Hello !
I start working on a arduino nano and i got a question, hope you guys can answer !
Thats my problem, my arduino board is powered by USB the whole time in my project, but i had to do something to make him autonomous in case of the USB is not powering my board for a while.
I wanted to use a supercapacitor 1.0F 5.5V (here the datasheet "KW-5R5C105-R", what i need to do is to load the supercapacitor when USB is ok then, if USB is unplugged or something else, the supercapacitor would power the Arduino. My project don't use any another components, only the Arduino Nano Board.
So Question is :
-Is it possible to make somethings like this ?
-If yes, how many time can my supercapacitor supply the board ? (I'll need around 2 days supply)
I'm not very good at electronic so it may be stupid question but can't find a clear answer.
Thanks by advance !
I believe that using a super cabacitor only has advantages for short periods of time, when the current is higher, and for long periods of time only if the current of consumption is very small.
To maintain the voltage at 5V, it will be necessary to use a dc-dc step-up converter, similar to the one used in a power bank.
I've already used a super capacitor in an RTC, and by the estimate it should arrive several days, but the consumption is very small.
The RTC chip I was working on is ISL12008. You can observe its datasheet, and you can find calculations to estimate the durability of the capacitor load.
Page 18: EXAMPLE 1: CALCULATING BACKUP TIME GIVEN
VOLTAGES AND CAPACITOR VALUE
But be careful that the capacitor discharged will operate as a short circuit, so it is necessary to use a current limiter. The use of the resistor is not very suitable for this type of situation, because the final current will fall, try to use an active current limiter with transistors or integrated circuit.
Hello, thanks for your quick answer !
I make a test, my arduino load the super capacitor for 1 hour, (5V,26mA). Then I tried to supply the Arduino with the SuperCapacitor (4.7V), I connect it for 1 minute then Voltage drop to 2.7V.
DO you have any link to a DC-DC step up converter ? I find this one but, delivery time is too long and i can't find a similar on RS component, maybe if you know any other good one.
I use a resistor only to load the supercapacitor, is it better to use an active current limiter as you said even if its only to load ?
What you need is a battery backup, li-po with charging/ discharging controller. Supercapacitor is too weak to power arduino for a couple of days. Just get small +5V power bank, as designing your own battery management circuitry is well above your head.
Darsys:
Hello, thanks for your quick answer !
I make a test, my arduino load the super capacitor for 1 hour, (5V,26mA). Then I tried to supply the Arduino with the SuperCapacitor (4.7V), I connect it for 1 minute then Voltage drop to 2.7V.
DO you have any link to a DC-DC step up converter ? I find this one but, delivery time is too long and i can't find a similar on RS component, maybe if you know any other good one.
I use a resistor only to load the supercapacitor, is it better to use an active current limiter as you said even if its only to load ?
[u]Here is a capacitor discharge curve[/u]. Notice it's the opposite of what you want... It's the opposite of an "ideal battery" because it discharges very-quickly at first then it levels-off the more it's discharged.
DVDdoug: [u]Here is a capacitor discharge curve[/u]. Notice it's the opposite of what you want... It's the opposite of an "ideal battery" because it discharges very-quickly at first then it levels-off the more it's discharged.
Yup. That's why a switch-mode converter is needed. To boost that sagging voltage back to 5V [or whatever the target voltage is].
A constant 26 mA - typical for an Arduino running in active mode - will drop a 1 Farad capacitor voltage at 26 mV per second. So you would lose one volt each 39 seconds. Clearly that will not be very useful. As a backup for a RTC drawing a microamp or less, usable.
DVDdoug: [u]Here is a capacitor discharge curve[/u]. Notice it's the opposite of what you want... It's the opposite of an "ideal battery" because it discharges very-quickly at first then it levels-off the more it's discharged.
Well, actually, it works slightly differently in practice. You wish to use the power - energy - stored in the capacitor. Let's say you have a (switchmode) power converter to compensate for the falling voltage. By the time the voltage falls to half the initial, the remaining energy is a quarter of the original so you have already used three-quarters of the available energy; if it drops to a quarter of the original, you have only a sixteenth left. So there is simply no point in scavenging at smaller voltages, it is not "levelling off"!
If you have a device that will function over a 2:1 voltage range, such as an Arduino that will run at 2.5 V, it makes no sense to use power converters.
Paul__B:
If you have a device that will function over a 2:1 voltage range, such as an Arduino that will run at 2.5 V, it makes no sense to use power converters.
Unless, of course, the Arduino is interfacing with a device that has a more limited voltage range. Also, in the case of Arduino flavors that use a linear regulator, as the dropout voltage increases, there will be more wasted power. Not sure if that's significant but perhaps worth considering...Paul?
Of course you bypass the regulator as you are at 5V already by using the 5V pin, not the Vin pin, so that's irrelevant.
A boost regulator will draw more current from the capacitor as the voltage drops, all the while losing 10-20% of power in the conversion, a loss that increases as the input voltage drops. If at 5V the output is 26 mA, the input will be about that. But at 2.5V input the regulator needs 5/2.5 * 1/0.9 * 26 = 58 mA to produce a 5V 26 mA output, assuming a very good 90% efficiency.
So quite likely your supercap will see a discharge that increases as the voltage drops. That 26 mV/s will have have increased to 58 mV/s when the charge reaches half, and a whopping 300 mA by the time the charge reaches 0.5V, the minimum of what your boost converter can work with.
This will give you about a minute of power from that supercap.
In contrast if you would clock down your Arduino you can let it run at much lower voltage, while taking much less current, and the current drops as the supply voltage drops (see curves in the data sheet). The ATmega328p can work at voltages as low as 1.8V, as long as the clock speed is low enough. That way you may be able to get 10-20 minutes or so out of that supercap.
wvmarle:
In contrast if you would clock down your Arduino you can let it run at much lower voltage, while taking much less current, and the current drops as the supply voltage drops (see curves in the data sheet). The ATmega328p can work at voltages as low as 1.8V, as long as the clock speed is low enough. That way you may be able to get 10-20 minutes or so out of that supercap.
Also, if the Arduino is only needed periodically, or sporadically [i.e not constantly], then putting it to sleep during those idle times, is another way to conserve power.
Hi, thanks for all these details !
Finally, i take a 3200mAh battery, i found a shield to reload it with another USB cable.
I got like 5hours autonomy with a 200mAh, so here around 80hours in theory.
Thanks again for all the helps and details on the use of a supercapa !
![[u]Here is a capacitor discharge curve[/u]](http://www.learningaboutelectronics.com/images/Capacitor-discharging-graph.png){kind=link}