# OP amp design - RF Cafe Forums

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song123

Post subject: OP amp design Posted: Sun Sep 18, 2005 10:59 am

Hi everyone,

I am going to design a zero crossing detector with using an op amp.

Then, I found a basic circuit from internet that the circuit uses voltage divider from the main source (230 V) as as input voltage (V+) for the op amp.

One thing that I am not sure is, why it uses coupling capacitor instead of resistor in the voltage divider ? What is the advantage of it ?

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song123

Post subject: Posted: Sun Sep 18, 2005 11:10 am

Sorry for the small image.

Here I make it bigger. Thanks.

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Guest

Post subject: OpAmp QuestionPosted: Mon Sep 19, 2005 11:53 am

It's a matter of numbers.

Resistors convert power=current*voltage into heat, capacitors don't.

Why waste power?

Good Luck!

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Guest

Post subject: Re: OpAmp QuestionPosted: Mon Sep 26, 2005 11:25 am

Guest wrote:

It's a matter of numbers.

Resistors convert power=current*voltage into heat, capacitors don't.

Why waste power?

Good Luck!

Good Try..

The capacitor removes any DC offsets from the generator and the 500k provides the ground reference.

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Guest

Post subject: Coupling capacitorPosted: Wed Sep 28, 2005 5:44 pm

In most areas, the 230 V mains voltage comes from a transformer. Since transformers do not pass DC, there should be *NO* DC offsets on the incoming voltage.

A resistor would, in fact, work in the circuit where the capacitor is shown.

Good Luck!

Posted  11/12/2012