Originally Posted by
itsajustme
I know what it's marketed for, but what is the actual difference?
I mean, electrically speaking, aren't we're talking about a simple voltage source? If they both put out 6V and the loads were the same, then wouldn't they both be delivering the same wattage?
A 6V 3W load is 12 ohms resistance. A 6V 2.4W load is 15 ohms resistance. Judging from the graphs, the 3W unit is designed to produce 6V at 15km/hr with a 12 ohm load while the 2.4W unit puts out 6V at 15km/hr with a 15 ohm load.
An ideal voltage source would put out its rated voltage no matter what the load resistance. The bicycle "dynamo" is not like this. It has a high internal resistance (actually more of a reactance to be precise). This means the voltage will change if you change the load resistance. To prevent premature bulb burn out, you should not use the 3W dynamo with a 2.4W load. If you use a 2.4W dynamo with a 3W load the bulbs will last a long time but will be dim. Best is to match dynamo to load; that is why they offer you the choice.