# Capcutor NEEDED

Hi, currently I am using an Arduino Nano with 7 WS2812B lights and a 10W water flow generator (5V and currently not 10W just arround 0.4W). I would like to use a capacitor so that the device keeps powerd (at least just the Arduino Nano) for arround 30 seconds. Which capacitor should I use?

Where would you connect the capacitor?

30 seconds, 100mA load? Need a diode also to prevent backdriving the generator.
Also keep in mind that a cap charged to 5V will rapidly (exponentially) drop in voltage as it discharges.
Might want to add a boost charger to hold the voltage up from whatever charge is in the cap:

For example, I used these 3 circuits to power up a supercap, which was then boosted to keep a '1284P alive long enough to store data into an SD card.

I don't recall the math for how we determined the supercap size, it was 9 years ago.

I would like to use a capacitor so that the device keeps powered (at least just the Arduino Nano) for around 30 seconds. Which capacitor should I use?

The relationship between the value of the capacitor, the current into or out of it and time is that for a 1F capacitor if you charge / discharge it at 1A then in 1 second the voltage will change by 1V.

So, you have to consider that the voltage on the capacitor will change as you discharge it, unlike a battery, which has near constant voltage, and you have to consider how big your capacitor needs to be to do what you want to do.

So, if you want 100mA for 30 seconds then the voltage change on a 2F capacitor will be (30 * 0.1) / 2 = 1.5V. I leave you to work out from that what you need.

pep_moyanofont03:
Hi, currently I am using an Arduino Nano with 7 WS2812B lights and a 10W water flow generator (5V and currently not 10W just arround 0.4W). I would like to use a capacitor so that the device keeps powerd (at least just the Arduino Nano) for arround 30 seconds. Which capacitor should I use?

A backup battery on trickle-charge is much simpler, no voltage droop. NiMH is a possible chemistry for this.

See the bit about emergency lighting and NiMH used at C/30 here: Nickel–metal hydride battery - Wikipedia

There's also https://batteryuniversity.com/

And also, if I would be using that capcitor (5.5V and 4F) how long would it take to charge them? With the generator of 5V and 40mAh?

Do the maths, I’ve given you the relationship you need.

https://www.electronics-tutorials.ws/rc/rc_1.html
I believe it will make clearer for you.
That large of a cap will look like a short across the generator, be sure to use a sufficiently sized resistor so that some voltage can be generated.
With R = 125 to limit current to 40mA (5V/125ohm = .04A), then Tau = RxC = 125 x 4 = 500 seconds.
If the time to fully charged is 5 x Tau, then Time = 2500 seconds.

PerryBebbington:
Do the maths

Yay, someone else who knows it's short for mathematics not mathematic

jubukraa:
Yay, someone else who knows it's short for mathematics not mathematic

Thank you.

I think, but I am not sure, that speakers of UK English say 'maths' and speakers of US English say 'math'.