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# OP amp design - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.

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
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