Electronic – Two resistors in series

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I know the summation equation for two or more resistors in parallel or series, and I know two parallel resistors will give more power.

But sometimes I saw some circuits that used two resistors in series, and I am wondering why that method was used and why they didn't use one resistor with a higher value (equal to total series resistors)?

Such as the following circuit diagram, two 33 kΩ resistors used in series. So why doesn't it use one 68K resistor?

Give it better results? I mean, noise filtering or something else?

Note: This circuit is an AC dimmer for a microcontroller.

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Best Answer

It's the voltage rating on the resistors that is important here. They are powered from rectified 230 V AC and they need to have the correct voltage rating to suit their application. Two resistors in series having an individual rating of 200 V gives a total voltage rating of 400 volts (near enough if you ignore tolerances on values).

Take a look at the good old MRS16 and MRS25 range from Vishay: -

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With 230 V AC present, the peak could be as high as 325 volts without even considering line transients. Clearly two resistors should be used. And, for SMT resistors this might be useful to consider: -

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