Connecting two bicycle dynamos together for extra power?

[iasonj]

I am not an expert on electricity, just have some experience with electronics, and would appreciate some help on this.

Description of a custom electrical system on my bicycle:
-Hub dynamo Shimano 3N70.
-After that, i drive the current in a small 'rectifying box'. This unit has one typical rectifying circuit with 4 diodes, and then 4 branches/circuits with L78S05, L78S06, L78S08 and L78S12 voltage regulators, for respective limiting of the voltage to 5, 6, 8, 12 volts, leading to 4 different outputs/jacks. (Note: the 12V output is rather theoretical, i don't think the voltage goes this high.)
So, according to the voltage of the device i want to use, i use the respective jack. Works fine!

My main purpose is charging AA Ni-MH batteries, since i find them to have the optimum weight/volume/capacity/price combination.
So i see that i have a need for more current, because if i connect a group of four, they take ages to charge.

I happen to have an old Sanyo NH-T6 dynamo as well.
Now the question is: can i use them both to increase the power?
I do not need extra volts, but extra milliamperes.
But then thinking crazy, it might be compicated:
Since the two dynamos generate alternative current, the current fron dynamo A might conflict with that from dynamo B (conflicting phase).
This might happen even if they are from the same type. They are not, so the phase is not the same.
I should probably rectify the two currents first, so after that the two DCs do not conflict. But then, how they should be connected? In series or in parallel?

Is this possible really? Or is it just stupid thoughts? Or - to put it practically - is it wise to think like that?
Can someone shed some light, please?
Thanks.

 

[Calina]

I'm no expert either, but why do you absolutely want to connect both dynamos to the same charging circuit ? Won't you have to isolate the dynamos from each other so that one doesn't try to run the other one as a motor ? Since you will have to rectify the current independantly anyway for each dynamo, wouldn't it be easier to just set up an independant charger for the Sanyo ?

 

[Steve K]

You should be able to full-wave rectify the Sanyo's output voltage and connect it in parallel with the rectified output of the hub dynamo. You should be using a large filter capacitor at the output of your existing rectifier, which reduces the ripple of the rectified AC power.

An alternative might be changing how you are charging the AA's. Are they wired in parallel, or in series while they are charging? The optimum arrangement would be to have the 4 AA's wired in series and connected to the output of the full-wave rectifier. No need for voltage regulation.. or at least the regulators that you are using aren't useful. Putting the full 500mA of the dynamo through the AA's should fully charge them in about 4 hours. You'll want to be careful that you don't overcharge the AA's though. I've used a basic shunt voltage regulator for that purpose, and it did well enough.

 

[find_bruce]

Steve has covered that there are other ways to skin a cat. I had held off responding as I was unsure about the effect of ripple current so am glad Steve covered that too.

In terms of your basic questions, your dynamo is capable of producing much more than 12v. In simple terms, the dynamo (it is actually a magneto, but I digress) produces relatively constant current & voltage increases with speed. A hub dynamo is capable of producing 12v under load at ~15km/h and 22v at over 30km/h.

If you have two DC sources of the same voltage, connecting them in parallel increases the current. If you have two DC sources with the same current, connecting them in series increases the voltage.

 

[iasonj]

I was experimenting in the meanwhile, just to discover my wrong approach: I was trying to charge AAs in a normal charger which accepts DC input. Silly.

You can just drive the rectified DC directly in them, in series, taking into account the ~500mA the hub gives and their nominated charging current.
I do not worry for the overloading, because a) they are so cheap and b) i can touch them and check temperature.

I also charged a 7,4V battery (from Nikon D200 camera) as well with the same method, just with the one normal dynohub.
I just keep an eye about overcharge, trying the battery in the camera and getting the idea of staying in safe time limits.

You are right, cat skinned, and no problem. Thanks!