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i just make a voltage rectifier as shown below

enter image description here

Four IN4007S diodes

one 15k ohm resistance

one 25v 2200uF capacitor

I have a 220v->13.5v transformer, and transformer output is supposed to convert to a 12v DC signal by the rectifier circuit. everything just went fine, but strange thing happen about output value, it's 17.97v DC!! do you have any idea why output is so hight, while it must be about 12vDC! I remove the capacitor and measure voltage of resistor and it was 12v, but when i put the capacitor, voltage change to about 17v! why is that?

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13.5 V is what's called the RMS voltage, a sort of average used with AC to make the math work out nicely. For a sinusoidal waveform, like what you get from the power mains or a transformer connected to it, the RMS voltage is equal to the peak voltage divided by the square root of two.

A rectifier with a capacitor will output the peak voltage minus the drop in the diodes, or \$13.5\ \mathrm{V} · \sqrt2 - 2 · 0.7\ \mathrm{V} = 17.69\ \mathrm{V}\$.

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  • \$\begingroup\$ the RMS voltage, a sort of average used with AC to make the math work out It's a bit more than that, suppose I have a resistor and I apply 13.5 V DC to it. That results in a certain power dissipation in the resistor. Now what AC voltage would I need to apply to that same resistor to get the same power dissipation? Answer: 13.5 V rms. \$\endgroup\$ – Bimpelrekkie Aug 21 at 13:35
  • \$\begingroup\$ @Bimpelrekkie Thus making the math work out nicely (P still equals V²/R). I don't want to go into too much detail when talking to someone who's never heard the term before. \$\endgroup\$ – Hearth Aug 21 at 13:37

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