noglider 

Now that I see there are cheap dynamo type lights that may be decent, I like the option of bolting lights onto the bike. Oddly, they seem moderately theft-proof. Even here in a bike-theft prone city, people don't steal lights that are bolted on. Has anyone powered a light intended for a dynamo (A/C) with a battery (DC)? 5V might just do the trick.

Of course, I could try it, and I might. I will report back.

PaulRivers

B&M makes ebike version of all of it's lights - that might be the way to go for the easiest way to do it:
https://www.bumm.de/en/products/e-bike-beleuchtung.html

That being said, seems to me like you'd be getting the worst of both worlds. You'd still need to attach and take off the battery right?

I'm sure you already know some of these, but if you're looking specifically for a shaped beam light, I know a few places sell them as all-in-one lights:
- B&M Space
- Fenix Bc35r
- Fenix Bc25r

steelbikeguy

a quick note of caution... most/all dynamos are limited to 0.5A output current. The only exception that I know of is the Velogical, and it relies on an external device to nominally limit the current. As such, many dynamo lights utilize this feature to limit current to the LED and the voltage clamping circuit. Connecting this type of light into a power source that can provide considerably more than 0.5A could damage the light.

I'd be interested in hearing from anyone who's dug into the design of current dynamo lights and knows whether this has changed or not.

Steve in Peoria

unterhausen

I feel like BuM doesn't rely on the limited input current. I'm positive that there are other brands that do rely on it. I should know, but that project is on the list.

I feel like battery powering dyno lights should work in most cases, with the caveat that you are losing a diode drop in voltage. I think I have seen circuits that use a capacitor for voltage doubling and that wouldn't work with a battery. Been a while, they might not block all current.

angerdan

I'd recommend to use B&M (or other brand) DC lights for ebikes like PaulRivers wrote.
https://translate.google.com/transla...treiben.46412/

fietsbob

I believe with a DC light powered by a Battery , It Draws, as a resistance load, what it needs from the battery pack ..

Simple Halogen bulb* in Union Headlight reflector , and an on the mudguard 0.6w bulb tail light wired to a 5D cell battery pack of Ni Cads..

In Series, got me home on hour ride for many months...
*It did nor seem to matter what it was rated at , as I Recall , just a Halogen flashlight bulb from Radio shack..

When I had a Schmidt e6 dynamo powered halogen headlight, that bulb selection had to be 3w on its own or 2.4w W an 0.6w tail light...




....

nazcalines

I just tried this with a v1 Edelux. I was expecting the light to work regardless of polarity, but it only works one way (with the power to the bare wire and gnd to the insulated). Seems to be easily bright enough to use at 5V and gets barely brighter going up to 6V. I measured 100mA at 5V and 200mA at 6V. I'm going to try using it with a 4 pack of AA nimh cells.

unterhausen

thanks for trying it and posting, that's very interesting

Tourist in MSN

I recall reading somewhere that someone blew out the electronics in a Luxos light with direct current, not sure if it was a Luxos U or B. I suspect 5v is safe, but I would be hesitant to go much higher.

Since the goal is to bolt on a light to avoid theft, any reason that you don't use a battery powered light and firmly attach it with a hose clamp or some other way of attachment that requires a tool that would take several minutes to remove?

Last summer I saw on Ebay a really cheap light (attached with rubber band, not bolted on) that was powered by USB. Took a month to arrive (shipped from China), but it worked pretty good off a power bank. It has a flashlight type beam, just as high as it is wide, but what do you expect for $5 USD?
https://www.ebay.com/itm/3000LM-XML-...t/201700089127

That Ebay light, I bought it to run off of a Sinewave Revolution, it did not work so good on that however. But for your purposes, something like this one that is held on with the rubber band could use zip ties instead or something else that would be difficult for a thief to steal.

nazcalines

I just had a look at Schmidt's website and they say the Edelux II is *not* to be powered by DC. Whether it'll blow it, or they just want to sell the DC version of the light, I don't know. I personally wouldn't risk trying it.

For me, I want to use a shaped beam light vs the a regular battery powered one. Depending on how long the nimh cells last, I might try to hide them in the steerer tube, or purchase a dyno hub.

2_i 

In my power system dynamo lights are regularly powered by a mixture of AC and DC, with the first taking over completely when riding fast and the second - when at rest. I remember that in the B&M lamps there was some functionality that employed only one polarity of the AC. I.e. when powering the lamp with DC, you need to test both polarities and pick the one for which the lamp works consistently.

P.S. It should be 6V but a bit up or down is OK.

unterhausen

I wonder if they are worried about a battery's ability to supply more current than a dyno.

Leisesturm

Portland is an incredibly bike theft prone city. It may be #2 behind NYC despite having fewer actual bikes than many other cities that rank lower. Bikes are one thing, bike accessories are another. I no longer bother to take my lights off my bike. I've had two bikes stolen in 10 years. No lights stolen ever. Not scientific and very small sample size but there you are. That said, my lights are MagicShines, they attach with O-rings. And they look for all the world like the ridiculously cheap lights you can find on Amazon for $20. For fancier battery lights they will ALWAYS have some kind of bayonet mount that allows them to be easily removed. The battery packs as well. Those are the lights that will be taken (maybe) if left on the bike, but since they are so easy to remove, I always do that, or did when I had expensive lights. I am not that certain that dynamo hubs produce A/C. I would doubt it. They all produce about 3W at 6V and the light electronics are designed to work with that. The issue with using batteries to power dynamo lights, besides the voltage mismatch, is the hook-ups which I have never actually seen never having owned a dynamo light but which I imagine are incompatible with the connectors often proprietary, from the various makes of battery packs in use. No matter how inexpensive a light might be modifying its cables becomes more work than simply buying a cheap battery light and replacing it if stolen. That's what I would do. FWIW.

Leisesturm

Again, for all the posters saying dynamo lights will be damaged by DC power I don't think it is the fact that it is DC that is the issue. It is the magnitude of the DC from even a small battery pack. When an AC device is presented with DC usually nothing happens! The physical construction of an AC device looks like an open circuit to DC current and there will not be any current flow unless the voltage potential is so great as to arc across the open circuit. DC battery packs don't have that huge a voltage potential which informs my opinion that the dynamo lights are DC lights albeit designed to run from a low current source of DC like the dynamo hub produces. I am not an electronics engineer, I could be off, but not too far off. The takeaway is still that it probably isn't worth the voiding of warranties and the risk of damage to the light to experiment with using batteries as power. And that is without considering the huge irony inherent in doing this since so many, many threads extol the virtues of dynamo's as a power source for head and taillights.

angerdan

Dynamo hubs always and only deliver A/C.
Sometimes less and rarely more than 3W.

Leisesturm

Ok, I'll take that as fact since I don't know any better. What would the frequency of the oscillation be? I thought about this while most of you guys were sleeping. Household AC alternates at 60 cycles per second, and it is maintained with such precision that you can use the alternation to keep clocks on time. How does a generator, dependent on the wildly varying speeds of a bicycle wheel maintain a constant alternating current?! Like I said, I have no idea. What I do know, however, is that LED bulbs (Light Emitting Diodes) must have DC in order to work. And the polarity of the DC matters, as an earlier poster discovered, since all diodes block current flowing in the 'wrong' direction, it's just that most diodes don't also emit light. Just about all the devices we use, incandescent (halogen are incandescent) lights excepted, most of them need DC to work properly. If the power source is AC (it usually is in a home) it must be turned into DC (rectified) using diodes in a unique arrangement. That is the purpose of the warts that take up two spaces on your powerstrip, to turn the 110V AC into DC. Usually at a lower voltage as well but the main thing is to stop the AC from alternating. So if the generator hub is producing AC then there is a bridge rectifier somewhere, either in the hub itself, or in the housing of the light, but you cannot present an LED light bulb with AC, it simply won't work. And I simply have to assume that it is an LED light that is in use. In 2019 there simply is no good reason to still be using halogen lights.

unterhausen

frequency varies with the rate the magnets pass by the windings. There is no fundamental reason why they shouldn't take dc, but most of the better designs take the AC component into account.

I think I posted upthread that dynos will only put out .5amp, and the lights probably don't have any protection against a battery's ability to source far more than that. They just don't need it. DC supply should be a current source, not a voltage source.

steelbikeguy

My measurements on my Schmidt SON28 dynamo (in a 700C wheel) show that it has a frequency of 27Hz at 10mph and 43Hz at 16mph.
The dynamo's open-circuit voltage is essentially proportional to the speed, which does present a challenge when trying to deliver a constant power to something as delicate as an incandescent bulb.
This is accomplished by the design of the dynamo and the selection of the bulb's resistance.
The dynamo has a large internal inductance. For my SON28, this is about 0.15 Henries. At 16mph, the dynamo produces an AC waveform with a frequency of 43Hz. The 0.15H inductance has an impedance of 39 ohms at this frequency. The bulb has a resistance of approximately 12 ohms. so we can see that the dynamo's inductance is the major portion of the overall impedance and largely determines how much current will flow.
The neat thing about inductance is that the impedance is proportional to the frequency, just like the dynamo's open-circuit voltage.
Therefore, as the dynamo speeds up, both the voltage and the impedance go up the same amount, resulting in nearly the same current delivered to the 12 ohm bulb. Such a clever design!

well, that's a nice thought, but it ignores that the LED could simply be powered by half of the AC waveform when no bridge rectifier is used.
It also misses the possibility of wiring two LEDs in parallel, but opposing polarity. Each LED could conduct during opposite halves of the AC waveform.
I would suggest that your future posts be phrased in a way that reflects your tentative understanding of electronics. i.e. less "I know", and more "I think that it works this way...."

Steve in Peoria

noglider 

In fact, that is what @rhm did when he made home-made lights. His headlight and taillight were butt-to-butt, and they flashed alternately. You can't see the flashing at high speed because the flashing ends up being steady. You can't see it at low speed because you don't look at both lights at the same time. Or maybe you do, and maybe you can see the alternation.

I think the theory that the LED uses half of the phase is plausible, but I don't know it as fact. And if it's true, then I shouldn't have trouble running dynamo lights off a 5C DC battery.

znomit

Dynamo basics, and not so basics here:
Bicycle Dynamos

Some circuits here
Dynamo LED Light Systems for Bicycles (electronic circuits)
There has been a small drop in LED Vf since Martin did these charts so equivalent speeds will be a bit less.

Summary:
Will deliver 500mA into an LED, or string of LEDs. You need to go faster to make more LEDs light up.
Without some smoothing the LEDs will blink annoyingly.

CliffordK

Why not take one of the external battery CREE lights, and just bolt it to the bike?

I think my current one is this one:
https://www.ebay.com/itm/The-Lamp-He...W/273525424570

The light comes with a switch. It is held on by a single screw near the back of the light.

I had to rebuild mine recently (cord pulled loose). It is reasonably sealed, and access through unscrewing lens, and it comes apart, so one could technically get access to the inside to alter bolting method.

The light has a button on/off, and 3 beam levels. I run on dim most of the time, and it will last quite some time.

You'd still have to figure out what to do with the battery pack, as it might be obvious to a light thief if they see the light and batteries.

Also, green lighted switch?

Have you had thieves mess with handlebar bags?

steelbikeguy

well... it's inefficient to run the light off of half of the AC waveform, and it's pretty inexpensive to use a full wave bridge rectifier, so I'd be surprised if a commercially produced light wasn't using a full wave bridge rectifier.

Regardless... this wouldn't be an indication of whether it is advisable to run the light from a 5V battery. Like any diode, there is a threshold voltage (called "forward voltage") for any diode.... if you apply a voltage below the forward voltage, almost no current will flow. If you apply a voltage greater than the forward voltage, lots of current will flow. A lot of white LEDs that you see in bike lights are rated for 1 amp or so, while batteries can supply a lot more than this. I think I regularly see over 5A out of AA nimh cells when shorted.

As such, the concern isnt' so much whether the power source is AC or DC, as the bridge rectifier will accept either. The bigger issue is whether anything is limiting the current that will flow through the LED(s). Some dynamo lights rely on the dynamo's internal impedance to limit the current, and would likely be killed if connected to a battery that could source a couple of amperes.

Other dynamo lights use a switching power supply to provide a better match of the dynamo to a single LED.
For instance, if you just rectify the power from a dynamo and feed it through a single white LED, you'll get a half amp of current. The LED's forward voltage is about 3V, so you'll get 1.5W of power into the LED. Not bad, but that's half of what you'd get with a standard 12 ohm load for the dynamo.
If you use a switching power supply, you can convert the dynamo power from 6V@0.5A into 3V@1A, letting you put 3W into the LED. Much better! A side effect of this is that the switching power supply (i.e. buck converter) is intrinsically current limiting, and should not mind being powered from a battery.

I haven't peeked inside many modern dynamo lights. A couple of years ago, I did fix a fellow's Supernova Triple headlight, and it did not use a switching power supply, and in fact was killed when connected to an e-bike's battery. Too much current flowed through the light, destroying almost everything inside.
The other light that I've looked at is a dead B&M Lumotec Eyc. It does appear to use a switching power supply, based on the use of a 22uH inductor...

(the square item with "220" on it)


The square integrated circuit above it is probably a microcontroller.. i.e. the brains of the light. There is a chance that it is looking for AC at the input, so it might just shut off the main light and only power the standlight if there is no AC present. For that reason, the light wouldn't be damaged by DC power, but would not operate from DC power.

All of this just goes back to my original comment.. (IIRC)... without detailed knowledge of a particular light's design, it's very difficult to predict whether or not it can operate from DC power.

Steve in Peoria

Tourist in MSN

Years ago on a sidewall alternator with incandescent bulb I wired in a couple zener diodes to trim off the voltage peaks when I went down hills. I think I used 8 volt diodes.

fietsbob

You probably actually should buy the E-Bike version of the light in question... They're made to draw off the battery running the motor..

Example:
https://nabendynamo.de/en/products/h.../for-pedelecs/
several versions shown..






...

Leisesturm

I said clearly, and more than once, that I was ignorant of the workings of dynamo hubs and generators. Did you miss that? I did not say I was ignorant of basic circuit electronics. Does it sound like I am? I was about to respond to your "it could be's" and then in a later post you discredit all of them for exactly the same reasons I was going to. So... how 'tentative' is my grasp of the commercial applications of what you and I both know to be true? I suggest you leave me to write my posts the way I want to, unless you have some actual misinformation to correct. Thank you.