Dynamo Lights Brightness PROBLEM

I would guess the reflectors inside the light head are different between your old dynamo light set and the new light set. Light is produced by a combination of the bulb, wattage, reflectors, etc. No way to make it brighter except by replacing the entire light head. There is a reason the Lumotec light heads sell for $30 and the Schmidt light heads sell for $100 and others sell for a few dollars.

Thanks for the replies,when i take the rear wire out the front light goes to full brightness and the same thing happens with the rear light,the dynamo is new and was working to full brightness but suddenly the front and rear lights are at half brightness after only being used

10 times,WHATS GONE WRONG?

snipped-for-privacy@yahoo.com wrote:

produced by a combination of the bulb, wattage, reflectors, etc. No way to make it brighter except by replacing the entire light head. There is a reason the Lumotec light heads sell for $30 and the Schmidt light heads sell for $100 and others sell for a few dollars.

Stop SHOUTING. Jeez. We can't see the thing for ourselves and tell you what the problem is.

There aren't that many possibilities. I'd look for loose connections first, including the ground (earth in the UK) connection. I'd check to make sure that the dynamo isn't slipping on the tire second. If you're running a standard .6W rear light, I'd check to make sure that the bulb is a 2.4W and not a 3W. I'd also be inclined to go with an LED rear light and use a 3W head lamp bulb. IMHO reducing the amount of wiring reduces the number of potential problems.

I thought I was one of the last bastions of filament rear lights when I changed a month or two ago. I'm amazed people still use them. The LED rears are bloody good these days and a lot now have the 'standlight[tm]' function which is worth its weight on gold(about 5g for the weight weenies).

However, when it fscks up, which the last one did (which is why I went filament) it's a new light time.

connections

Thinking about it, isn't the dynamo actually an alternator? Rotating magnet, stator coil? In which case one could probably correct the power factor with a judiciously selected capacitor and do wonders for the output. Or am I imagining it all?

John

Yes, you are. A filament is a nearly pure resistance load, so the pf is already equal to 1.

In message , dated Thu,

17 Aug 2006, David Kerber writes

The alternator has internal resistance and inductance. To match its impedance for maximum power transfer, you need a resistor AND a capacitor. The capacitor resonates with the inductance, but the resistances apply heavy damping.

As I indicated a while back, doing this can seriously damage your permanent magnet.

My power engineering courses were a long time ago, but I don't believe you can increase the power transfer into a resistive load (the filament) by adding a capacitance to a circuit; the cap just acts as an

*additional* load (even though it's reactive load).

Having said that, any idea how much inductance? Given the size of the coils, I would think it's pretty low, but have never measured it. However, I suppose if the filament had enough twists, it could have a measurable inductance, and in that case a few pf of capacitance might help a little.

Well, I tried it a few years ago. IANA Electrical Engineer but I had one helping me.

We tested three different generators and found similar results with all. Here's some data for a Soubitez bottom-bracket generator. I'll just give the values for 14 mph, 12 ohm load (i.e. standard generator headlight bulb), with and without some capacitance. The capacitance was chosen to give max power boost at about 12 mph, IIRC, and was 100 microFarads. (What's the ASCII abbreviation for "micro"? I'll go with "mu".)

RMS Voltage: With 100 muF: 7.3 V without: 6.4 V

Current: With 100 muF: 0.6 A without: 0.53 A

Power: With 100 muF: 4.4 W without: 3.4 W

Efficiency: With 100 muF: 42% without: 39%

The efficiency figures are the least reliable, BTW. Our method of measuring did not take into account the losses resulting from the interface between the rubber tire and the generator roller surface, and those can be considerable.

I'll also note, the September 1995 issue of "Electronics World + Wireless World" (a British magazine) has a letter to the editor, plus response, on p. 770 that deals with this issue. For their own reasons, they used a resistance of 24 Ohms instead of 12 Ohms, with and without a 50 microFarad capacitor. Above 10 mph, they got more power with the capacitor than without. A graph on that page shows their results.

So you can get more out of a bike generator by adding capacitance. But it's not much more, it varies quite a bit with speed, and we judged it not worth the trouble.

- Frank Krygowski

It appears that the current and power measurements were for the bulb. So what has the capacitor done? It raised the voltage at the load and hence there was more power to the load. Fair enough. The efficiency figures are, as indicated, questionable. In the case where the capacitor is included, there will be a higher total current from the generator and more electrical losses. In addition to the extra power to the load, there will be extra losses and the power source has to provide both.

In this case, I have to agree with your conclusion-why bother?

Out of curiosity, what was the frequency at which you took the measurements?.

Generally in power system applications, maximum power transfer is not a meaningful thing except as a situation to avoid due to inefficiency and stability problems. Having a source impedance which is much lower than the load impedance is more beneficial.

Well, we actually used 12 Ohms worth of power resistors for the load. But yes, voltage, current and power were all as delivered to the load.

That's true. In my notes, I see that we had difficulty with some sort of instability when measuring generator torque with the capacitor in the circuit. That is, the torque measurement fluctuated - I can't say why. The torque values (and, hence, efficiency values) probably contain more than the normal amount of "eyeballing."

Frequency of the generator's AC output is proportional to road speed, and can be surprisingly high. For this generator, 12 MPH gave about

190 Hz, IIRC. We tested it (and other generators) from about 8 MPH to about 25 MPH.

- Frank Krygowski

Thanks- that is enough info to determine the inductance but not the resistance of the generator. I should also have asked for the open circuit voltage and the DC resistance of the generator to get a better handle on the model. Also I have been assuming the capacitor is in parallel with the load resistance. Is it?

Out of curiosity - why 12 ohms?

As for fluctuations in torque due to the capacitor- there shouldn't be any other than double frequency components which would have an average of 0. Fluctuations of this nature will also be present without the capacitor. As for demagnetisation, that is unlikely as the generator probably can handle heavier loads and also a more leading pf will reduce demagnetisation.

We measured the resistances and inductances directly. We had the instruments.

Union bottle generator: 7.9 Ohms, 5.45 mH (up to 6.2 mH)

Soubitez bottom bracket generator: 3.8 Ohms, 6.76 mH (down to 4.9 mH)

Inductance measurements were done using an impedance bridge. They varied with angular position of the generator shaft... whether the generator was in one of it's "notches" or held in a different position. Ultimately, we decided it didn't matter much which value we used; we were measuring just to calculate a roughly appropriate capacitor size, and if it was off 30%, it didn't matter much for our purposes.

Well, I think I've got those open circuit voltages somewhere...

Nope. In series.

Briefly, a standard 3 Watt generator bulb (assuming only headlight, no taillight) is 12 Ohms. A 2.4 Watt bulb used with a 0.6 Watt taillight in parallel has a combined R of 12 Ohms.

Bike generators are, roughly speaking, constant current devices. Open circuit, their output current is zero, and their output voltage is roughly proportional to their rpm (up to a certain limit).

When given a resistive load, they will do their darndest to put out their rated current. Most bike generators are designed to produce 0.5 Amp. But their rating is invariably stated as 6 Volt, 3 Watt. That rating depends on having a 12 Ohm resistance in the load.

It's interesting that you can get more power out of a generator by giving it more resistance. For example, seeing a 24 Ohms load, the generator will try its darndest to put out 0.5 Amp. To do that, it will generate 12 volts, and produce 6 watts. Same generator, twice the power. (This only works if the speed is high enough.)

Problem is, most generators won't succeed at that job, because their drive wheels need about twice the torque as usual. They'll slip. One reason I like the Soubitez bottom bracket generator is that it can pull this off without slipping. So, of course, can the hub generators like the SON (or Schmidt). Bottle generators usually can't do it.

The fluctuations we saw weren't at anything like double the frequency. They had a period measured in (by memory) a second or so - i.e. frequency of 1 Hz or less.

To make this clear: We measured reaction torque by having the generator mounted in a sort of gimbal arrangement, with a long torque arm pressing on a digital scale. (We used a balloon between the arm and scale to absorb vibrations.) Anyway, the measuring system worked well in "normal" mode, but when we used it with the capacitor in the circuit, scale readings (i.e. torque readings) varied quite a bit.

Again, as with the inductance, we were just checking to see if adding capacitance was possibly worthwhile. Even without precise results, we learned enough to say capacitance was not worthwhile.

- Frank Krygowski

If the internal resistance of the dynamo source is about 6 ohms and the load resistance applied (the bulb) is 12 ohms or more, the source is more akin to a Thevenin source (a voltage source) than a Norton source (a current source). NOT a constant current device, but a constant voltage device. For a true (zero resistance) Thevenin source with a small series resistance in addition to the load (your measured values of 3.8 and 7.9 ohms) the voltage will be only about 40 percent higher with the load removed, than with the load. But the power will be zero because the current into an open circuit is zero.

For a current source the open-circuit voltage would become much higher (theoretically infinite) with the load removed.

For the generators you cite, the situation is more like a matched load, where about teh same power is dissipated in the source internal resistance and in the load resistance. In this case the dissipations are within about 50%, which practically is pretty well matched.

Sorry for all the dweeby stuff, but I have fun with this as well as with bikes!

Ken

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I thought that small alternators were constant current sources?That when all cars manufacturers switched to alternators (from shunt DC generators) the battery could be constantly charged even when the motor was idling?No?

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I have a cassete tape head demagnitizer, bought it back in Kozani (northwest Greece) maybe 10 years ago.Amazing piece of engineering (can post a link to a photo if you'd like).Recently changed for the first time the battery (50 cents, button cell, alkaline)

-- Tzortzakakis Dimitrios major in electrical engineering,freelance electrician mechanized infantry reservist dimtzort AT otenet DOT gr

This is not a constant voltage source except at a constant speed. voltage rises with speed as does the internal inductive reactance. If this reactance is dominant, then it can appear as if it is nearly constant current over a wide range of speeds. At a given speed, the load resistance does have an effect. This is consistent with the data given in the reference paper.

As to a source being Thevenin or Norton- please note that Thevenin or Norton refer to equivalent ways to a "non-ideal" source. Being equivalent- there is no way to differentiate them from any point outside the black box(i.e. from outside the terminals) so it is meaningless to say that a source is more like a Thevenin source than a Norton source. The load doesn't know the difference. Don't confuse the model with the physical device. If you want to say "nearly constant voltage" source vs "nearly constant current" you are welcome to do so but that is not saying it is a Thevenin or Norton source.

Yes, I am being pedantic but recognising the difference between a linear circuit model theorem and the actual device is of importance.