Dynohub + LED

lance house wrote in message ...

Quoted message said:

hi

has anyone tried building an LED lamp to be powered by the old
Sturmey-Archer Dynohub? I found one in fantastic condition and ,ahem,
fried one of my LED front lights on an, ah, test ride. I'm figuring now
that I definitely need a bridge rectifier and maybe some batteries as a
kind of voltage sink. Can I just rig it up with about the right number
of LEDs in series to accommodate the voltages which I expect to get out
of it?

cheers
lance

LEDs burn out unless current limited, usually to a max of about 30mA
No need for a rectifier. LEDs are diodes, have one connected one way round
and another the other way, each LED requires its own limiting resistor.
Take the forward voltage of the LED and subtract it from your average DC
voltage,(assume 6V) use Ohms law to obtain limiting resistor value.

Trevor

Trevor said:

lance house wrote in message ...

Quoted message said:

hi

has anyone tried building an LED lamp to be powered by the old
Sturmey-Archer Dynohub? I found one in fantastic condition and ,ahem,
fried one of my LED front lights on an, ah, test ride. I'm figuring now
that I definitely need a bridge rectifier and maybe some batteries as a
kind of voltage sink. Can I just rig it up with about the right number
of LEDs in series to accommodate the voltages which I expect to get out
of it?

cheers
lance

LEDs burn out unless current limited, usually to a max of about 30mA
No need for a rectifier. LEDs are diodes, have one connected one way round
and another the other way, each LED requires its own limiting resistor.
Take the forward voltage of the LED and subtract it from your average DC
voltage,(assume 6V) use Ohms law to obtain limiting resistor value.

Trevor

How do you know about the maiximum current unless you know the model of the LED?
What are the specifications of the Dyno and LED(s) involved?

lance house said:

hi

has anyone tried building an LED lamp to be powered by the old
Sturmey-Archer Dynohub? I found one in fantastic condition and ,ahem,
fried one of my LED front lights on an, ah, test ride. I'm figuring now
that I definitely need a bridge rectifier and maybe some batteries as a
kind of voltage sink. Can I just rig it up with about the right number
of LEDs in series to accommodate the voltages which I expect to get out
of it?

cheers
lance

I have my Shimano Dynamo Hub hooked up to a luxeon III led but the
system is fairly complex, going through a FET rectifier (low voltage
drop), a very low voltage drop regulator which can be switched out if
necessary, a 700mA constant current regulator, as well as being mounted
in a Energiser 6v light with rechargeable batteries which charge when
the light is off. It works O.K but is overly complex when a simpler and
cheaper system might have been just as good. If anyone else is
interested in going this route I will be happy to share any
problems/solutions I encountered

daveornee wrote in message ...

Quoted message said:

Trevor said:

lance house wrote in message ...

Quoted message said:

hi

has anyone tried building an LED lamp to be powered by the old
Sturmey-Archer Dynohub? I found one in fantastic condition and

,ahem,

Quoted message said:

fried one of my LED front lights on an, ah, test ride. I'm figuring

now

Quoted message said:

that I definitely need a bridge rectifier and maybe some batteries as

a

Quoted message said:

kind of voltage sink. Can I just rig it up with about the right

number

Quoted message said:

of LEDs in series to accommodate the voltages which I expect to get

out

Quoted message said:

of it?

cheers
lance

LEDs burn out unless current limited, usually to a max of about 30mA
No need for a rectifier. LEDs are diodes, have one connected one way
round
and another the other way, each LED requires its own limiting
resistor.
Take the forward voltage of the LED and subtract it from your average
DC
voltage,(assume 6V) use Ohms law to obtain limiting resistor value.

Trevor

How do you know about the maiximum current unless you know the model of
the LED?
What are the specifications of the Dyno and LED(s) involved?

Not always available. If it don't burn your finger, its probably OK quick
method saves bothering with manufacturers data. Useful output can come from
as low as 5mA. The forward voltage is checked by measurement and is around
about 2V in most cases.

Remember when riding, the LEDs are probably going to be cooled anyway so it
matters less that the current is spot on when the voltages increase to high
speed. It may be viable to use two in series so there is a volt drop of 2V
across the limiting resistor. Whether there would be sufficient voltage at
low speeds is one for trial. R=V*V/I 4/0.030=133ohms 130 ohm low
power resistor req.

Trevor

Trevor said:

daveornee wrote in message ...

Quoted message said:

Trevor said:

lance house wrote in message ...

>hi
>
>has anyone tried building an LED lamp to be powered by the old
>Sturmey-Archer Dynohub? I found one in fantastic condition and

,ahem,

>fried one of my LED front lights on an, ah, test ride. I'm figuring

now

>that I definitely need a bridge rectifier and maybe some batteries as

a

>kind of voltage sink. Can I just rig it up with about the right

number

>of LEDs in series to accommodate the voltages which I expect to get

out

>of it?
>
>cheers
>lance

LEDs burn out unless current limited, usually to a max of about 30mA
No need for a rectifier. LEDs are diodes, have one connected one way
round
and another the other way, each LED requires its own limiting
resistor.
Take the forward voltage of the LED and subtract it from your average
DC
voltage,(assume 6V) use Ohms law to obtain limiting resistor value.

Trevor

How do you know about the maiximum current unless you know the model of
the LED?
What are the specifications of the Dyno and LED(s) involved?

Not always available. If it don't burn your finger, its probably OK quick
method saves bothering with manufacturers data. Useful output can come from
as low as 5mA. The forward voltage is checked by measurement and is around
about 2V in most cases.

Remember when riding, the LEDs are probably going to be cooled anyway so it
matters less that the current is spot on when the voltages increase to high
speed. It may be viable to use two in series so there is a volt drop of 2V
across the limiting resistor. Whether there would be sufficient voltage at
low speeds is one for trial. R=V*V/I 4/0.030=133ohms 130 ohm low
power resistor req.

Trevor

Hey, thanks for the info.

I took the bike out for a ride today with a multimeter attached to it.
At an average cruising speed it outputs 8-12V, but down a light incline
it got up to 14V. Down a big hill I could easily see it topping 20 V,
(Ill check it out next time im near a big hill) so I think I need to
allow for a possibility of putting that much through the LEDs at least
occasionally (although extra cooled by speed, they might also be
operating in hot tropical climates.)

So in otherwords the LEDs will be recieving 0-20V

I was thinking if linking it to a battery as well so that some power
went to the lights and the excess went to charge a batttery which then
powered the lights when I wasnt moving. I think this is why I thought I
need a rectifier, as mentioned on sheldon browns site.

My multimeter only measures up to 200mA and it maxes out on that at
pretty low speeds, so I guess that explains why it blew the last LEDs so
quickly.

I also wouldnt mind hooking up some red LEDs for the back, I guess as
long as I make the string of reds serial and equal to a single white, I
should be able to just run a wire back there.

Incidentally, I tried hooking up a halogen to it and got a depressing
orange glow even at reasonable speeds. It says its a 2.5V 0.5A bulb, so
perhaps the dynohub outputs somewhere between 200-500mA, but if so it
seems that the Amp output is not linear with speed.

cheers
Lance

[<SNIP>
Hey, thanks for the info.

I took the bike out for a ride today with a multimeter attached to it.
At an average cruising speed it outputs 8-12V, but down a light incline
it got up to 14V. Down a big hill I could easily see it topping 20 V,
(Ill check it out next time im near a big hill) so I think I need to
allow for a possibility of putting that much through the LEDs at least
occasionally (although extra cooled by speed, they might also be
operating in hot tropical climates.)

So in otherwords the LEDs will be recieving 0-20V

I was thinking if linking it to a battery as well so that some power
went to the lights and the excess went to charge a batttery which then
powered the lights when I wasnt moving. I think this is why I thought I
need a rectifier, as mentioned on sheldon browns site.

My multimeter only measures up to 200mA and it maxes out on that at
pretty low speeds, so I guess that explains why it blew the last LEDs so
quickly.

I also wouldnt mind hooking up some red LEDs for the back, I guess as
long as I make the string of reds serial and equal to a single white, I
should be able to just run a wire back there.

Incidentally, I tried hooking up a halogen to it and got a depressing
orange glow even at reasonable speeds. It says its a 2.5V 0.5A bulb, so
perhaps the dynohub outputs somewhere between 200-500mA, but if so it
seems that the Amp output is not linear with speed.

cheers
Lance[/QUOTE]

I think you will need some voltage regulation and likely want something to keep the voltage up as you slow down. I think your idea of a battery is good.
For some LED arrays and specifications, take a look at the following URL:

<http://autolumination.com/specifications.html>

You can get other configurations from other sources, but this gives you some voltage and current specifications to deal with. Voltage regulating and dropping resistors provide protection, but they disapate the power via heat.
If you can get the desired regulation with minimal heating by using a battery and some combination of LED arrays, you will get more light for the drag of the dyno.
I think your unloaded dyno may produce more voltage than when it is loaded with an appropriate load. The voltage and current readings you took indicate a 10 watt peak output. NightHawk and Performance Bicycle are selling a LED unit that uses a very high power LED that takes 6 volts from a series of 4 AA batteries. I don't know if they use any internal circuitry to boost the voltage, but I know that you can get very good light from a single LED with 6 volts.

lance house said:

I took the bike out for a ride today with a multimeter attached to it.
At an average cruising speed it outputs 8-12V, but down a light incline
it got up to 14V. Down a big hill I could easily see it topping 20 V,
(Ill check it out next time im near a big hill) so I think I need to
allow for a possibility of putting that much through the LEDs at least
occasionally (although extra cooled by speed, they might also be
operating in hot tropical climates.)

So in otherwords the LEDs will be recieving 0-20V

I would be quite surprised if the voltage goes that high when there is
a load on the output.

Quoted message said:

I was thinking if linking it to a battery as well so that some power
went to the lights and the excess went to charge a batttery which then
powered the lights when I wasnt moving. I think this is why I thought I
need a rectifier, as mentioned on sheldon browns site.

That sounds complicated. You could get some of the benefits of
batteries by using supercapacitors. A Maxcap LV110505A is rated for
11V and 5 farads. If I understand correctly, this unit stores over
600 watt-seconds of energy at its rated voltage, and should store
about 180 watt-seconds if charged at 6V. That may cause too long an
interval between setting in motion and the lights coming on, but there
are other similar units with smaller capacitance that would charge and
discharge more quickly.

Quoted message said:

My multimeter only measures up to 200mA and it maxes out on that at
pretty low speeds, so I guess that explains why it blew the last LEDs so
quickly.

I don't know about red LEDs' tolerance for being overdriven, but white
ones are routinely driven at 50mA in commercial flashlight
applications even though their specifications stipulate a nominal
current rating of 20mA and a maximum of 30mA.

Quoted message said:

I also wouldnt mind hooking up some red LEDs for the back, I guess as
long as I make the string of reds serial and equal to a single white, I
should be able to just run a wire back there.

In my experience, the forward voltage for 5mm white LEDs is almost
exactly twice that for 5mm red LEDs. This rule of thumb does not
apply to Luxeon high flux LEDs.

Quoted message said:

Incidentally, I tried hooking up a halogen to it and got a depressing
orange glow even at reasonable speeds. It says its a 2.5V 0.5A bulb, so
perhaps the dynohub outputs somewhere between 200-500mA, but if so it
seems that the Amp output is not linear with speed.

There's a clue that the voltage under load will not be as high as you
measured. Unfortunately, it's also a sign that the dynohub may be
unable to put out its rated 3 watts.

Chalo Colina

Chalo said:

lance house said:

I took the bike out for a ride today with a multimeter attached to it.
At an average cruising speed it outputs 8-12V, but down a light incline
it got up to 14V. Down a big hill I could easily see it topping 20 V,
(Ill check it out next time im near a big hill) so I think I need to
allow for a possibility of putting that much through the LEDs at least
occasionally (although extra cooled by speed, they might also be
operating in hot tropical climates.)

So in otherwords the LEDs will be recieving 0-20V

I would be quite surprised if the voltage goes that high when there is
a load on the output.

It won't, I'm sure. Bike generators are pretty close to constant
current devices. IOW, typical ones put out 0.5 amp, no matter the
resistance - or at least, they try to. The more resistance they see,
the more voltage they develop. I've measured as much as 35 volts off
certain ones in an open circuit condition (IIRC).

But you won't get one to put out more than 0.5 amp (at least, not by
much). And IIRC, the Sturmey-Archer Dynohub put out even less, maybe
0.3 amp instead. I've got the figures stashed somewhere - I'll double
check later.

Quoted message said:

Quoted message said:

I was thinking if linking it to a battery as well so that some power
went to the lights and the excess went to charge a batttery which then
powered the lights when I wasnt moving. I think this is why I thought I
need a rectifier, as mentioned on sheldon browns site.

That sounds complicated. You could get some of the benefits of
batteries by using supercapacitors. A Maxcap LV110505A is rated for
11V and 5 farads. If I understand correctly, this unit stores over
600 watt-seconds of energy at its rated voltage, and should store
about 180 watt-seconds if charged at 6V. That may cause too long an
interval between setting in motion and the lights coming on, but there
are other similar units with smaller capacitance that would charge and
discharge more quickly.

Quoted message said:

My multimeter only measures up to 200mA and it maxes out on that at
pretty low speeds, so I guess that explains why it blew the last LEDs so
quickly.

I don't know about red LEDs' tolerance for being overdriven, but white
ones are routinely driven at 50mA in commercial flashlight
applications even though their specifications stipulate a nominal
current rating of 20mA and a maximum of 30mA.

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to just
put the proper number of LEDs in parallel, to share the current properly?

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
..030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

In any case, has anyone tried this idea - just using the inherent
current regulation of a generator to drive LEDs?

Quoted message said:

Quoted message said:

Incidentally, I tried hooking up a halogen to it and got a depressing
orange glow even at reasonable speeds. It says its a 2.5V 0.5A bulb, so
perhaps the dynohub outputs somewhere between 200-500mA, but if so it
seems that the Amp output is not linear with speed.

For a dynohub, try http://www.reflectalite.com/lightmodel.html#anchor1
to get a workable bulb. They recommend a GH 105 or 107.

BTW, I haven't tried this, so take it as a citation, not a recommendation!

Quoted message said:

There's a clue that the voltage under load will not be as high as you
measured. Unfortunately, it's also a sign that the dynohub may be
unable to put out its rated 3 watts.

Again, it's not rated for 3 watts. Modern generators are, but old
Dynohubs weren't.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Frank Krygowski said:

Chalo said:

lance house said:

I took the bike out for a ride today with a multimeter attached to
it. At an average cruising speed it outputs 8-12V, but down a light
incline it got up to 14V. Down a big hill I could easily see it
topping 20 V, (Ill check it out next time im near a big hill) so I
think I need to allow for a possibility of putting that much
through the LEDs at least occasionally (although extra cooled by
speed, they might also be operating in hot tropical climates.)

So in otherwords the LEDs will be recieving 0-20V

I would be quite surprised if the voltage goes that high when there
is
a load on the output.

It won't, I'm sure. Bike generators are pretty close to constant
current devices. IOW, typical ones put out 0.5 amp, no matter the
resistance - or at least, they try to. The more resistance they see,
the more voltage they develop. I've measured as much as 35 volts off
certain ones in an open circuit condition (IIRC).

But you won't get one to put out more than 0.5 amp (at least, not by
much). And IIRC, the Sturmey-Archer Dynohub put out even less, maybe
0.3 amp instead. I've got the figures stashed somewhere - I'll double
check later.

Quoted message said:

Quoted message said:

I was thinking if linking it to a battery as well so that some
power went to the lights and the excess went to charge a batttery
which then powered the lights when I wasnt moving. I think this is
why I thought I need a rectifier, as mentioned on sheldon browns
site.

That sounds complicated. You could get some of the benefits of
batteries by using supercapacitors. A Maxcap LV110505A is rated for
11V and 5 farads. If I understand correctly, this unit stores over
600 watt-seconds of energy at its rated voltage, and should store
about 180 watt-seconds if charged at 6V. That may cause too long an
interval between setting in motion and the lights coming on, but there
are other similar units with smaller capacitance that would charge and
discharge more quickly.

Quoted message said:

My multimeter only measures up to 200mA and it maxes out on that at
pretty low speeds, so I guess that explains why it blew the last
LEDs so quickly.

I don't know about red LEDs' tolerance for being overdriven, but
white
ones are routinely driven at 50mA in commercial flashlight
applications even though their specifications stipulate a nominal
current rating of 20mA and a maximum of 30mA.

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to
just put the proper number of LEDs in parallel, to share the current
properly?

Putting diodes in parallel is generally considered a bad idea because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Quoted message said:

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
.030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

There is no current in a bridge without a load. Might as well connect
an anti-parallel pair. Nothing wrong with that.

Jim Smith said:

Frank Krygowski <[email hidden]> writes:

Quoted message said:

Quoted message said:

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to
just put the proper number of LEDs in parallel, to share the current
properly?

Putting diodes in parallel is generally considered a bad idea because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Hmmm. (Thinking slowly, since I'm an ME, not an EE) OK, that makes
some sense to me. But are the differences likely to be significant?
Another post referred to LED flashlights significantly overdriving the
LEDs. If you were closer to spec by my method, would the (apparent)
safety factor in the ratings be good enough?

A problem, I guess, is that failure might be quickly progressive. One
LED might fail and take out the whole array.

But I think of resistors as energy losers. Also, each resistor is
another circuit element to try to fit into the package. Would it be
better to use the standard trick of opposed Zeners to limit voltage?
Could the internal resistance of the generator then be all you'd need in
the circuit?

(I never wanted to know more about EE until I started playing with
generators!)

Quoted message said:

Quoted message said:

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
.030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

There is no current in a bridge without a load. Might as well connect
an anti-parallel pair. Nothing wrong with that.

My idea of an LED bridge was to possibly reduce the LED count compared
to 10 facing East, plus 10 facing West. The load would be the rest of
the LEDs. I think.

--
--------------------+
Frank Krygowski [To reply, remove rodent and vegetable dot com,
replace with cc.ysu dot edu]

Alun Thomas said:

lance house said:

hi

has anyone tried building an LED lamp to be powered by the old
Sturmey-Archer Dynohub? I found one in fantastic condition and ,ahem,
fried one of my LED front lights on an, ah, test ride. I'm figuring
now that I definitely need a bridge rectifier and maybe some batteries
as a kind of voltage sink. Can I just rig it up with about the right
number of LEDs in series to accommodate the voltages which I expect to
get out of it?

cheers
lance

I have my Shimano Dynamo Hub hooked up to a luxeon III led but the
system is fairly complex, going through a FET rectifier (low voltage
drop), a very low voltage drop regulator which can be switched out if
necessary, a 700mA constant current regulator, as well as being mounted
in a Energiser 6v light with rechargeable batteries which charge when
the light is off. It works O.K but is overly complex when a simpler and
cheaper system might have been just as good. If anyone else is
interested in going this route I will be happy to share any
problems/solutions I encountered

do you have any suggestions as to howa simpler solution might work?

Frank Krygowski said:

It won't, I'm sure. Bike generators are pretty close to constant
current devices. IOW, typical ones put out 0.5 amp, no matter the
resistance - or at least, they try to. The more resistance they see,
the more voltage they develop. I've measured as much as 35 volts off
certain ones in an open circuit condition (IIRC).

But you won't get one to put out more than 0.5 amp (at least, not by
much). And IIRC, the Sturmey-Archer Dynohub put out even less, maybe
0.3 amp instead. I've got the figures stashed somewhere - I'll double
check later.

OK, I checked the results of tests on the NOS S.A. Dynohub I have.
Using its original headlight bulb, at an rpm roughly equivalent to 15
mph road speed (this was actually a bench test), I got 7.2 V at about
0.27 amps. That's about 2 watts.

It seems the Dynohub isn't as well regulated as modern generators. I
think most modern generators wouldn't exceed about 6.5 volts with their
stock bulbs. But if, as I suspect, it performs generally the same as
they do, then the 0.25 to 0.3 amp will be a constant, decreasing only
slightly with increased load resistance. More resistance will cause
more output voltage, and open circuit will give max. output voltage.

--
--------------------+
Frank Krygowski [To reply, remove rodent and vegetable dot com,
replace with cc.ysu dot edu]

Jim Smith said:

Frank Krygowski said:

Chalo said:

lance house <[email hidden]> wrote:

>I took the bike out for a ride today with a multimeter attached to
>it. At an average cruising speed it outputs 8-12V, but down a light
>incline it got up to 14V. Down a big hill I could easily see it
>topping 20 V, (Ill check it out next time im near a big hill) so I
>think I need to allow for a possibility of putting that much
>through the LEDs at least occasionally (although extra cooled by
>speed, they might also be operating in hot tropical climates.)
>
>So in otherwords the LEDs will be recieving 0-20V

I would be quite surprised if the voltage goes that high when there
is
a load on the output.

It won't, I'm sure. Bike generators are pretty close to constant
current devices. IOW, typical ones put out 0.5 amp, no matter the
resistance - or at least, they try to. The more resistance they see,
the more voltage they develop. I've measured as much as 35 volts off
certain ones in an open circuit condition (IIRC).

But you won't get one to put out more than 0.5 amp (at least, not by
much). And IIRC, the Sturmey-Archer Dynohub put out even less, maybe
0.3 amp instead. I've got the figures stashed somewhere - I'll double
check later.

Quoted message said:

>I was thinking if linking it to a battery as well so that some
>power went to the lights and the excess went to charge a batttery
>which then powered the lights when I wasnt moving. I think this is
>why I thought I need a rectifier, as mentioned on sheldon browns
>site.

That sounds complicated. You could get some of the benefits of
batteries by using supercapacitors. A Maxcap LV110505A is rated for
11V and 5 farads. If I understand correctly, this unit stores over
600 watt-seconds of energy at its rated voltage, and should store
about 180 watt-seconds if charged at 6V. That may cause too long an
interval between setting in motion and the lights coming on, but there
are other similar units with smaller capacitance that would charge and
discharge more quickly.

>My multimeter only measures up to 200mA and it maxes out on that at
>pretty low speeds, so I guess that explains why it blew the last
>LEDs so quickly.

I don't know about red LEDs' tolerance for being overdriven, but
white
ones are routinely driven at 50mA in commercial flashlight
applications even though their specifications stipulate a nominal
current rating of 20mA and a maximum of 30mA.

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to
just put the proper number of LEDs in parallel, to share the current
properly?

Putting diodes in parallel is generally considered a bad idea because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Quoted message said:

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
.030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

There is no current in a bridge without a load. Might as well connect
an anti-parallel pair. Nothing wrong with that.

ok,

so obviously this is going to be a bigger project than I had
anticipated. Is it easier - if at all possible - just to make the
dynohub charge the batteries which then power the LED. that way I could
skip the concerns about blowing 10buck LEDS and basically just have a
bicycle powered battery recharger? (Very handy for touring methinks,
since I am constantly dissapointed with my solar battery charger...)

I have several light bodies which I will probably construct this around.
If I was able to construct a battery recharger sufficiently well I
might even run a 4.8V0.5A halogen instead of the LED (just becasue I
already have one...)

cheers
Lance

"lance house" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Jim Smith said:

Frank Krygowski said:

Chalo wrote:

>lance house <[email hidden]> wrote:
>
>
>>I took the bike out for a ride today with a multimeter attached to
>>it. At an average cruising speed it outputs 8-12V, but down a light
>>incline it got up to 14V. Down a big hill I could easily see it
>>topping 20 V, (Ill check it out next time im near a big hill) so I
>>think I need to allow for a possibility of putting that much
>>through the LEDs at least occasionally (although extra cooled by
>>speed, they might also be operating in hot tropical climates.)
>>
>>So in otherwords the LEDs will be recieving 0-20V
>
>I would be quite surprised if the voltage goes that high when there
>is
>a load on the output.

It won't, I'm sure. Bike generators are pretty close to constant
current devices. IOW, typical ones put out 0.5 amp, no matter the
resistance - or at least, they try to. The more resistance they see,
the more voltage they develop. I've measured as much as 35 volts off
certain ones in an open circuit condition (IIRC).

But you won't get one to put out more than 0.5 amp (at least, not by
much). And IIRC, the Sturmey-Archer Dynohub put out even less, maybe
0.3 amp instead. I've got the figures stashed somewhere - I'll double
check later.

>>I was thinking if linking it to a battery as well so that some
>>power went to the lights and the excess went to charge a batttery
>>which then powered the lights when I wasnt moving. I think this is
>>why I thought I need a rectifier, as mentioned on sheldon browns
>>site.
>
>That sounds complicated. You could get some of the benefits of
>batteries by using supercapacitors. A Maxcap LV110505A is rated for
>11V and 5 farads. If I understand correctly, this unit stores over
>600 watt-seconds of energy at its rated voltage, and should store
>about 180 watt-seconds if charged at 6V. That may cause too long an
>interval between setting in motion and the lights coming on, but there
>are other similar units with smaller capacitance that would charge and
>discharge more quickly.
>
>
>>My multimeter only measures up to 200mA and it maxes out on that at
>>pretty low speeds, so I guess that explains why it blew the last
>>LEDs so quickly.
>
>I don't know about red LEDs' tolerance for being overdriven, but
>white
>ones are routinely driven at 50mA in commercial flashlight
>applications even though their specifications stipulate a nominal
>current rating of 20mA and a maximum of 30mA.

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to
just put the proper number of LEDs in parallel, to share the current
properly?

Putting diodes in parallel is generally considered a bad idea because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Quoted message said:

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
.030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

There is no current in a bridge without a load. Might as well connect
an anti-parallel pair. Nothing wrong with that.

ok,

so obviously this is going to be a bigger project than I had
anticipated. Is it easier - if at all possible - just to make the
dynohub charge the batteries which then power the LED. that way I could
skip the concerns about blowing 10buck LEDS and basically just have a
bicycle powered battery recharger? (Very handy for touring methinks,
since I am constantly dissapointed with my solar battery charger...)

I have several light bodies which I will probably construct this around.
If I was able to construct a battery recharger sufficiently well I
might even run a 4.8V0.5A halogen instead of the LED (just becasue I
already have one...)

cheers
Lance

I think if you ran into a battery you could share it with other bicycle
essentials such as MPG player and mobile phone and rear flashing lights. A
speed sensor running off the speedo could switch LEDs in and out according
to your speed, you don't need hi beam when you're pedaling up a steep hill.
What about a dynohub on the rear and run them in parallel?

Marty

Frank Krygowski said:

Jim Smith said:

Frank Krygowski <[email hidden]> writes:

Quoted message said:

Quoted message said:

Here's what I wonder: Since the generator is inherently current
limited, and since LEDs need current limitation, is it possible to
just put the proper number of LEDs in parallel, to share the current
properly?

Putting diodes in parallel is generally considered a bad idea
because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Hmmm. (Thinking slowly, since I'm an ME, not an EE) OK, that makes
some sense to me. But are the differences likely to be significant?
Another post referred to LED flashlights significantly overdriving the
LEDs. If you were closer to spec by my method, would the (apparent)
safety factor in the ratings be good enough?

Should work, it is just a light after all. I think a factor of 2
different in current between the diodes would not be surprising.
Again, not considered a good idea, but should work for some value of
"work." When people do this for real they usually run each LED off
its own current source, or else put them in series and use a voltage
kicker.

Quoted message said:

A problem, I guess, is that failure might be quickly progressive. One
LED might fail and take out the whole array.

This is a possibility if they are close to the current limit.

Quoted message said:

But I think of resistors as energy losers. Also, each resistor is
another circuit element to try to fit into the package. Would it be
better to use the standard trick of opposed Zeners to limit voltage?
Could the internal resistance of the generator then be all you'd need
in the circuit?

Yeah, the resistors do waste power. So do the zeners functioning as a
shunt regulator. LEDs ideally like a constant current supply. A
voltage regulator and a resistor is a common approximation of this.
Something like the zeners and a Luxeon with a current limiting
resistor would probably work and be safe. ISTR that one can buy
little modules for about $10 that are designed to serve as a constant
current source for LED lights. Should be more efficient than the
zener and resistor. Also, white LEDs usually drop about 3.5 volts, so
maybe the generator can drive two in series.

Quoted message said:

(I never wanted to know more about EE until I started playing with
generators!)

Quoted message said:

Quoted message said:

Unfortunately, we're talking about roughly 10 LEDs (i.e. 0.30 amp /
.030mA/LED) ... and since they're diodes, those 10 light during only
half the cycle. You'd want another 10 facing the other way.

Alternately, you could do a diode bridge, using the LEDs themselves as
diodes.

There is no current in a bridge without a load. Might as well
connect
an anti-parallel pair. Nothing wrong with that.

My idea of an LED bridge was to possibly reduce the LED count compared
to 10 facing East, plus 10 facing West. The load would be the rest of
the LEDs. I think.

Do you mean four LEDs hooked up as a bridge and the others connected
as the load? I can't think of any advantage to that.

Cannibalizing a LED flashlight might be a good starting point.

Frank Krygowski said:

Jim Smith said:

Putting diodes in parallel is generally considered a bad idea because
chances are very good that they will have different voltage drops and
not share the current equally. One can get around this a bit by
putting an equalizing resistor in series with each diode.

Hmmm. (Thinking slowly, since I'm an ME, not an EE) OK, that makes
some sense to me. But are the differences likely to be significant?
Another post referred to LED flashlights significantly overdriving the
LEDs. If you were closer to spec by my method, would the (apparent)
safety factor in the ratings be good enough?

A problem, I guess, is that failure might be quickly progressive. One
LED might fail and take out the whole array.

Right; the problem relates to an LED's resistance going _down_ as it
gets hotter, just like any other semiconductor's. A small difference
in forward voltage becomes exaggerated in a self-stoking cycle that
can lead to premature failure of the hottest-running LED in a parallel
array. Then the next hottest one goes, and so forth. It's a common
phenomenon among the cheapest and simplest multi-LED flashlights,
accelerated there because the LEDs are overdriven.

Chalo Colina

08 Sep 2004 12:27:39 -0500, Jim Smith:

Quoted message said:

LEDs ideally like a constant current supply. A
voltage regulator and a resistor is a common approximation of this.
Something like the zeners and a Luxeon with a current limiting
resistor would probably work and be safe. ISTR that one can buy
little modules for about $10 that are designed to serve as a constant
current source for LED lights. Should be more efficient than the
zener and resistor.

Bicycle generators are current limited - not voltage limited. Just use a
rectifier and LED(s) which can handle the 500-600mA (Luxeon 3W or 5W
type), maybe add a fat capacitor for smoothing and a goldcap for
"standlight" and you are done. A voltage regulator is only a waste. If you
want to use "classic" 20-50mA LEDs the best way is to hook two in series,
add a small series resistor and bundle 10 to 25 of this strings.

Quoted message said:

Also, white LEDs usually drop about 3.5 volts, so
maybe the generator can drive two in series.

Voltage doesn't matter. A bicycle dynamo will provide 3Volt to a 6 Ohm
load, 6Volt to a 12 Ohm load, 12 Volt to a 24 Ohm load (if speed is high
enough).

Andreas

Chalo said:

Frank Krygowski said:

Hmmm. (Thinking slowly, since I'm an ME, not an EE) OK, that makes
some sense to me. But are the differences likely to be significant?
Another post referred to LED flashlights significantly overdriving the
LEDs. If you were closer to spec by my method, would the (apparent)
safety factor in the ratings be good enough?

A problem, I guess, is that failure might be quickly progressive. One
LED might fail and take out the whole array.

Right; the problem relates to an LED's resistance going _down_ as it
gets hotter, just like any other semiconductor's. A small difference
in forward voltage becomes exaggerated in a self-stoking cycle that
can lead to premature failure of the hottest-running LED in a parallel
array. Then the next hottest one goes, and so forth. It's a common
phenomenon among the cheapest and simplest multi-LED flashlights,
accelerated there because the LEDs are overdriven.

OK, a related question: Are current limits for LEDs based entirely on
management of heat, or is there something else besides?

IOW, what can you get away with in duty cycles, where an LED is
significantly overdriven but only for a small percentage of a short
repeating cycle time, and shut off for the rest?

With tungsten filament bulbs, AFAIK, the thermal lag ensures that it's
just RMS current that matters, at least at reasonably high cycle
frequencies. I imagine "reasonably high frequencies" may need to have a
larger minimum for LEDs (since they operate almost instantaneously) but
is the situation similar, or is there some maximum current even for an
infinitesmal duration?

I guess I'm also curious about the relationship between current and
light output - is it directly proportional, or does output increase more
slowly than current?

I suppose I could go look all this stuff up. But Usenet is more fun.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Frank Krygowski said:

Chalo said:

A small difference
in forward voltage becomes exaggerated in a self-stoking cycle that
can lead to premature failure of the hottest-running LED in a parallel
array.

OK, a related question: Are current limits for LEDs based entirely on
management of heat, or is there something else besides?

My understanding is that it is solely a heat transfer issue. However,
the critical junction is between the LED die (chip) and its substrate,
which imposes an upper power limit even with infinite heatsinking.

Quoted message said:

IOW, what can you get away with in duty cycles, where an LED is
significantly overdriven but only for a small percentage of a short
repeating cycle time, and shut off for the rest?

That method is used to make blinky lights flash more brightly than
they could burn steadily. As long as the LED die remains within its
tolerable temperature limits, no problem.

Quoted message said:

With tungsten filament bulbs, AFAIK, the thermal lag ensures that it's
just RMS current that matters, at least at reasonably high cycle
frequencies. I imagine "reasonably high frequencies" may need to have a
larger minimum for LEDs (since they operate almost instantaneously) but
is the situation similar, or is there some maximum current even for an
infinitesmal duration?

At some current I imagine the resistance in the tiny leads would
impose a limit. But your question exceeds my understanding of the
relevant issues.

Quoted message said:

I guess I'm also curious about the relationship between current and
light output - is it directly proportional, or does output increase more
slowly than current?

Efficiency drops off with increasing power, within normal power
ranges. You get more lumens per watt by underdriving LEDs than you do
by running them to spec, and overdriving them yields much less change
in luminosity than the change in power input.

Overdriving LEDs is a cost-cutting measure (fewer LEDs to get a
desired light output) for manufacturers, who are not feeding
replacement batteries to the devices after all.

Chalo Colina