[Moved] Calculate the value of Filter capacitor for 5V DC Power supply

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SyedFahad

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Hi, I am trying to make a power supply of 5 V as we all know it used a bridge circuit, then capacitors and LM7805 .

But i want to know how the valu of capacitance is calculated. What is tha formula for it?

If the formula is C=It/V

>Then what will be the value of 'I' and 'V'
>What is 't'?

I have 220V, 60Hz main supply. What are the calculations?
 

You need 1,000u or each amp and then the LM7805 will reduce the ripple by 1,000. You will get an output ripple of between 1mV and 5mV on full load.
 

Hi,

t: with a 60Hz frequency and a full bridge rectifier you get a frequency of 120 Hz (for charging the capacitor). This means a time of about 8ms.
I: the max current your load draws (averaged over 8ms)
U: the max alowable ripple voltage.

example:

Maybe a voltage regulator needs min 7 V to operate correctely.
You need an current of 1A
You have an transformer rated with 7V RMS. this gives about 9.9V peak. subtracting 1.5V of the rectifier, you get a max voltage of 8.4V
So ripple max can be 8.4V -7V = 1.4V.
here U: = 1.4V
C = I * t / U = 1A * 8ms / 1.4V = 5700 uF

This is theretically or mathematically right. but nobody wants to go to the limits.

Klaus
 

Thanks Klaus . I have searched for this every where but didnt find any thing. Your answer is very helpful . But i have a question again that why we used 1000uf capacitor as there is a great difference between the calculated value 5700uf and 1000uf.
Because i have made a supply of 5V having 1000uf, i just want to justify this problem of difference in calculated and used value of capacitor. Thanks
 

Hi,

Why 5700uF instead of 1000uF.

Maybe you use a 12V transformer or you need lower current.
My calculation is just an example, we all are only guessing, because you don't give us your values.
How can we know your situation?.

Klaus
 

KlausST said:
Hi,

Why 5700uF instead of 1000uF.

Maybe you use a 12V transformer or you need lower current.
My calculation is just an example, we all are only guessing, because you don't give us your values.
How can we know your situation?.

Klaus
Click to expand...

"Maybe you use a 12V transformer "

What has the voltage of the transformer to do with it ?????
 

Hi colin,

See my calculations .
If you use a 12V transformer you get about 15.5V rectified voltage.
the regulator needs at least 7V then ripple voltage may be 8.5V

C = l * t / U.= 1A * 8ms / 8.5V = about 1000uF.

Klaus
 

Let's explain how the filter capacitor works.
I have suggested 1,000u for a 1 amp supply.
A 1,000u will perform like this: when you are drawing 1,000mA, the ripple will be 5v.
That means the minimum voltage across the electro must be 8v as the 7805 requires 3v across it to operate the electronics inside the regulator.
8v + 5v = 13v.
The transformer must deliver 13v + 2v across the rectifier = 15v.
A 12v transformer is 12v AC and this will produce 12v x 1.4 = 16.8v
This will be suitable for the project.
Now, the losses in the 7805 when 1 amp flows, will be 14.8 - 5 = 10 watts

What Klaus is adding is this:
When the input voltage is much higher than needed by the 7805, the 5v ripple is above the voltage needed by the regulator and we don't have to worry about it.
 
Last edited:

Hi, I am using 220V-6V transformer for the 5V DC power supply. Sorry i didnt tell the values before.
With the same 7805 regulator.
220V 60Hz main supply. Thanks
 

SyedFahad said:
Hi, I am using 220V-6V transformer for the 5V DC power supply. Sorry i didnt tell the values before.
With the same 7805 regulator.
220V 60Hz main supply. Thanks
Click to expand...

A 6v transformer is not high enough
 

Its a good information,
Capacitor value should be large enough that it can provide enough voltage(+2 volts means 7v for 7805) to the regulator IC while discharging, means voltage across capacitor should not go below 7v.
I have found two article where Capacitance calculation has been explained well:
Capacitor value calculation 1 and
Capacitor value calculation Here 2
 

A 6VAC transformer has a peak of 8.5V and the bridge rectifier output will be a maximum of 6.5V. The filter capacitor is not HUGE so it produces ripple that reduces the 6.5V to maybe only 1.5V. The 7805 regulator MINIMUM input is 7V for it to regulate fairly. Then your 6VAC is too low.
 

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