Hub Dynamo-DIY Voltage regulator questions

[bikes4two]

> I have a built wheel with an old HB-NX30 that needs replacing for which I am considering the Shimano DH-3N80 (are the hub flanges the same diameter?)
> I want to use the hub dynamo simply for charging a battery pack from which I can charge up my other toys
> Yes, I know about the B&M E-Werk but I'm an inveterate tinkerer with an electronics background, and there are so many voltage regulator packages at under £10 from eBay, that I'm very tempted to make my own charger first
> So, has anyone out there done this and if so, what components did you use?

> My initial thoughts are to build a simple bridge rectifier circuit (with smoothing capacitors) to convert the AC output of the hub dynamo to (an unregulated) DC voltage and feed this into a USB Voltage regulator package (such as this one or similar). The spec says the input voltage needs to be 6V-35V DC
> Not having had a (working) hub dynamo before, I'm interested to know what the AC voltage output is, i.e. does it start at zero volts and build up to a maximum (of what)?

Any guidance in this please?

[Brucey]

with many hub generators the open circuit (i.e. no or low current drawn) voltage increases pretty much linearly with speed. You can get about 50V quite easily and (in lab tests at speeds that are not entirely realistic for road use) over 100V has been seen.

Anyway this is one problem to overcome; the safe voltage rating of the parts is probably a lot more than most people suppose. In general terms making something that will work at all is easy enough if you are even moderately electronically literate, but making it both robust and efficient is less easy.

IIRC the hub flanges are not the same diameter BTW.

cheers

[Reflector]

with many hub generators the open circuit (i.e. no or low current drawn) voltage increases pretty much linearly with speed. You can get about 50V quite easily and (in lab tests at speeds that are not entirely realistic for road use) over 100V has been seen.

Anyway this is one problem to overcome; the safe voltage rating of the parts is probably a lot more than most people suppose. In general terms making something that will work at all is easy enough if you are even moderately electronically literate, but making it both robust and efficient is less easy.

IIRC the hub flanges are not the same diameter BTW.

cheers

I was very interested in your reply and depending upon your response to this missive I may be back for more.
1. Do I understand correctly that a hub dynamo, namely a SON, produces a regulated 6v DC voltage at the output terminals to the lamp.
2. I presume the voltage from bottle dynamo such as a B&M Dymotec 6 is 6volt, do you know if this is DC or AC.

[Brucey]

1. Do I understand correctly that a hub dynamo, namely a SON, produces a regulated 6v DC voltage at the output terminals to the lamp.

-absolutely not.

2. I presume the voltage from bottle dynamo such as a B&M Dymotec 6 is 6volt, do you know if this is DC or AC.

Again no, and most generators produce AC. A few (like a handful, ever) produce DC.

With all these generators the output varies with the speed and load. LED headlights typically contain a rectifier and a regulator of some kind. The regulation assumes that (unlike the open circuit voltage) the power of the generator doesn't increase linearly with speed, which is a good assumption; most generators both become less efficient at speed and have internal impedance. The combination of these two things means that the regulator might only have to dissipate a few watts.

Most hub generators exhibit a current limit (in part because of the internal resistance) and won't produce any more than about 0.75A even into a dead short. However many will produce ~10V @ ~0.4A at speed so you can often run two '6V' LED headlights in series once you are doing more than about 15mph.

cheers

[bikes4two]

> My understanding is that the voltage from hub dynamos is AC. As alluded to in my OP, I've not owned a working hub dynamo but I'm 99% sure that it's an AC voltage, not DC
> See this link and this link to see simple DIY circuits - the key point is that the circuits depict the hub output as AC
> And Sheldon Brown says here in the 5th paragraph that dynohubs (aka hub dynamos?) are AC voltage devices

[edocaster]

You can see my build experiences here: viewtopic.php?f=5&t=79811&start=15#p811210

Mk I had no overvoltage protection. Fortunately, the linear regulator had fairly benign behaviour, simply cutting out on overvoltage.

Mk II has protection as linked to from the above page. I have only done a handful of tests, including on the road, but it seems to work as wanted - when the protection kicks in the dynamo voltage is chopped and, from the point of view of anything post-smoothing capacitor, is instantly halved. As speed increases beyond this, voltage falls yet further as the thyristor chops earlier and earlier in the wave. So this means the regulator still supplies 5V even after the protection kicks in, until a relatively high speed is reached.

Note: Voltage only rises when the dynamo is unloaded. So, in normal (charging) usage the protection circuit actually lies dormant.

Also, for my circuit I used a linear regulator for ease - but they may also be more robust?

[bikes4two]

IIRC the hub flanges are not the same diameter BTW.

cheers

You are right - the NX30 flange is 80mm whilst the DN-3N80 flange is 70 mm (as stated here)

[MikeF]

Most hub generators exhibit a current limit (in part because of the internal resistance) and won't produce any more than about 0.75A even into a dead short. However many will produce ~10V @ ~0.4A at speed so you can often run two '6V' LED headlights in series once you are doing more than about 15mph.

cheers

You don't need to be going as fast as that to produce a decent light output for two LED lights in series - well under 10mph in my experience.

> My understanding is that the voltage from hub dynamos is AC. As alluded to in my OP, I've not owned a working hub dynamo but I'm 99% sure that it's an AC voltage, not DC
> See this link and this link to see simple DIY circuits - the key point is that the circuits depict the hub output as AC
> And Sheldon Brown says here in the 5th paragraph that dynohubs (aka hub dynamos?) are AC voltage devices

As far as I know all cycle "dynamos" are AC generators and 3N80 definitely is.

[Brucey]

You don't need to be going as fast as that to produce a decent light output for two LED lights in series - well under 10mph in my experience.

I think it varies with the lamps and the generator that you are using.

...As far as I know all cycle "dynamos" are AC generators and 3N80 definitely is....

there have been some DC cycle generators but they are very much the exception. I don't know of any presently available (I was going to say 'current' but this is just confusing in this context... ) hub generators that are this way.

cheers

[jacksonz]

The simplest way is to use four NI-MH batteries to passively regulate the voltage. Sheldon Brown mentions the idea here: http://sheldonbrown.com/dynohubs.html

Basically, the dynamo output is passed via a bridge rectifier to 4x NI-MH batteries in series. Add a USB female connector to batteries. The output is around 5 volts.

Yes, I actually use this on tour. I run a Garmin GPS from the pack and charge concurrently a smart phone.

[Reflector]

1. Do I understand correctly that a hub dynamo, namely a SON, produces a regulated 6v DC voltage at the output terminals to the lamp.

-absolutely not.

2. I presume the voltage from bottle dynamo such as a B&M Dymotec 6 is 6volt, do you know if this is DC or AC.

Again no, and most generators produce AC. A few (like a handful, ever) produce DC.

With all these generators the output varies with the speed and load. LED headlights typically contain a rectifier and a regulator of some kind. The regulation assumes that (unlike the open circuit voltage) the power of the generator doesn't increase linearly with speed, which is a good assumption; most generators both become less efficient at speed and have internal impedance. The combination of these two things means that the regulator might only have to dissipate a few watts.

Most hub generators exhibit a current limit (in part because of the internal resistance) and won't produce any more than about 0.75A even into a dead short. However many will produce ~10V @ ~0.4A at speed so you can often run two '6V' LED headlights in series once you are doing more than about 15mph.

cheers

Oh dear, now I am now out of my depth and need to leave this topic. I will have to give some thought as to what I want to know and submit a topic more suited to my ability.

[MikeF]

You don't need to be going as fast as that to produce a decent light output for two LED lights in series - well under 10mph in my experience.

I think it varies with the lamps and the generator that you are using.

...As far as I know all cycle "dynamos" are AC generators and 3N80 definitely is....

there have been some DC cycle generators but they are very much the exception. I don't know of any presently available (I was going to say 'current' but this is just confusing in this context... ) hub generators that are this way.

cheers

Yes it may well depend on the lights. I have only one series pair of lights so that's not much of a sample. My Philips Saferide also gives a very good light, but that has 2 LEDs, but I don't know the internal connections.

I was going to use the word current as well, but deleted it. Presently always means "in a little while" to me, but I expect the meaning depends on where you live and I avoid the word.

[[XAP]Bob]

If you are always charging then the open circuit need never arise - the lack of OV protection on my Biologic ReeCharge led me to fit a couple of switches - one to disconnect power from the ReeCharge (so I get all the power to lights) and one to the headlight (so I can turn that off in daytime).

The tail light wasis/ my "nominal load" to ensure that the voltage didn't go crazy at high speed.

[wjhall]

It is interesting that this works for you. In the 1970s, when dynamo sets were sold without regulators, the inevitable result of the headlamp bulb blowing was that the output voltage would rise, due to the much lower current drain of the rear lamp, and the rear lamp would blow shortly afterwards. Are you using a high power rear lamp, or one with overvoltage protection?

If you are always charging then the open circuit need never arise - the lack of OV protection on my Biologic ReeCharge led me to fit a couple of switches - one to disconnect power from the ReeCharge (so I get all the power to lights) and one to the headlight (so I can turn that off in daytime).

The tail light wasis/ my "nominal load" to ensure that the voltage didn't go crazy at high speed.

[Brucey]

....Yes it may well depend on the lights....

although having said that, if the lights themselves are reasonably efficient, and the generator isn't too weird, the power into the lights at low-ish speeds might be very similar regardless of whether you have one lamp or two. For example

@ 10 mph driving one lamp might be 5V, 0.4A (say) and

@ 10mph driving two lamps in series might be 9V, 0.2A

-which is a similar power overall. I don't know how likely such an outcome is but one thing is for sure; because of the lack of forward conduction below a threshold voltage, and the efficiency of LEDs (that convert low currents into light far better than bulbs ever did; in fact LEDs are often most efficient at half or less of their maximum rating...) a series LED lamp setup will be head and shoulders above a series tungsten lamp setup at lower speeds.

I guess the snag is that at very low speeds neither lamp will come on with a hard-wired series setup.

cheers