Hi,
I want to power a project with a 2S Lipo that has a nominal voltage of 7.4 but realistically is somewhere between 6V and 8.5V. I need 5V and 3.3V for an Arduino chip, microSD and some sensors and was wondering if I should choose a linear voltage regulator or a buck converter. I am using the lipo to power two servos and the project won't be running more than 15min at a time so energy efficiency is not a big factor. Is a buck converter overkill here?
-Mark
MarkGoingToSpace:
Is a buck converter overkill here?
That would depend on the project, are you able to provide details ?
When you get up to around 1A it's time to think about a switching regulator. With the servos you're probably at that point.
And when you think about the total cost of your project the extra cost of a switching regulator is probably insignificant.
1/ A lipo cell has a fully charged voltage of about 4.2v, and by the time it gets down to 3.7 volts it's nearly dead. < 10% life left. Hence your minimum working voltage is 7.4 V. Running it down below this will shorten the cell life.
2/ The LM1117 regulator used on most arduinos has a dropout voltage of 1.2 V
3/ hence to get 5v on the arduino (And it'll run OK down to 4V) you need to supply it with at least 6.2 V . .
result : don't worry. You don't need a regulator.
Allan
The microSD card will need a 3.3V regulator, they are power hungry and the Arduino boards'
built in 3.3V regulator is probably not able to cope. A switch mode regulator from the LiPo
pack down to 3.3V will be more efficient than a linear regulator, probably worth it to avoid heat
build-up.
Some SD modules will already have a 3.3V regulator built in so they can run from 5V.
Unless you have high current 5V sensors just use the Arduino's 5V rail for 5V stuff.
Don't forget that SDcard signal must be level shifted, 3.3V is the maximum voltage SDcards
can tolerate. Again some modules may do this for you, check it.
Thank you guys for your quick replies!
DVDdoug:
When you get up to around 1A it's time to think about a switching regulator. With the servos you're probably at that point.
I am feeding the servos directly from the lipo. They have a operating voltage of 6-8.5V. Sorry I should have been more clear on that.
allanhurst:
1/ A lipo cell has a fully charged voltage of about 4.2v, and by the time it gets down to 3.7 volts it's nearly dead. < 10% life left. Hence your minimum working voltage is 7.4 V. Running it down below this will shorten the cell life.2/ The LM1117 regulator used on most arduinos has a dropout voltage of 1.2 V
3/ hence to get 5v on the arduino (And it'll run OK down to 4V) you need to supply it with at least 6.2 V . .
result : don't worry. You don't need a regulator.
Allan
srnet:
That would depend on the project, are you able to provide details ?
I want to make my own PCB with an Atmel 32bit chip, accelerometer, gyro, barometer, flashmemory and SD-slot.
I have read online that you calculate the powerloss by (U_in - U_out)*I. Is there a rule of thumb to calculate how much heat this will produce?
-Mark
Temperature rise (C) = power dissipation (W) x heatsink thermal resistance (C/W).