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# Capacitor voltage storage

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#### Naveed Ahmed

##### Member level 4
Hello dears!

I have designed a Three phase wireless smart energy meter using ATMEGA32 MCU. For energizing the circuit I have used DC power supply but not any battery backup in case of power failure.

When power failure occurs or Load shading occurs my circuit can not send the data due to power loss.. And the Database is not informed about load shading and its time.

Is there any technique by which circuit runs for minimum 15sec after the power failure?so that i can inform database abt failure of power? Should i use a big capacitor with power supply?

What are the charging, discharging time equations.? which equations are used??

Cdv/dt???

or E=(1/2)CV²???

Thanks

Regards
NAveed khowaja

how much does your whole circuit consumes? Moreover 15 secs is too much for a capacitor

Thanks nandhu015 for interest

My circuit board consumes 50mA, while Wireless mesh module is of 500mA. Circuit consumes a total of 550mA.

So for how long a capacitor can store charge/voltage??

Thanks

Regards
Naveed Khowaja
coolnaveed1@hotmail.com

E = 0.5*CV^2

So if your supply voltage is 5V, and it is allowed to drop to 4.5V, you can calculate what you need as follow :

0.55*5V = 2.75W for 15 seconds = 15*2.75 = 41.25Joule

So, 4.75 = 0.5*C*(5^2 - 4.5^2) => C = 2 Farad!

The capacitor is a bit high, but you can do two things (or both):
- supercapacitors : with these you can easily reach this level of capacitance
- choosing a higher voltage to start with, and use a regulator to create the 5V (for example a buck converter) : example 24V allowed to drop to 8V gives a capacitance of 0.019 Farad (~20000µF), still a bit impractical in size (and I didn't include the efficiency of the converter in the calculation).

You can also try to reduce the time you need the backup, what do you need to do during the 15 seconds?

Stefaan

ya, forget the capacitor. i would diode OR your supply with a small button battery (watch batery).

Mr.Cool

Naveed Ahmed said:
I have designed a Three phase wireless smart energy meter using ATMEGA32 MCU. For energizing the circuit I have used DC power supply but not any battery backup in case of power failure.
When power failure occurs or Load shading occurs my circuit can not send the data due to power loss.. And the Database is not informed about load shading and its time. Thanks
Regards ...NAveed khowaja
You may have already programmed the pulses per Kwh which the Meter IC generates. You may log these pulses in an EEprom, and then send the data wirelessly to the base station for every Unit of power consumed. In case the power is shut down, the part of the pulses are stored in the EEprom and when power resumes, and the pulses adds up to make a Unit, then you may again send the data to the base station intimating the consumption of a Unit. Thus eleiminate any power requirement during power failure. Cheers

Mr. cool, If we use battery with supply. The battery has its life and charging Cycles. Thats why i decided to use capacitor

Naveed

Added after 22 minutes:

Dear Stefaan.

For large capacitors the size of the board will be much higher Can i use RC time constant??

15sec time can be reduced to 6 secs.
T=RC

for that instant of 6 sec, i will read all the phases from IC, will write EEPROM, read RTC, send the data through Gsm. it takes time

Naveed

Added after 15 minutes:

Dear Pranam77.

I think you are talking about Calibration frequency pulse output, ADE7754 provides that.

But there is a requirement to indicate the power failure, and reports must be generated at the base station and on Website.

Naveed

Thats why i decided to use capacitor
If you want to avoid batteries (for understandable reasons) a supercap or high voltage capacitor with a DC/DC
converter, as suggested by Stefaan, are the only meaningful solutions in my opinion. Some board space has to
be calculated for it.

Regarding possible reduction of backup time. Data collection, nonvolatile storage and calculations effectively occur in "no time",
but it's difficult to set a maximum amount for transmission of the GSM alarm message, because it depends on the actual
network traffic. In most cases, it's finished in 1 or 2 seconds, but I fear, the service provider won't guarantee it.
If you only expect a limited number of alarms during instrument service intervals, a replaceable long-life lithium battery
may be a more reliable solution.

Dear Fvm,

How much can be the size of super cap for 6 sec???

Current consumption = 550mA
Voltage = 5V

Regards
Naveed Khowaja

For maximum flexibility in options on how to send the data or store it in EEPROM. It depends also on the requirements : do the user need to be informed when the power is down? or can it wait until the power lines are live again?

I should go for the supercap approach, the board space needed for them is relatively small, but be sure you don't go over there speced maximum voltage (mostly 2.5V).

It should be interested to know where your logic voltage comes from? Can you put a backup before your ultimate low-voltage regulator? It really depends on how your power-supply structure looks like. If possible, upload a block-diagram.

Stefaan

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