Wednesday, March 4, 2020

60 watt 5 amp 12 volt bulb powered by bicycle via DC-DC buck converter t...

I tried doing 120 watts as 10 amps - the wiring was not very good so I had some shorting - and so I could not get near full power. yeah when I was approaching 80% on the Chafon lithium battery inverter generator then my DC-DC buck converter amps dropped down to 0.18 - no matter what speed I was pedaling! Wow.
Gene's Green Machine Voidisyinyang Voidisyinyang The load (felt resistance when trying to pedal) is defined by the demand of the devices plugged into the circuit. My iPhone SE draws about 5 watts through an automotive charger (12v to 5v DC-DC conversion), the automotive laptop charger for my Lenovo draws about 65 watts, and so on. My build is different than yours, I chose the motor/generator to match the targeted volts I wanted to output, based on anticipated RPM. This simplifies things quite a bit, I don't need the DC-DC step down converter you have, and as long as I maintain the voltage output between 11-15 volts, all works well! I think for you system to work better, you may want to incorporate (4) 12v lead acid batteries in series (getting you 60v peak) with a blocking diode between the motor and your DC-DC buck. In my design, the bridge rectifier acts as a blocking diode. As far as AMP, VOLTS and WATTS go, the load (watts - going to the phone, laptop) remains constant, volts vary with my pedaling speed. The higher the volts, the lower the amps. Playing with this calculator helps me figure things out:="
 https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html

I was wondering about that automotive charger - thanks. Yes the DC-DC buck converter claims to be over 90% efficient. So what you're recommending - that would probably change the "back emf" resistance at slower volts? When I get up to 60 volts then there is a nice feedback on the DC motor so that the resistance goes down. Anyway I had not thought of that - great idea - to wire in series. I'm still wondering - how the DC-DC buck converter would increase the resistance at slower speeds? It seems like it's just the DC motor doing it - although the buck converter works through a magnetic induction back emf resistance - but that is just a volts step down in the circuit. So that should not effect the pedaling resistance. So with the DC-DC buck converter I have the option of being able to pedal at any speed and then the output is at around 12 volts. So yes I do think there is more resistance because I had the DVD player plugged into the lithium generator/inverter. So also of course if the amps go down to .18 at 80% charge - I didn't notice a resistance difference much though. So I'll have to just get more experience with the set-up I guess.

 Ok so assuming the resistance is based on the load demand - I looked up my Chafon again - it's 36 cells but wired in parallel for 9 packs. So each pack of 4 cells at 3.7 is 14 volts about. So then I used your handy calculator link - and sure enough - the resistance at 14 volts with 3 amps is quite high but then at 5 amps the resistance goes down. So when I pedal faster my amps go up via the DC-DC buck converter but my resistance goes down. Actually the "back emf" should go down also at faster speed. At lower speed the torque as "back emf" is much stronger - so it almost equals the voltage going in as rpm. Anyway but it's true that if there is no load then even at slow speed there's no resistance on the motor. So yes that confirms what you say - that the resistance is from the demand load. Thanks! I had forgotten that the "back emf" on the motor is just voltage - not current. So the current is just from the demand load.

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