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I need to use this controller with an external applied voltage to control a fan: Currently it is controlled manually by a knob. I need to see the internal circuit to interfere.

How can I find the schematics of this product? Is this a DC to AC converter?

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That looks like an SCR-based speed control for induction motors. They work just like a lamp dimmer, delaying the turn-on during each AC half-cycle, except that they use SCRs instead of a triac (because of the inductive load).

The turn-on of the SCRs are typically done by the rising voltage of the AC line charging a cap on each AC cycle, at a rate determined by the speed-pot (also like a lamp dimmer). There is probably nothing inside there that is low voltage.

To control it from an applied voltage, you could replace the pot/cap delay with some other form of delay circuit, but it would need to be able to trigger the SCRs and survive in a 230 volt environment.

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    \$\begingroup\$ Do you mean the circuit changes the AC amplitude but not the freq? \$\endgroup\$ – user16307 Apr 21 '16 at 12:50
  • \$\begingroup\$ please see my related new question thanks: electronics.stackexchange.com/questions/229730/… \$\endgroup\$ – user16307 Apr 21 '16 at 17:54
  • \$\begingroup\$ Yes. The line frequency is passed straight through, with the duty-cycle (and therefore the average voltage) determined by when the SCRs fire each half-cycle. I will check your related question now. \$\endgroup\$ – Mark Apr 21 '16 at 22:07
  • \$\begingroup\$ Please see the photos from the circuit here electronics.stackexchange.com/questions/229730/… II understand why there is full wave rectififer. It is not like a typical dimmer circuit. Is this circuit is similar to postimg.org/image/carep5kkx ? Why do you think there is a rectifer? \$\endgroup\$ – user16307 May 14 '16 at 14:54

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