I would do this:
simulate this circuit – Schematic created using CircuitLab
simulate this circuit
By putting two capacitors in series you double your cost, energy capacity, and maximum voltage.
You could use a reverse-biased zener for the diodes also, but using a forward-biased LED also gives you a visual indication of "fully charged".
The diodes are indicator LEDs you add, not the one already in the torch. They protect the capacitors from overcharging. You want to select the LEDs to do two things:
- have a forward voltage less than the absolute maximum voltage of the caps. I'd leave a 20% margin to be safe.
- absorb all the energy you can generate with the crank after the caps are charged without failing. If you can count on the operator to not continue cranking hard after the LEDs light, then this is substantially easier.
You say the dynamo voltage is about 4V, but really its a function of how fast the crank is turned. How hard the crank is to turn is a function of the current through the dynamo, so in the case where the caps are charged, so there is no charging current, the dynamo will be very easy to turn until you turn it fast enough to generate more than the LED forward voltage, at which point it will become markedly harder to turn. When the caps aren't charged, then the current/difficulty of turning/torque will be high even at low speeds/voltages.
Why is this? Consider the law of conservation of energy. Neglecting frictional losses, the mechanical power (torque times angular velocity) must equal the electrical power (voltage times current). Mechanical power is angular velocity times torque.
There are more complicated solutions, but I think they would be harder than buying a different product that operates the way you'd like without modification:
- remove turns from the dynamo or decrease the strength of its permanent magnets to reduce the dynamo's output voltage. The dynamo's output voltage will be lower for a given speed, and the crank will be easier to turn for a given current, just as if you changed the mechanical advantage with a transmission.
- use a buck-boost converter to reduce the voltage. With a switch you could use the same converter to upconvert the voltage on C1 to what's needed to drive the torch. You will still need something to absorb all the crank's energy once C1 is charged.