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This is a four-component-dimmer (R, C, diac, triac) with an extra diode bridge and resistors R2 and R3. What are they for? I've never seen those before. It looks like the resistors are just parallel to R1 + VR1.

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    \$\begingroup\$ That looks familiar :-) I get the feeling that circuit might have been incorrectly copied from somewhere. If you check the page it came from, the is no mention of R2 or R3 values, and R1 is listed as "3.3kµF/50V elect" !? Also the diodes are not set up in a bridge rectifier configuration (maybe a mistake in the schematic) and if this is intended, it's difficult to see what the point of them is. If they were in a BR, then it could be to limit the DIAC firing to only positive swings, so half the frequency. \$\endgroup\$ – Oli Glaser Nov 13 '11 at 11:35
  • \$\begingroup\$ ..however this still wouldn't explain R2 and R3, as they would not be necessary. \$\endgroup\$ – Oli Glaser Nov 13 '11 at 11:37
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    \$\begingroup\$ The way the bridge is placed makes absolutely no sense IMO \$\endgroup\$ – stevenvh Nov 13 '11 at 14:22
  • \$\begingroup\$ kilo-micro-Farads gave me a good laugh! \$\endgroup\$ – DarenW Nov 14 '11 at 8:28
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This dimmer circuit allows better control over power to the load than the basic circuit. The same circuit can be found as figure AN1003.12 in this PDF file which seems to be written by sane people. At least, they measure their resistors in ohms. It's an application notes for triacs from Teccor Electronics published in 2002.

The extra diodes and resistors work the charges on the capacitor in such a way as to eliminate the hysteresis effect. This is when a light dimmer is first turned up, nothing happens until a certain threshold, then you get a medium-brightness glow from the bulb. The dimmer can be backed down for a dim glow. But coming up from dark, you can't get striaght to a dim glow but must go up to that theshold. By fiddling with the charge on the capacitor at the right times, this effect is reduced. I don't totally get it, but suffice it to say that it's not fruitful to think of those diodes as a bridge rectifier, and that an explanation can be found in the PDF.

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