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no enlightenment [b]aracer [/b]??
Sorry - missed that you'd replied.
OK, so you want to jump in the deep end? Let's rewind to where you first seemed to have a problem before I started trying to simplify it:
"never allowing the dynamo voltage to rise enough to supply the required power."
??? if it's stuck hard on it isn't stopping the dynamo from doing anything it can't do for lack of revs
"The voltage can't rise above this, as in order to do so it would need to supply more current"
I'm sorry, but this is twaddle
First of all, lets ignore the non-linear nature of the LED - all that matters is that as you increase the current into the entire circuit (rectifier, driver and diode) the voltage increases and vice versa. Let's ignore the detailed nature of the dynamo entirely - instead modelling it as voltage limited (dependent on speed) current source. Get that - if not I'll have to revert to slow step-by-step stuff?
So, as the speed increases from stationary, the voltage of the dynamo will increase until it reaches a point the control circuit has enough voltage to start working, the FET has enough voltage across the gate to start turning on and the LED enough voltage across it to start passing current. With a little more speed and voltage you reach the point where the LED is shining brightly with 500mA flowing through it and the FET is hard(ish) on, since the driver control cirtcuitry senses not enough current so keeps it in direct drive. At this point the dynamo current source is saturated - the voltage of it can't rise any more, since to do so it would have to supply more current, and it can't do that. Hence "The voltage can't rise above this, as in order to do so it would need to supply more current" and "never allowing the dynamo voltage to rise enough to supply the required power."
Let me know if you need more simple steps in there to explain the latter two points you had a problem with before.
At this point the dynamo current source is saturated - the voltage of it can't rise any more, since to do so it would have to supply more current, and it can't do that
isn't this a circular argument - "It can't because it can't" ?
I guess at any particular speed there'll me a maximum current available where its back EMF + the load voltage will equal the open circuit voltage...
isn't this a circular argument - "It can't because it can't" ?
No - the voltage can't rise because it's a current source which can supply a maximum of 500mA. It would be basic Ohm's law if things were linear (hence why I mentioned that). The fact the voltage/current curve for the load is non-linear doesn't change the fact that for a given current the voltage is fixed.
I guess at any particular speed there'll me a maximum current available where its back EMF + the load voltage will equal the open circuit voltage...
Yes - 500mA for a "6V" dynamo at any speed. Hadn't realised the gap in your understanding was as basic as not getting that the dynamo can supply a max of 500mA - or have I not mentioned that figure before in this thread? 😉
Hadn't realised the gap in your understanding was as basic as not getting that the dynamo can supply a max of 500mA - or have I not mentioned that figure before in this thread?
I dunno, but why is this the case (if it is)? And were it to be so, why were you rabbitting on about voltages ? What physical process limits the current ?
@simonfbarnes - the physical process which limits the current is the inductance of the dynamo coil. The faster the hub turns, the higher the frequency of the AC which is generated. The internal inductance of the coil means that the higher current which would be generated is reduced because the frequency is higher. This is generally A Good Thing as it stops the dynamo burning out itself or whatever is attached to it.
Each type of dynamo has a current at which the power transfer is highest. This varies a little with speed, but not a huge amount. More info and testing of a couple of dynamos here: http://bit.ly/eg4dj6