigel, thank you so much for your invaluable advice!