Dynamo bike lights – do they vary in power draw and drag?

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Brucey
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby Brucey » 8 Sep 2015, 9:20pm

in a word; 'yes'

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ConRAD
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby ConRAD » 9 Sep 2015, 11:19am

just now made a test powering-up an Edelux at 20km/h with a very normal dynohub and I've got something like 8.52V-0.51A ... now my doubt is are they 4.35W or 4.35VA, in other words how can I assess the possible reactive component (capacitive I guess) involved in this type of measurement ?
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Brucey
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby Brucey » 9 Sep 2015, 3:56pm

I think that once the lamp is drawing its maximum current its impedance seems to increase, hence 8.5 V appears on your meter.

I would suggest that if you don't have two lights to try, put a 12ohm resistor (or a tungsten 6V 0.5A bulb) in series with the lamp and try again.

I suspect that you will see about 10V and about 0.45A, increasing more with speed.

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ConRAD
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby ConRAD » 18 Sep 2015, 7:10pm

Using a very cheap dynohub and some led based lights I did something different and at 20 km/h I’ve got the following results:
- Edelux + Toplight (alone): 8.6V/0.513A => 4.4W
- Lumotec Hopal (alone): 10.3V/0.490A => 5.05W
- Edelux + Toplight + Lumotec (connected in series): 13.4V/0.362A => 4.85W

It’s evident that Lumotec is not the best from an efficiency point of view, it’s also evident that talking about complete led based lights and leds alone is totally different … to the point that two lights in series sometimes seem to draw less power than a single one !! Any comment to this somehow primitive analysis ?

FARO = HEADLIGHT, ACCESO = ON, SPENTO =OFF, LUCE POSTERIORE = REARLIGHT, DINAMO AL MOZZO = HUB DYNAMO

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Brucey
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby Brucey » 18 Sep 2015, 7:28pm

I think it only fair to mention that the 'power figures' you quote may not be accurate, either in terms of the power used in the lights, or the drag induced in the generator.

The reason for this is that the current and the voltage figures are each RMS ones and with any load that is even remotely complicated, the current and voltage may not be in phase any more. Now this is unlikely to be a completely perverse relationship in this instance but in the case of the Lumotec lamp, it'll most likely draw about 0.5A @ 6V rms and it is designed to draw no more current than that even if the voltage increases considerably. The effect of limiting the current drawn is in turn to increase the voltage still further at the generator (at any speed you will see more volts across a higher impedance load).

So anyway, to work out a true power figure you should measure the voltage across the LED and the current through the LED using a 'scope or digital data sampling and then multiply the one waveform/data set by the other to derive a power value (useful power in the lamp). You can do the same thing at the generator, to derive output power. The difference between the two is the power wasted in the lamp electronics.

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MikeF
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby MikeF » 18 Sep 2015, 8:44pm

Samuel D wrote:
By the way, does anyone here get irrationally afraid of the dark when in deep countryside at night?
Not for me, but there are plenty that do. Some even panic when the street lights go out. :wink:
"It takes a genius to spot the obvious" - my old physics master

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ConRAD
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby ConRAD » 18 Sep 2015, 8:51pm

Brucey wrote:...the 'power figures' you quote may not be accurate, either in terms of the power used in the lights, or the drag induced in the generator...

I think that there's no "drag induced" error in my measurements simply because I'm taking all measurements directly on the dynohub itself (TEST WHEEL) ... the DRIVING WHEEL is something totally indipendent and is maintained into rotation by a separate three-phase motor/inverter. LOOK ALSO HERE PLEASE

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Brucey
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby Brucey » 18 Sep 2015, 10:08pm

I think you have missed the point of what I am trying to say.

Suppose for sake of argument that the waveform is comprised of two parts (A and B) and that there is an equal amount (in time) of each.

In the first part A the voltage averages (say) 4V and current averages 0.7A .
In the second part B the voltage averages 14V and the current averages 0.3A.

If you use a simple meter to read the voltage you will read 9V rms and similarly the current reads 0.5A rms. Multiply the two together and you get 4.5W, right?

Wrong.

The power in part A is 2.8W
The power in part B is 4.2W

The average power is 3.5W, not 4.5W.

This is the kind of thing that happens when the voltage and current are out of phase with one another. If you take simple meter readings of complex circuits (in which current and voltage are not in phase with one another) you can get everything horribly wrong.

This is why you need to monitor volts and amps using an oscilloscope or digital data capture and then process the results carefully. Your 'power measurements' will be useless otherwise.

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edocaster
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby edocaster » 19 Sep 2015, 12:28am

The two lights in series will probably draw more power if you take them beyond 20km/h.

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ConRAD
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby ConRAD » 19 Sep 2015, 8:30am

@Brucey
as you can see from the pictures below the waveforms are in all cases perfectly simmetrical, yet no sinusoidal
- Picture 1 shows the typical waveform at dynamo output at no-load conditions, dynamo claw-pole design is responsible in this case for 3rd harmonic distorsion
- Picture 2 shows the change in waveform once the dynamo output is loaded on a pure resistive/ohmic load
- Picture 3 shows the change in waveform once the dynamo output is loaded on Edelux exhibiting in this case the double zener chopping effects
- Picture 4 shows that the voltage reading of the scope matches the readout of the TRMS (true rms) voltage meter

.... different story is the voltage-current power factor for which of course I must find a solution !!

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@edocaster
yes, at 25km/h the two lights in series draw 15.8V/419mA for an "APPARENT" power of 6.62VA
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Brucey
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Re: Dynamo bike lights – do they vary in power draw and drag

Postby Brucey » 19 Sep 2015, 8:49am

that is pretty much what I'd expect to see. If you monitor current using the 'scope as well, I'd expect you to find that the two are broadly in proportion to one another with a resistive load, but not necessarily with an Edelux lamp as the load.

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