What causes the physical resistance in a dynamo?

Thread Starter


Joined Feb 10, 2020
I have a new old timey bicycle dynamo ( i think thats what it was built for) that when spun by contact friction from the bicycle tyre turns the DC generator and lights your light proportional to your pedal speed - standard old timey bicycle lighting system (thank you Mr Max Plank et al for inventing theories that invented LED,s TBH).

Anyhoo I have put the bike on rollers so it's stationary and putting lots of different light bulbs on it because reasons.

When I put lots of bulbs on the power required of my legs to light the bulbs obviously increases but i was amazed how hard it becones to pedal with the increased power requirements.

What physically makes it harder to pedal when trying to supply the power. It's like pedalling in quicksand.


Joined Mar 14, 2008
The increase in bulbs requires more power, thus the dynamo requires more input power to deliver the required output power, which your legs have to supply.
There's no free lunch.


Joined Jun 5, 2013
The current in the windings on the dynamo produce a magnetic field. This field repels the magnets. More load means more current means stronger field means more force.


Glenn Holland

Joined Dec 26, 2014
It's a case of Lenz's law where the field induced in the output of the generator wingdings is opposite to the field of the magnets in the rotor. The field is proportional to the output current so as this current increases, the opposition is also increased.