Electrical question (tailight)

There's several possibilities for the cause of the taillight not working:

1. When you connected the two leads for the headlight together, you shorted out the circuit and all
the power from the generator flows through it relatively unimpeded and so the feed to the
taillight won't carry current through the higher-impedence of the filament of the bulb.

2. When you first disconnected the headlight, all the current from the generator went through the
taillight, burning out the bulb.

3. Apparently, since two leads were used for the headlight, that may mean that you have a
non-conductive frame?? With metal frames, as with cars, the DC current uses the chassis for the
return circuit and only one lead is required. So, if this is the case, make sure that your
taillight has two leads that are working to complete the circuit with the generator.

4. It's possible that one of the leads that connected to the headlight, may actually be the wire
that feeds power to the taillight. In this case, the power would go through the headlight, then
to the tailight and the circuit would be completed for both lights by returning through a
conductive frame or by a separate wire back to the generator. If so, by removing the headlight,
you disconnected the power source for the taillight. This would be a series connection and if
either light was not working, neither would the other. I've never seen this with a bike
generator, but I haven't used that many different types.

This should give you a few ideas to explore.

Steve McDonald

[email hidden] (Steve McDonald) wrote in news:24265-3F63D7D1-
[email hidden]:

Quoted message said:

There's several possibilities for the cause of the taillight not working:

1. When you connected the two leads for the headlight together, you shorted out the circuit and
all the power from the generator flows through it relatively unimpeded and so the feed to the
taillight won't carry current through the higher-impedence of the filament of the

bulb.

Quoted message said:

2. When you first disconnected the headlight, all the current from the generator went through the
taillight, burning out the bulb.

3. Apparently, since two leads were used for the headlight, that may mean that you have a
non-conductive frame?? With metal frames, as

with

Quoted message said:

cars, the DC current uses the chassis for the return circuit and only one lead is required. So, if
this is the case, make sure that your taillight has two leads that are working to complete the
circuit with the generator.

4. It's possible that one of the leads that connected to the

headlight,

Quoted message said:

may actually be the wire that feeds power to the taillight. In this case, the power would go
through the headlight, then to the tailight

and

Quoted message said:

the circuit would be completed for both lights by returning through a conductive frame or by a
separate wire back to the generator. If so,

by

Quoted message said:

removing the headlight, you disconnected the power source for the taillight. This would be a
series connection and if either light was not working, neither would the other. I've never seen
this with a

bike

Quoted message said:

generator, but I haven't used that many different types.

This should give you a few ideas to explore.

Steve McDonald

Thanks Steve -1) I tried it first with the leads seperated and it didn't work. Then I tried
connecting them as a 2nd attempt

2) seems like a good possibility

3) tailight does have two leads still solidly connected

4) that's what I thought it might be when I tried the leads connected.

MLB said:

My Kettweisel has a bottle generator connected to a headlight and taillight. I took off the
headlight to mount my Trailhead light (battery powered and much brighter) but now the tailight
doesn't work. Two spade terminal leads to the headlight. I tried connecting them together but
that doesn't work either. What do I need to do to replicate the headlight being there to enable
the tailight?

In general, it is not possible (or at least, not reliably and sensibly) to run just a rear light off
a bicycle bottle dynamo.

Dynamos have a behaviour closer to a constant current source than a constant voltage source. This is
the oppositte of batteries, which are a set voltage and the current adjusts to suit teh voltage.
What this means, is that it's almost guaranteed that you'll blow a rear light very quickly if it's
the only load on a dynamo.

Generally a front dynamo bulb is a nominal 6V 2.4W, and a rear is 6V .6W. That mean you have 0.4A
flowing to teh front bulb and 0.1A to teh rear. The dynamo is designed such that it generally
produces .5A, and in practice teh voltage will fluctuate widely while the current stays about the
same, at teh right level to drive one front and one rear lamp.

If the dynamo were an ideal current source (and teh lamp an ideal ohms law load), when you just
hook your rear lamp to it the dynamo would generate 30V to try and get 0.5A flowing through teh
rear lamp. In practice, the voltage wouldn't get that high, but the bulb blows well before it
would anyway.

This is why most dynamo instructions contain teh warning taht you should switch off teh dynamo if
the front bulb blows unexpectedly. Cheap rear lights are not a problem - the bulb blows quickly and
that's that. More expensive rear lights have overload protection that protects teh rear bulb for a
short while, but if you continue cycling then you risk blowing the overload protection, and then
your expensive rear light either stops entirely, or behaves like a cheap one. If you've a standlight
function in teh rear then you might blow those electronics too.

The solution (if you really want a rear only dynamo light) are to either add something in parralle
to dissipate teh extra power - a front light bulb would be ideal (!), but a 15 ohm resistor with at
least 2.5 W power rating would do. The down side of this is you're wasting all that power - and teh
drag on a dynamo is related to teh power generated. Also, bear in mind that teh resistor will get
very hot (melt plastic, blister your skin type hot).

Alternatively, hook up five rear lights in parallel, then at least you're benefitting from teh power
you're generating. You may get mistaken for a berserk UFO from behind, but that's not necesarily a
bad thing.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

Have you checked the tail light bulb to make sure it still works?

Remember, its only a .4 or .5 watt bulb.

Lewis.

MLB <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

My Kettweisel has a bottle generator connected to a headlight and taillight. I took off the
headlight to mount my Trailhead light (battery powered and much brighter) but now the tailight
doesn't work. Two spade terminal leads to the headlight. I tried connecting them together but that
doesn't work either. What do I need to do to replicate the headlight being there to enable the
tailight?

Ian Smith must be edykated coz e writed:

Quoted message said:

MLB said:

My Kettweisel has a bottle generator connected to a headlight and taillight. I took off the
headlight to mount my Trailhead light (battery powered and much brighter) but now the tailight
doesn't work. Two spade terminal leads to the headlight. I tried connecting them together but
that doesn't work either. What do I need to do to replicate the headlight being there to enable
the tailight?

In general, it is not possible (or at least, not reliably and sensibly) to run just a rear light
off a bicycle bottle dynamo.

Dynamos have a behaviour closer to a constant current source than a constant voltage source. This
is the oppositte of batteries, which are a set voltage and the current adjusts to suit teh
voltage. What this means, is that it's almost guaranteed that you'll blow a rear light very
quickly if it's the only load on a dynamo.

Generally a front dynamo bulb is a nominal 6V 2.4W, and a rear is 6V .6W. That mean you have 0.4A
flowing to teh front bulb and 0.1A to teh rear. The dynamo is designed such that it generally
produces .5A, and in practice teh voltage will fluctuate widely while the current stays about the
same, at teh right level to drive one front and one rear lamp.

If the dynamo were an ideal current source (and teh lamp an ideal ohms law load), when you just
hook your rear lamp to it the dynamo would generate 30V to try and get 0.5A flowing through teh
rear lamp. In practice, the voltage wouldn't get that high, but the bulb blows well before it
would anyway.

This is why most dynamo instructions contain teh warning taht you should switch off teh dynamo if
the front bulb blows unexpectedly. Cheap rear lights are not a problem - the bulb blows quickly
and that's that. More expensive rear lights have overload protection that protects teh rear bulb
for a short while, but if you continue cycling then you risk blowing the overload protection, and
then your expensive rear light either stops entirely, or behaves like a cheap one. If you've a
standlight function in teh rear then you might blow those electronics too.

The solution (if you really want a rear only dynamo light) are to either add something in parralle
to dissipate teh extra power - a front light bulb would be ideal (!), but a 15 ohm resistor with
at least 2.5 W power rating would do. The down side of this is you're wasting all that power - and
teh drag on a dynamo is related to teh power generated. Also, bear in mind that teh resistor will
get very hot (melt plastic, blister your skin type hot).

Alternatively, hook up five rear lights in parallel, then at least you're benefitting from teh
power you're generating. You may get mistaken for a berserk UFO from behind, but that's not
necesarily a bad thing.

regards, Ian SMith

Or stick a small trickle charge circuit for your front light in the loop.

--
Ian

catrike.co.ukcatrike.co.ukOpen ↗

Does the output voltage of a dynamo hub increase with speed, or just the current and frequency
(which some circuits might rectify to DC before letting it pass through the bulbs anyway). The
reason I suspect that the voltage increases is that different dynamos are made for different size
wheels. Such being the case, at what speed is the rated voltage achieved: 15kph, 20mph, ?. I will
never forgot the time when, as a teen, a bulb popped as I flew down a hill in total darkness;
fortunately the lamp had a low beam switch which changed over to a spare bulb.

--

Joel Wilson Fort Lauderdale
=================================
Proud 2 B a pioneering satellite radio subscriber AI4I is always on the trailing edge of technology
=================================

"ai4i" <[email hidden]> wrote in :"]news:[email hidden]:

Quoted message said:

Does the output voltage of a dynamo hub increase with speed, or just the current and frequency
(which some circuits might rectify to DC before letting it pass through the bulbs anyway). The
reason I suspect that the voltage increases is that different dynamos are made for different size
wheels. Such being the case, at what speed is the rated voltage achieved: 15kph, 20mph, ?. I will
never forgot the time when, as a teen, a bulb popped as I flew down a hill in total darkness;
fortunately the lamp had a low beam switch which changed over to a spare bulb.

Thanks guys, makes it easy to decide to just take it all off. I never liked the drag of the dyno
anyway, so i'll just get a battery rear.

Most tire-rubbing generators do increase their voltage proportional to wheel speed. Most decent
generator/light combos have a diode somewhere that dumps the extra power to ground after it reaches
a certain voltage [usually 7.2V for a "6V" system].

If you're driving the generator on a tire, then it doesn't matter what size wheel you have - the
speed of the tire is the same for all tires at a given speed. If you use a hub generator, then you
might want to adapt for rim size.

Bobby

MLB said:

"ai4i" <[email hidden]> wrote in :"]news:[email hidden]:

Quoted message said:

Does the output voltage of a dynamo hub increase with speed, or just the current and frequency
(which some circuits might rectify to DC before letting it pass through the bulbs anyway). The
reason I suspect that the voltage increases is that different dynamos are made for different
size wheels. Such being the case, at what speed is the rated voltage achieved: 15kph, 20mph, ?.
I will never forgot the time when, as a teen, a bulb popped as I flew down a hill in total
darkness; fortunately the lamp had a low beam switch which changed over to a spare bulb.

Thanks guys, makes it easy to decide to just take it all off. I never liked the drag of the dyno
anyway, so i'll just get a battery rear.

I should have specified, the incident that happened to me back in the early 1970's was with a cheap
tire rubber. The modern ones I have been reading up on are all dynahubs.

--

Joel Wilson Fort Lauderdale
=========================================
Proud 2 B a pioneering satellite radio subscriber AI4I is always on the trailing edge of technology
=========================================
<[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Most tire-rubbing generators do increase their voltage proportional to

wheel

Quoted message said:

speed. Most decent generator/light combos have a diode somewhere that

dumps

Quoted message said:

the extra power to ground after it reaches a certain voltage [usually 7.2V

for

Quoted message said:

a "6V" system].

If you're driving the generator on a tire, then it doesn't matter what

size

Quoted message said:

wheel you have - the speed of the tire is the same for all tires at a

given

Quoted message said:

speed. If you use a hub generator, then you might want to adapt for rim

size.

Quoted message said:

Bobby

Ok, here's a somewhat related question. I have one of those ultra-bright 18 led no name tailights
that I had been hanging from a seat cross brace with zip-ties. Now, on my GTO I just rigged up a
rigid bracket. Here's the problem: the light "cycles" itself off (from the flashing mode) probably
from the extra vibration it now absorbs. What's causing this? Weak battery contacts? A bad switch?
Whaddya think?

BentJay

My experience it is always lose battery contacts. Mostly from AAA(1,2v/1,5v) batterys. Just adjust
the contacts a litlle with a small screwdriver or something. On 15 Sep 2003 09:00:43 -0700,

(BentJay) said:

Ok, here's a somewhat related question. I have one of those ultra-bright 18 led no name tailights
that I had been hanging from a seat cross brace with zip-ties. Now, on my GTO I just rigged up a
rigid bracket. Here's the problem: the light "cycles" itself off (from the flashing mode) probably
from the extra vibration it now absorbs. What's causing this? Weak battery contacts? A bad switch?
Whaddya think?

BentJay