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In a circuit like the one below, why is capacitor C1 needed? I mean I can make do with just a resistor and pot to attenuate the voltage to some level suitable for switching.

What does using capacitors add to the circuit, and what is it's advantages and disadvantages over just using simple resistors voltage dividers instead?

Also in the circuit in the 2nd graph I am not so sure what the purpose of adding C2 and R3.

Dimmer 1

Dimmer2

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    \$\begingroup\$ Because what you need to do is delay the turn-on point, and a capacitor together with a resistor form a delay, characterised by the RC time constant. \$\endgroup\$ – user_1818839 Oct 1 '20 at 17:32
  • \$\begingroup\$ @BrianDrummond What I need to do ( at least the way I see it ) is turning on the triac in a specific point on the waveform by just attenuating the main's to some degree I can make do with Voltage dividers to attenuate the main's for the diac to the point where it reaches half of it's first interval or It's peak for instance \$\endgroup\$ – Mahmoud Salah Oct 1 '20 at 17:35
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    \$\begingroup\$ That approach cannot delay turn-on beyond 90 degrees for obvious reasons. Which doesn't give an adequate dimming range. \$\endgroup\$ – user_1818839 Oct 1 '20 at 17:48
  • \$\begingroup\$ @BrianDrummond, please expand your comments into an answer so I can upvote it. \$\endgroup\$ – TonyM Oct 1 '20 at 17:55
  • \$\begingroup\$ @BrianDrummond LOL I totally missed that So the capacitor is there to make sure that you can Dim the Waveform to the extent beyond just it's peak voltage I mean you can turn on the triac on the falling edge of the main's too Did I get it right Now? Or still something is missing? \$\endgroup\$ – Mahmoud Salah Oct 1 '20 at 17:59
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Because what you need to do is delay the turn-on point, and a capacitor together with a resistor form a delay, characterised by the RC time constant.

The delay should be adjustable anywhere between 0 degrees (full brightness) and as close as you can get to 180 degrees (minimum brightness), to cut out almost all of each half cycle.

The suggested approach of using a potential divider, with no capacitor, will allow delays up to 90 degrees (the peak of the AC waveform) and no further. Which doesn't give an adequate dimming range for most applications.

Resistive divider will also be quite sensitive to noise and spikes on the rising edge of the AC waveform, which doesn't always resemble a nice clean sinewave. Uneven dimming and random flickering is likely to result, so you'll realistically need a capacitor to suppress noise anyway.

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  • \$\begingroup\$ And what about the R3 And C2 of the second circuit? \$\endgroup\$ – Mahmoud Salah Oct 2 '20 at 8:50
  • \$\begingroup\$ Perhaps the designer thought they give a better dimmer response. \$\endgroup\$ – user_1818839 Oct 2 '20 at 10:18

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