# Puzzle, enough to charge the power bank?

#### Rangineer

Joined Sep 16, 2020
4
I have a strange set up and wondering if people can help me figure out how to solve part of it.

The output needs to be enough to power a power bank. Spec on the device is 5V 2.0A.
The input is variable... Runs off a DC motor. The charge is continuous but changeable. It will however be higher than 5V 2.0A.
I am wondering if anyone has any ideas for a filter circuit to guarantee my output is exactly enough to charge the power bank?

I have a breadboard, 1kohms resistors, wires, LED's and capacitors.

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#### Irving

Joined Jan 30, 2016
1,162
What is changable? The voltage or the current? If the voltage is fixed, the powerbank will take whatever current it needs to charge. If there's more than 2A available it simply won't use it. However, if the voltage isn't fixed and exceeds 5v then you stand a high chance of damaging the powerbank, there's no way to tell how it'll respond without stripping it down and investigating what's inside. You can't solve that problem with what you have, it needs something much more complex...

#### Rangineer

Joined Sep 16, 2020
4
Thanks for getting back. Sorry I was away the weekend so only getting to reply now.

Basically uses a waterwheel to charge the power bank.
So it would depend on the flow rate of the water... Which can be different each time it rains. I just want to see if someone knows how to filter this variable current into something that would be known so it can be set as 5V.

#### Papabravo

Joined Feb 24, 2006
15,515
Thanks for getting back. Sorry I was away the weekend so only getting to reply now.

Basically uses a waterwheel to charge the power bank.
So it would depend on the flow rate of the water... Which can be different each time it rains. I just want to see if someone knows how to filter this variable current into something that would be known so it can be set as 5V.
The concept of filtering here is not applicable. Filtering is a process of selecting a group of frequencies for amplification or attenuation. Using a DC motor as a generator is a dubious enterprise at best. With such a variable input I can't see a reliable way to generate current for a charger at a fixed voltage. Depending on the construction of the DC motor you may get nothing or an AC output. If you get an AC output you can rectify it, store the charge on a capacitor, and use a voltage regulator or DC-DC converter. Here is the thing. To get any reasonable amount of power out you need a high shaft speed. What kind of shaft speeds did you have in mind?

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#### Irving

Joined Jan 30, 2016
1,162
Thanks for getting back. Sorry I was away the weekend so only getting to reply now.

Basically uses a waterwheel to charge the power bank.
So it would depend on the flow rate of the water... Which can be different each time it rains. I just want to see if someone knows how to filter this variable current into something that would be known so it can be set as 5V.
A generator based on a DC motor generates a voltage based on shaft speed. With waterwheels, as with windmills, it is usual to gear up the waterwheel so your generator shaft speed is sufficiently high even when the wheel is turning slowly, and limit the top speed with a brake or by controlling the waterflow with a sluice gate/bypass channel. Once your generator is producing more than 5v then it is relatively simple to convert this into something useable by your powerbank. The size of your generator will determine the power output and therefore the charge rate that can be achieved.

#### Papabravo

Joined Feb 24, 2006
15,515
A generator based on a DC motor generates a voltage based on shaft speed. With waterwheels, as with windmills, it is usual to gear up the waterwheel so your generator shaft speed is sufficiently high even when the wheel is turning slowly, and limit the top speed with a brake or by controlling the waterflow with a sluice gate/bypass channel. Once your generator is producing more than 5v then it is relatively simple to convert this into something useable by your powerbank. The size of your generator will determine the power output and therefore the charge rate that can be achieved.

I'm just wondering if the type of DC motor will have an effect on this experiment. Do we have any details on the proposed motor?

#### Irving

Joined Jan 30, 2016
1,162
No, nothing said about the 'generator', or the size of the waterwheel. Knowing the parameters of the wheel will give power and torque available and together with the motor specs will tell us required gearing and likely voltage & current. From there a MPPT converter will do the job... though this won't necessarily be cheap...