Series connecting 6volt generators

[Callemanbager]

Hello!

Disclaimer - i basically have NO understanding of electronics, just the crazy basic stuff.


I recently got an idea that i wanted to have an old car headlight on my old bicycle so it would look like a motorcycle. So i bought an old car light!

now the problem is how i will get it to work. Here in sweden, almost all old bicycles have ”dynamos” which are essentially just small 6-volt generators that you flick in so that they roll on the side of the tire - and then are able to power a 6volt light. (Maybe ”dynamos” are common elsewhere too, i have no idea). I want to power my car light with ”dynamo-power” so that i wont need any batteries.

of course the headlight is 12 volts, so powering it with one dynamo didn’t work (i found out the hard way)

but i asked one of my teachers who seems like he has some kind of idea of how stuff like this works, and he told me that if i series-connect two of these dynamos (so plus from one and minus from the other and then connect the two remaining poles to eachother) that it should make a 12 volt signal!

I tried it today and there was no light in the headlight. I did tape some cables together, so maybe that was why it didn’t work.

should it work???

(I have tried the headlight with a car battery so it does work.)

 

[FvM]

Car head lights consume up to 50 watts and can be hardly powered by any kind of bicycle dynamo. How about mounting a bicycle LED lamp in the head lamp housing?

 

[D.A.(Tony)Stewart]

With tungsten car lamps and low efficiency, you would never be able to generate enough power from a bike dynamo to power your lamp. Keep it for future use. Go to the car junk yard and find an LED headlight (Pick-A-Part) with the regulator. Or roll your own with a CREE LED based flashlight and figure out how to charge it. The problem being figure out how to design a matched impedance generator converter like SOLAR you need an MPPT regulator which matches impedance to the source. With your leg power being the prime mover for lower impedance.

To do this test your generator at various energy and speed for Voc (open circuit voltage) and briefly pulse for Isc short circuit current which may make it hot.

Do not burn out the generator wire, so only briefly measure short circuit current.

Then graph this in a spreadsheet to compute Voc/Isc= Z mpt (max power transfer optimum point) and this is your generator impedance while Impt *Vmpt=Pmax However when charging a battery you won't need to use max power unless the battery is dead but the voltage converter ideally will load the generator at Vmpt and not lower. This may be around 75% to 80% of the Voc.


This may make no sense at first (even to experts) but as you study more, it will .

I don't know your dynamo specs but I would expect 5~10W max.

 

[c_mitra]

If course the headlight is 12 volts, so powering it with one dynamo didn’t work (i found out the hard way)

You have done the work systematically.

Your lamp is ok, it works but does not light up with a dynamo.

The dynamo produces AC of indeterminate frequency. The power, frequency and the voltage depends on the speed.

It is tricky to put AC voltage sources in series.

My suggestions:

1. Try with a lower power lamp (say the turn signal lamps or the inside lamp. They are still 12V but needs lower current and power and will light up even at 6V.

2. Then try the series connection of two dynamos: it may work but may not be reliable.

3. Use a diode and convert the AC to DC; also add a capacitor. Do thsi for both the dynamos and then connect them in series.

But even with two generators you may not be able to get 50W power. But the lamp should light up.

Best of luck!

 

[D.A.(Tony)Stewart]

https://www.bikeradar.com/advice/buyers-guides/bike-dynamos/

3 watts of drag at normal riding speeds (20km/h) with the light switched off.
6W if going 30 kph downhill

 

[KlausST]

Hi

I agree with the others, you may need 10-20 dynamos to run a halogen headlight.

To connect dynamos in series...I don't recommend to connect these AC sources in series. I'd use full wave rectifier, bulk capacitor, plus an anti reverse diode for each dynamo.
But even then one should consider some kind of voltage regulation.

Or .. consider a modern LED light which consumes much less power.

Klaus

 

[betwixt]

... or maybe a better strategy if you already have two dynamos is to wire them in parallel, each with a rectifier in series so you get 6V for LEDs but with twice the current available.

To expand on earlier advice, the dynamo doesn't produce anywhere close to enough power to operate a normal headlight but a further problem is most produce AC. A bike headlight doesn't care whether it is fed AC or DC, as long as enough current flows through the filament it will glow. However, an AC voltage will (as named) alternate its polarity one or more times per revolution of the dynamo shaft. If you just join them in series there is a very high probability the voltages won't line up, at best they peak positive and negative simultaneously to give 12V but at worst, one peaks positive while the other peaks negative so you get total cancellation and no voltage at all. Keeping them synchronized would be almost impossible so the only practical way to make them work is to convert the AC to DC from each dynamo then join the DC outputs in series.

Note that LEDs need DC, if you apply AC you risk damaging them permanently.

Brian.

 

[c_mitra]

Just to have some idea: most common dynamos used in cycles may have a max output power of 5W. It may be possible to push it to 10W but its life will be severely reduced.

A normal adult at rest consumes power about 40-60W (depends on body mass and also gender and age). While at rest, all the metabolic energy appears as body heat.

If you connect a 50W generator to your cycle, you may find it tough to pedal.

 

[Externet]

And if the dynamos have only one wire connection, means the other terminal is the bicycle chassis with no possibility to wire two in series. Really, really, use a LED instead of an automotive headlamp which converts 90% of the pedaling energy into heat and 10 % into useable light. The LED is about the opposite, and can be powered by a single rectified dynamo if AC.

 

[Callemanbager]

Alright! Thanks for the replies!

I’m starting to understand that this idea of powering the headlight directly with 2 series connected dynamos might be quite difficult.

the thing is that I would really like to use a non LED light because of the yellow-ish light that ”old” lights give. (To look cool was the reason that I bought the car light in the first place

so, I actually have some backup-plans, because I was a little bit skeptical about this first idea all along..

Plan B: To change the bulb in the headlight for a 6volt bulb that is made to be powered by a dynamo, to make it seem like my bicycle has a car headlight. I guess that it wouldn’t be as bright, but if 6volt lights are bright enough for other bicycles it shouldn’t be a problem, right? And it would probably also be a bit tricky getting a 6volt bulb to fit in the headlight due to different sockets, but could you fix that somehow?

Plan C: To mount a motorcycle-battery on the bike to power the headlight as is, and charge the battery every once in a while. I found a motorcycle battery that is ”3 ah” (feel free to correct me, but isn’t that ”ampere hours?) , how long would that battery be able to power the headlight before I need to charge it?

and also if you were curious my bicycle dynamos say 3watts each

thanks!

/Calle with the old swedish military bike

 

[c_mitra]

I’m starting to understand that this idea of powering the headlight directly with 2 series connected dynamos might be quite difficult.

the thing is that I would really like to use a non LED light because of the yellow-ish light that ”old” lights give. (To look cool was the reason that I bought the car light in the first place

so, I actually have some backup-plans, because I was a little bit skeptical about this first idea all along..

Plan B: To change the bulb in the headlight for a 6volt bulb that is made to be powered by a dynamo, to make it seem like my bicycle has a car headlight. I guess that it wouldn’t be as bright, but if 6volt lights are bright enough for other bicycles it shouldn’t be a problem, right? And it would probably also be a bit tricky getting a 6volt bulb to fit in the headlight due to different sockets, but could you fix that somehow?

Plan C: To mount a motorcycle-battery on the bike to power the headlight as is, and charge the battery every once in a while. I found a motorcycle battery that is ”3 ah” (feel free to correct me, but isn’t that ”ampere hours?) , how long would that battery be able to power the headlight before I need to charge it?

and also if you were curious my bicycle dynamos say 3watts each

thanks!

/Calle with the old swedish military bike

You are right. Connecting dynamos in series may end up in a big mess.

But you can get LED lamps that are yellowish; just look for warm white. Warm refers to more yellow and cool implies more blue. You can also refer to their color temp (warm white will have a lower color temp).

The apparent brightness depends on the surface energy density of the radiator and LEDs can be brighter than a halogen lamp. The actual emitter surface can be rather small for a LED but not so for a WI lamp. Also, a 5W LED lamp may output same amount of visible light as a 25W halogen lamp.

The 3AH suggests that the battery can deliver 3A current for 1 hour. Normally a 3AH battery is expected to be discharged at 0.3A and it may last 10 hours. If your lamp is rated at 6W and 6V, it may take about 1A current and you may end up with a dead battery after 2-3 hours. They (the common batteries used in bikes) are lead-acid type and cannot stand much abuse.

Better to get s bright LED cycle head lamp run with 18650 Li-ion cells (2-4 in parallel). You can get warm white color although most prefer cool daylight.

 

[Callemanbager]

Alright, ill try a LED light then i guess

I Found this bulb on Ebay, it seemes quite perfect to me but it says DC and i have now learnt that my dynamo generetas AC current so i suppose it wouldn't work?

The Bulb i Found

 

[D.A.(Tony)Stewart]

You have done the work systematically.

Your lamp is ok, it works but does not light up with a dynamo.

The dynamo produces AC of indeterminate frequency. The power, frequency and the voltage depends on the speed.

It is tricky to put AC voltage sources in series.

My suggestions:

1. Try with a lower power lamp (say the turn signal lamps or the inside lamp. They are still 12V but needs lower current and power and will light up even at 6V.

2. Then try the series connection of two dynamos: it may work but may not be reliable.

3. Use a diode and convert the AC to DC; also add a capacitor. Do thsi for both the dynamos and then connect them in series.

But even with two generators you may not be able to get 50W power. But the lamp should light up.

Best of luck!

Not a chance if the dynamo has high impedance under 10W max under 16V at 30 Ohms Requiv. Better to try a switching PFC circuit to load with proportional power load limited by LED and current sensing to a battery with RPM sensing to match the lower Requiv with rising speed and power output V^2/Req and Voc proportional to RPM and thus power proportional to kinetic energy 1/2 mv^2

 

[dick_freebird]

I would recommend a LED or N of them in parallel (w/ suitable ballasting) and a small buck / boost or simple boost converter set up for current mode control matched to LED current specs. Could be very simple and crude.


Opportunities to harvest multiple LED chips or encapsulated LEDs are many, from little throwaway flashlight to monitor illuminator panels - although the series / parallel config in this case favors a short stack for low input voltage.

 

[c_mitra]

Alright, ill try a LED light then i guess

I Found this bulb on Ebay, it seemes quite perfect to me but it says DC and i have now learnt that my dynamo generetas AC current so i suppose it wouldn't work?

The Bulb i Found

Yes, you need to use a simple diode in series to feed the lamp with DC. If space permits, just also add an electrolytic capacitor across the lamp terminals.

It should work; even without the rectifier. LEDs are diodes and will light up only in one direction.

A series diode is recommended because it will also act like a series ballast. If it does not work, just reverse the diode.

Do you have a bell? You can also add an electronic bell; they come with musical tones!

Happy biking!

 

[Externet]

If your generator is AC, use a couple of white LEDs in counterparallel instead of rectifying :

Ignore the wrong "+" , gn, rt in the schematic.

 

[betwixt]

Many moons ago (more than half a century!) I did something similar but LEDs hadn't been invented then. To keep the incandescent lamp, I think a 3W one, running and to make it safer when the bicycle stopped, I converted the dynamo output to DC and connected it to a small 6V Pb battery. That utilized any excess power (yes, in those days I still had some!) to charge the battery and it kept the lamp lit up for a while when not in motion. If you did that with an LED it would be quite efficient.

Brian.

 

[Taihung]

Fully Classical Style

The best solution is changing light bulb to 6V type.

The second solution is increase output voltage of dynamos.
You open dynamos, remove old coil turn by turn. You count the number of coil - N turn.
The copper wire will have diameter D, you find another wire with diameter D/3 and re-wire the dynamos 3xN turns.
Close it again and try..
This solution will keep the "bike" true Classical Style

The third solution is ... still open dynamos.
Replace old rotor - weak magnetos with new strong modern magnetos.
Need 3D printed frame & Epoxy Glue to mount strong magnetos on rotor shaft.
Stronger output voltage and power, also take more damage to bike's tire because of increasing friction force.