Linear Voltage Regulator vs. Buck-Boost (Step Up/Down) Converter for Bike Powered USB Charger

Thread Starter

AdelCraft

Joined Jul 9, 2020
3
Hi,

I am making a bike powered USB charger. The output must be 5V@2.1A. Considering that the wheel won't have a constant speed, what is the best way of regulating the output voltage? As I know that a linear voltage regulator will make much more losses of energy in heat, I wanted to use a buck-boost (step up/down) converter, but every tutorial I found on internet for similar projects were using a linear voltage regulator.

What should I use?

Thanks.
 

dendad

Joined Feb 20, 2016
3,924
First, what is your generator voltage and current capacity?
If it will only output around the required voltage , then a linear will be ok. But if it will supply a fair bit higher, go switch mode.
For instance, if it is 12V out, a linear will waste almost 2/3 the power generated in heat. It is hard enough riding a bike without having to add extra wasted load to your effort.
Just a buck would be ok if your normal riding speed produces enough volts.
 

LowQCab

Joined Nov 6, 2012
1,188
First things first ......
The Bike Generator probably has an AC Output, so you need a LDO Bridge Rectifier .......
make one out of these Diodes : Super LOW FV Bridge Diode SM74611 ,
with a large Bulk-Storage Capacitor, maybe a 470uf @ 35V .

Then figure out how much Power the Generator can put out at normal cruising speeds.
Do this by first, figuring out the Load, 5V X 2A = 10 watts, 5V/2A = 2.5 Ohms,
so get a 2.5 Ohm, 10 watt Power Resistor and connect it across the Bridge Rectifier Capacitor.
Now, go for a ride, with a DVM strapped to the Handlebars, and see what you get.
You'll probably need ~6 to ~7 Volts for either type of Regulator,
a proper LDO Linear Regulator may need slightly less than 6V.

Here's what I think you'll find .......
10 watts takes serious effort and physical work to generate on a Bike,
you'll have to keep up a fairly fast speed, probably in the range of ~20mph,
and the amount of Drag created by the Generator will be VERY noticeable, and will wear you down quickly.
If you can actually make this much Power,
the Generator will wear the Rubber off of the side of your Tire in less than about ~10 hours.
Your Phone needs at least a couple of hours to charge fully,
do you think you can keep-up the pace for that long ???
If you play with your Phone in traffic, you may get seriously injured or killed, don't do it.

All this is ONLY if the Generator is actually capable of producing a full ~10-Watts, (plus losses).
It's probably designed to produce about ~2-Watts of Output Power.
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