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MintyBoost mod

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moosh

New Member
Hello,

I've designed a small modification to the great Minty Boost circuit to work with the dyno-hub of my bicycle (which outputs 2-6v DC). The goal is to power my cell phone while riding. The original MintyBoost circuit can be seen here: minty-boost-circuit.png. My modification is here: minty-buck-boost-circuit.png. The mod places a zener diode across the input to clamp the voltage there to 4.7V. Additionally two of the USB output lines need to be tied high to tell the phone to enter a charge state. I accomplish this by using two voltage dividers as can be seen in the modified circuit.

I'm fairly new to circuit design and am wondering if the voltage dividers will have side-effects that I don't know about. Should the voltage divider resistors be a bit larger to reduce the current to the data lines?

Thanks in advance!

Michael
 

duffy

Well-Known Member
The divider resistors look way too low - you are wasting 45ma through those for no reason. Try 10k and 12k.

The 4.7V zener will short the dyno if it goes over 4.7V. Also, a lot of bike dynos are actually alternators so they really need a bridge rectifier and a filter cap. Need to know the voltage output of that thing. You have a boost converter in there now, but you may need a buck convertor if you are going down instead of up with the voltage.
 

ronsimpson

Well-Known Member
Most Helpful Member
Moosh,
Are you shure you have the circuit wired for boost-buck? It looks like boost to me. You do need boost-buck. Change your zener to 7 or 8 volts.
I think you could only use pull up resistors on the data lines.
 

moosh

New Member
Thanks for the replies! Reading the hub specs awhile ago led me to believe it was variable DC with a fixed (or clamped) output power. But I will verify.

Initially I was going to add a buck stage but read the efficiency of such a combo was fairly low. Since most of the time the output voltage of the hub was going to be 5v or less (assuming I don't go DOWN too many hills ;-) I thought the z-diode would be a simpler choice. Assuming the hub outputs DC and I were to stay the course and use the zener diode, wouldn't the internal resistance of the device prevent shorting of the hub? Or would you recommend adding a resistor in series just in case?

Regarding the voltage divider I agree the values are too low. I should have added a "K" to the values when I created the circuit in Eagle. I'm intrigued by simply using pull-up resistors instead of the full voltage divider. So is it safe to apply a full five volts to the USB data lines instead of the ~3V applied by the original circuit?

Thanks again for your replies -- this is a great site for beginner hobbyists like myself!
 

duffy

Well-Known Member
Zener diodes don't have much internal resistance. The big issue is wattage, if it's a 1-watt zener and you are putting 3/4 of an amp through it going downhill you will blow it. Also, there will be a drag on your bike (always annoying) when you exceed that voltage and the zener shorts the windings.
 
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