noglider 

I'm not that handy with electronics, but I can solder. I've used multimeters, but maybe I need to learn more. My taillight isn't working, though my headlight is. I put an ohmmeter over the leads coming out of the headlight intended for the taillight. The beep indicates continuity, but it shouldn't, should it? But wait, there's more. I put the ohmmeter on the leads from the dynamo hub, and that beeped, too. But the hub works. 1. How is this possible, and more importantly, 2. how do I find the problem with this short circuit in place?

Tourist in MSN

I assume this is not a SuperNova. I mention that because there is something about their taillights that is different than all others. If it is a Supernova, I can't help.

If your taillight is grounded to the frame or a rack, there could be a polarity problem, switch the wires at the headlamp. But if your taillight is on a plastic mount, then it can't be grounded to the frame.

I would not worry about your meter. I would expect continuity on the dynohub, as the wire has to be continuous for that to work. The headlight, it does not surprise me that your meter beeped.

The wires to your taillight, do you have a Li Ion battery that is rated for 3.7 volts that has contacts on it? See if you can make the taillight work with that. I used my 3.7v phone battery to check a taillight, and it worked great. But I am one of those lucky ones that has a phone where you can remove the battery. (Gee, what a concept, allow the user to replace batteries.)

Or try some other low voltage batteries, I suspect a 1.5V AA would not have enough voltage to light it but you could try. Two AA in series for 3v might be enough?

If you do not have a low voltage battery, can you use a USB charger for a 5v power source to try it with? I would not try anything higher than 5v, but 6v might still be safe?

If powering the light with the wires at the light work, but if the light does not work when you power the light with the wires up near the headlamp, then you have a broken wire or broken connection.

And you might have a corroded wire connection that does not allow enough power through. Can you visually check for that, that actually might be the first thing to try.

Your taillight might be toast. A lot of dyno powered taillights have poor water resistance at the bottom, is your light where tire spray could have gotten into it?

If I think of anything else, I will add that.

ADDENDUM:

Remove your taillight, plug the taillight into the wires at the headlight and see if that makes it work. It might be hard to check that way, you need to spin the wheel with one hand while the other hand holds the light, but ... you get the idea.

unterhausen

I agree with the above comment, but it was long enough I think it buries the lede. My first question is, has the taillight ever worked? Even if it has, it could be wired backwards, and finally got grounded to the frame through a process of fretting somewhere. I would physically disconnect it from the bike, hang it in air, and see if it starts working. Of course, the short to ground might not be at the taillight mount. But since many taillights are meant to return current through the frame, the mounting point is the most likely place for a short.

It seems like your multimeter puts a little too much voltage through the circuit it's being used to test when it tests for continuity. Maybe it will also test diodes on that setting? Probably need to use the resistance measuring function, if any. I can check the resistance across a known-good hub if you tell me what hub it is.

steelbikeguy

Hub dynamos are just some magnets spinning around a coil of wire (albeit with some steel innards that direct the magnetic flux, but that's not relevant at this time). The wires in my Schmidt hub dynamo have a resistance of about 2 ohms. On most meters, that's going to look like a short circuit. I'd only be worried if you measured a high resistance across the dynamo terminals.

Steve in Peoria

noglider 

Thanks, folks. I'll try what you suggest. And thanks for letting me know that dynamos should have low resistance across the terminals. I don't know why I looked there anyway. Just curiosity, I guess.

foxyloxy

If the headlight's working, there isn't a short. That would pull the voltage down and the headlight wouldn't work either. Go to your tail light and use the 20volt a.c. scale on your meter.
If your tail light terminals have a voltage between them, the light is faulty, replace the bulb or the whole light if there's no removable bulb. If there's no voltage you need to check continuity on the live wire going to your tail light (the one that's insulated from the frame) . If that's ok then there's a bad connection to the frame. Some bikes use 2 wires to eliminate the possibility of a bad frame connection, in that case you need to check continuity of both wires. Tail lights often have poor quality touching metal connections, so I always bypass these and solder the wires directly to the inside of the light - if I can get into it. Hope this helps.

Troul 

have you tried reversing the leads at the tail light? LEDs are directional with electricity. If you are feeding juice into the negative side of the LED tail light, it will not work.

SpedFast

Just read it 's LED since that is all Spanninga makes. Good luck and please keep us posted.

Tourist in MSN

Dyno powered taillights are designed to work with AC power, has nothing to do with polarity of the LED. Exception, I am not sure if that is the case with Supernova brand taillights.

The reason I suggested reversing the wiring in a previous post had nothing to do with LED polarity, but some dynohubs have one lead grounded to the frame and if the taillight is one of those that ground one lead to the frame, that could cause a short if the dynohub was also grounded to the fork.

Troul 

I didn't catch if they are using a taillight from an aftermarket source thats not devoted to that particular products system.

unterhausen

Op didn't tell us anything about his system, so we're just speculating. One thing I know for sure is checking the resistance across the taillight leads shouldn't show a short. Unless this is an incandescent system, which would be unlikely, but good to know.

noglider 

It's a system that previously worked, and the taillight has stopped. Hub: Velo Orange, Headlight: B&M, Taillight, Spanninga. I'll test the taillight, but I suspect the wiring broke somewhere. I haven't had time to try anything since posting. I'm also using a very bright battery taillight, so I'm safe. I just want the taillight to work.

Tourist in MSN

I am using Spanninga taillights and B&M headlights on two bikes, they are compatible which you already know because it previously worked.

If you decide you need to replace the entire length of wire, I have re-used wire from chargers from various devices that I previously discarded for wiring dyno lighting. I mean the chargers where there is a transformer with built in plug that plugged into the outlet and used low voltage wiring to the device. If you have any such plugs and wires you have not discarded yet, that could be the wire source.

noglider 

Thank you, @Tourist in MSN. I stocked up on low-current wiring, and I believe I have enough.

I lost the mounting hardware for the taillight long ago, so I jury-rigged the current mount, and it's ugly. Maybe it's time to buy a new taillight and mount it properly to the fender.

In the meantime, my Cygolite Hypershot 350 makes me feel more secure than any taillight I've ever used. The low output from dynamo taillights doesn't give me warm fuzzy feelings anyway, and it doesn't blink, either. This is a less-than-urgent problem, and I don't have much time to tinker because I'm working and attending grad school.

Velo Mule

If you have to replace the wire, I like using speaker wire because it is mostly clear. I haven't actually rewired a dynamo bike since Radio Shack when out of business though. I also like running two wires rather than relying on the bike and brackets to provide the ground path.

znomit

I used to use this a lot for the bling factor when I was building my own dynamo lights. But the shimano wire is a good choice..

ericoseveins

I think LEDs being diodes are sort of directional by definition and only take direct current. But various brands of lighting will put a rectifier in different places.

steelbikeguy

The Supernova does use a dedicated tail light that uses DC power, but every other one that is general purpose will use AC. These include rectifiers... usually configured as a bridge rectifier.

An example is a Herrmans light that I fixed for a friend. Here's a shot of the board once removed from the housing....



A closer look shows diodes D1 through D4 that form a bridge rectifier to convert the AC voltage to a lumpy DC voltage....


This particular light was subjected to a higher voltage than intended, and some of the resistors didn't get along with the extra power that they were being asked to handle.

Steve in Peoria

unterhausen

I'm surprised it doesn't have more capacitance, but I suppose you don't really need it. Or is there some on the other side?

steelbikeguy

There is a 1F (Farad) supercapacitor on the back of the board, along with the LEDs....



You could argue that this still isn't a lot, but this is fairly typical, I believe.

Steve in Peoria

2_i 

In my memory, these were the capacitances for the supercaps in the B&M tail lights I was fixing, with a 5.5V rating.