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### Topic: Using capacitors for power continuity (Read 826 times)previous topic - next topic

#### rturk3

##### Mar 03, 2012, 03:38 am
Hello all, thanks so much for all the help on here. I am powering a cellular module which needs bursts of 2A+ at a time. I have a good regulated 5V source and have seen several people include capacitors in line between the output of the 5V source and the input of the cellular shield.

Do these just provided the requisite voltage for a blip in power? Are these recommended? Can anyone specify a good value for me please. If I install one, it's just wired in series between the power supply and shield?

Thanks very much

#### winner10920

#1
##### Mar 03, 2012, 04:03 am
You wire it in parrallel and the bigger the better, for 2a id say in the 1F range if you can manage, they are a bit expensive tho
and make sure the voltage is rated

#2
##### Mar 03, 2012, 04:08 am
Digikey has "supercaps" that are rated for 5V, and high capacitance.
They can take a while to charge up.

How long of a burst of 2A current do you need?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### jackrae

#3
##### Mar 03, 2012, 09:22 am
You also need to include a "surge control" resistor between the 5 volt supply and the capacitor, since the capacitor will appear like a dead short at the instant you power up the 5 volt supply.  I suggest you use something in the order 2 ohm.  However when your load demands power, this resistor would introduce a voltage drop of 4 volts (at 2 amps) if using only the 5 volt regulator, but the capacitor will then supply the demanded load current at a full initial 5 volts.

#### retrolefty

#4
##### Mar 03, 2012, 10:07 am
Relying on 'hanging a big old cap' on the output of a voltage regulator is most likely not going to fix the 'problem' and likely to interject new problems. Perhaps if you could supply the following data, then there is a chance, using some math to actually determine if a properly sized cap is pratical or useful or not.

Actual current max 'burst' amount.
The duration and period of the 'bursts'.
Your voltage sources max current capacity and output impedance.
Capacitance value and ESR value of the cap proposed to use.

It's always preferred to size the voltage source to handle the maximum current demands in the first place. 'Hanging caps' is the shade tree mechanic version of electrical engineering.

Lefty

#### Constantin

#5
##### Mar 03, 2012, 02:17 pm
It's always preferred to size the voltage source to handle the maximum current demands in the first place. 'Hanging caps' is the shade tree mechanic version of electrical engineering.

Thought some products like the innovative "Tweet-a-Watt" from Adafruit make use of such caps for very good reasons. Bursty power demands like the tweet-a-watt radio transmissions are actually a pretty good reason to use a big capacitor as a storage battery though I agree that using a adequate power supply is a better approach. But even a big power supply would likely appreciate a big smoothing capacitor to help with bursty transients...

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