I've seen a few threads noting dynamo lights for MTB use, namely some of the c2010 stuff from Ktronik - however, has anyone really put up an optimized circuit for riding at MTB speeds (e.g. decent light for climbing at super low speeds without sacrificing brightness during the screaming down hills)?
There are seemingly satisfactory commercial versions:
http://www.exposurelights.com/product/000098/revo-dynamo-mk1-(new)/#.UIPnDG_A83V
http://cncdelite.com/
I got a nice deal on a disc brake capable Shimano dynamo hub (dh-3d72) and I'm doing some 24 hour races this year, so I thought I'd try my hand at a DIY light setup to minimize reliance on battery chargers. This will be on a 29" wheel (700c size with big knobby tires), so low frequency at low speed is going to be a big problem.
Here are my initial thoughts:
1) LC circuit a la Martin's circuit 6 (http://www.pilom.com/BicycleElectronics/DynamoCircuits.htm#Boost) with a "too small" tuning capacitor to maximize the low speed boost (i.e. reach a resonant frequency at low RPMs) - I've seen this idea in a few places before (it looks like this may be what Ktronik is using from that second link above)
2) auto switch from LC circuit to full-wave rectified circuit that isn't (necessarily) optimized to maintain a smooth curve, but is optimized to provide ideal output at several key speeds (say 4-6 kmph for really grinding uphills at 4 AM, 13 kmph for cruising speed on flats, 30kmph for dangerously fast downhill sections) - this ideal output would take into account hub drag. Ideally, I'd want to minimize drag going uphill and on the flats, possibly sacrificing some brightness, while having no restrictions on brightness at usual downhill speeds. Additionally, It seems that there would be a tradeoff between having good low speed power while not losing power at cruising speeds (due to exceeding the frequency of the LC circuit, I think).
3) A bright stand-light feature that need last no more than 10-15 seconds - to help keep the light steady during short, really grindy sections and to keep the bike visible after a crash. The light must come on immediately after starting to pedal, so the stand light feature must have a limit to the input.
4) A way to switch off the power during the day that doesn't kill circuitry due to open circuit voltages of 100V
An alternative to #2 is using a design I've seen from Steve K, which uses two banks of lights, only one of which is on at low speeds. The restriction on any design is that it must rugged - likely calling for a limit to the use of electrolytic capacitors, namely.
My questions are:
1) What else would you consider when designing an MTB dynamo light?
2) Are there other options I'm neglecting to consider (aside from an LC circuit and Steve K's solution of having multiple light banks)?
There are seemingly satisfactory commercial versions:
http://www.exposurelights.com/product/000098/revo-dynamo-mk1-(new)/#.UIPnDG_A83V
http://cncdelite.com/
I got a nice deal on a disc brake capable Shimano dynamo hub (dh-3d72) and I'm doing some 24 hour races this year, so I thought I'd try my hand at a DIY light setup to minimize reliance on battery chargers. This will be on a 29" wheel (700c size with big knobby tires), so low frequency at low speed is going to be a big problem.
Here are my initial thoughts:
1) LC circuit a la Martin's circuit 6 (http://www.pilom.com/BicycleElectronics/DynamoCircuits.htm#Boost) with a "too small" tuning capacitor to maximize the low speed boost (i.e. reach a resonant frequency at low RPMs) - I've seen this idea in a few places before (it looks like this may be what Ktronik is using from that second link above)
2) auto switch from LC circuit to full-wave rectified circuit that isn't (necessarily) optimized to maintain a smooth curve, but is optimized to provide ideal output at several key speeds (say 4-6 kmph for really grinding uphills at 4 AM, 13 kmph for cruising speed on flats, 30kmph for dangerously fast downhill sections) - this ideal output would take into account hub drag. Ideally, I'd want to minimize drag going uphill and on the flats, possibly sacrificing some brightness, while having no restrictions on brightness at usual downhill speeds. Additionally, It seems that there would be a tradeoff between having good low speed power while not losing power at cruising speeds (due to exceeding the frequency of the LC circuit, I think).
3) A bright stand-light feature that need last no more than 10-15 seconds - to help keep the light steady during short, really grindy sections and to keep the bike visible after a crash. The light must come on immediately after starting to pedal, so the stand light feature must have a limit to the input.
4) A way to switch off the power during the day that doesn't kill circuitry due to open circuit voltages of 100V
An alternative to #2 is using a design I've seen from Steve K, which uses two banks of lights, only one of which is on at low speeds. The restriction on any design is that it must rugged - likely calling for a limit to the use of electrolytic capacitors, namely.
My questions are:
1) What else would you consider when designing an MTB dynamo light?
2) Are there other options I'm neglecting to consider (aside from an LC circuit and Steve K's solution of having multiple light banks)?
