November 28, 2012

Inrush Current in single phase rectifier

Hello friends, this post is about simple full bridge rectifier.

As, we know that un-rectified output from single phase full bridge rectifier is some what like shown below.
full bridge rectifier

But we or clearly speaking our devices don't like this type of voltage so what we do is simply put a capacitor at the output of rectifier called the "Filter capacitor" this capacitor works fine for smoothing the voltage output as shown in below figure.

full bridge rectifier capacitor filter

it is taught in school or college that as we increase the capacitor or decrease the capacitor value the smoothness changes.

By increasing the capacitor..
full bridge rectifier capacitor filter
  By decreasing the capacitor...
full bridge rectifier capacitor filter

Now some smart ass would say that lets make the capacitor value infinity and we get perfect DC Output from the rectifier.

Huh that's nice, if we ignore initial current flowing from the source to the capacitor.
now have a look at the value of initial current flowing from source to charge the capacitor in initial time because at the start time the capacitor would be fully discharged....

When the capacitor is small

inrush current because of rectifier filter

When capacitor is huge
inrush current because of rectifier filter

Now that's a problem huh!!!!!

so what to do to solve the problem?
There is a magical device called NTC (Negative Temperature Coefficient) resistor
as the name suggest when resistor becomes hot it's resistance decreases.
Initially it helps it softly charge the capacitor but when current passes from it it becomes hot and resistance decreases so that there is not any drop across the resistor...
inrush current supressor

By increasing the value of soft-charge resistor we are able to charge capacitor even slowly.... as shown in below figure.

soft charge of capacitor in rectifier
this can help you protect your equipments and fuses......    ;)

if any problem or suggestion please feel free to post your comments....


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