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Testing a Nexus HB NX23 Generator Hub

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Old 06-12-18, 01:33 PM
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Testing a Nexus HB NX23 Generator Hub

I recently bought a bakfiets with a Nexus generator hub. Interestingly, when you search for "Nexus" on this forum, a thread from someone else who bought a bakfiets with a Nexus generator hub comes up. He also seemed to have a burned out bulb and it war recommended that he gets a new light rather than just a new bulb. OK, I'm on board with that.

Hub is HB-NX23, and a Google search didn't turn up much. It's a single wire design with a spade connector. Shimano's website found doucmentation, but only in Japanese, and it wouldn't let me download it anyway. I turn the wheel, and there's no light. The bulb has a frosted, knobbled texture, so I can't see the filament, but it looks intact, I'm just making the assumption it's bad because that's the most obvious. However, before I invest in a new light, I'd like to be sure.

How do I test to make sure my hub works, first? What do I put the little tester leads onto?
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Old 06-12-18, 04:09 PM
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if you have a voltmeter, you should be able to measure between the spade lug and some point on the frame that doesn't have paint. Maybe a brake bolt?
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Old 06-12-18, 04:59 PM
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Got it. So the ground goes through the frame. Thanks.
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Old 06-12-18, 08:04 PM
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Dyno hubs are alternating current, if your meter is set for direct current it might not give you what you need to know.

A dynohub that is not supplying current to anything other than your meter can run high voltage. Going down a hill at 25 mph I measured over 30 volts AC on my hub.

Many Shimano hubs (possibly all?) ground to the frame.
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Old 06-13-18, 02:05 AM
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Originally Posted by bajajoaquin View Post
Got it. So the ground goes through the frame. Thanks.
This is often a problem as the connections through the frame are often poor. Flip the bike upside down, connect a voltmeter set to AC between the spade connector and axle, turn the wheel and see whether there is a reading. Without a meter, you could try to short-circuit the dynamo by connecting a wire between the spade connector and axle. If the dynamo works, you should sense a highly increased drag for a short-circuited dynamo. You can test the lamp and bulb separately with a voltage source of about 6V or a meter set to ohms. If the dynamo works, you should aim at tossing the incandescent lamp and getting an LED, preferably with a standfunction. They went down in prices so much that there is no excuse in persisting with a dim inefficient incandescent. Also you may consider a washer connected to a wire on the axle to take out the unreliable connection through the frame.
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Old 06-13-18, 08:52 AM
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Thank you. That's what I planned on doing, but the extra detail is helpful. I had planned on waiting for my multi-meter to arrive, but I think I'll grab a couple pieces of wire and do a jump test to see if I get light. Of course, the light bulb might still be bad!

Yes, a new LED light is coming. I want to get the generator sorted first, then buy a light. If this generator doesn't work, my caliper brakes aren't very good either, so I'll be buying a Sturmey-Archer XL-FDD 90mm drum brake Dynohub.
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Old 06-13-18, 10:36 AM
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You said bulb, a bike headlight bulb is 2.4w. what is the hub output?

typically a bike generator is 3w, with an 0.6w bulb rail light and a 2.4w headlight bulb,

one burns out the other is overpowered..

Now, LED lights get an IC, with a rectifier (AC to DC)
and an overvoltage regulator in the circuit..
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Old 06-13-18, 02:20 PM
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That is correct. The light is also a Nexus, but it's an older bulbed version. I reassembled it all, and I don't have the model number handy, but it was a 2.4w bulb. There's a taillight, but the wire was just run into the rubber grommet on the rear drum brake linkage.... in other words nowhere. My guess was that the bulb was burned out, but I decided to try to get a light on the bulb by running a ground wire from the light bracket to the axle. It didn't work.

So I'm back to waiting for Amazon to deliver my multi-meter, and I'll test the output of the hub by touching from the spade connector (for the light to hook up to) and to the axle or other parts of the hub. If I find that the hub puts out it's 3w at 6v that I expect, I will probably keep it and add a new LED light/taillight. If the hub doesn't work, I'll probably buy a Sturmey-Archer XL-FDD.
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Old 06-13-18, 05:57 PM
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If you get a 3 watt bulb at the hardware store, then skip the taillight. A good Ace hardware store or a True Value hardware store should have an assortment of flashlight bulbs.

Some people have wired in 7.5 or 8 volt Zener diodes to reduce the chance of blowing out an incandescent bulb on downhills. But since almost all bike lights now are LED with over-voltage protection it is getting hard to find the instructions to do that, but googling zener diode bicycle dynohub might give you some help. WIth the demise of Radio Shack it is getting harder to find things like Zener diodes.
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Old 06-13-18, 06:48 PM
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Thanks. I may try that.

Theres very little danger of my blowing out bulbs on any downhills on this bike!
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Old 06-14-18, 11:11 AM
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The 3 watt rating on dynamos is nominal. Power output varies a lot with speed.

Modern LED lights have very good over-voltage protection, so there is virtually no worry of damaging them with too much.
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Old 06-14-18, 11:20 AM
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I am glad I read this thread, I was planning on building a new front wheel with a Nexus dynamo hub but now that I know it is a frame ground I need to rethink, carbon front fork. Even though unrelated to the OP's issue I still learned something that will save me a bunch of time and money on something I wouldn't get to work. Thanks.
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Old 06-14-18, 01:44 PM
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I think I would check on that first, they probably have made different versions.
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Old 06-14-18, 02:01 PM
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Don't most of those hubs have two wires coming out of them?
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Old 06-14-18, 05:45 PM
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Originally Posted by WheresWaldo View Post
I am glad I read this thread, I was planning on building a new front wheel with a Nexus dynamo hub but now that I know it is a frame ground I need to rethink, carbon front fork. Even though unrelated to the OP's issue I still learned something that will save me a bunch of time and money on something I wouldn't get to work. Thanks.
It should not matter if you have a hub grounded to the frame or not. If your fork is not conductive, then run two wires. The hub connector is designed to use two wires. If you bought an SP hub instead, that also has two wires but is not grounded to the frame. An interesting side note - the SP and Shimano wire connectors on the hubs are interchangeable. I have one bike with a Shimano hub and two bikes with SP hubs, the connectors on all the bikes are essentially the same except for the labeling, in some cases I am using an SP connector on a Shimano hub and the reverse is also true.

Where you can get in trouble is if you have a metal fork and a hub grounded to the frame, and a light also grounded to the frame. In that case, there is a 50/50 chance that when you wire it, you will get the wires reversed and the light won't work. In that case, switch the wires.


Originally Posted by noglider View Post
Don't most of those hubs have two wires coming out of them?
Yes, I think all hubs have two connections for wires. But often one of those two wires is redundant if you have a metal fork and have a headlamp that is also grounded to the frame.

I tried an experiment, I wired in a headlamp that was grounded to the frame. Shimano hub, so it also was grounded to the frame. I only needed one wire from the hub to the light, see photo. I no longer run the light on the fender mount, the light worked fine but was too low to the ground and I got weird shadows from tiny little things sitting on the ground, it was a short term experiment and I decided I needed the light up higher. But that was the easiest wiring job I ever did, less than a food of wire.

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Old 06-14-18, 07:20 PM
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Yeah, great wiring, bad lighting. I tried that mount point, too. No good at all. However, the fork crown is a perfectly good height for me. I love my headlight there.
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Old 06-14-18, 07:29 PM
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This is an old model Nexus hub. I don’t know the age, but I’ve posted the model number above if anyone can use that to find out better than I did.

In other words, don’t take my hub description to be the current state of the Nexus art.
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