Questions about a dimmer circuit

Question

Below is a simple dimmer circuit using a triac, a cap and resistors. As you can see, the resistor Poti changes with time and the load current change can be seen in the plot. It seems like one can use this circuit to control a light bulb light or a single phase AC motor according to the simulation.

But in many literature or tutorials, always a diac is connected to the gate of the triac (and sometimes a fuse added to the Line). Here is an example:

http://www.eleccircuit.com/wp-content/uploads/2010/09/ac-lights-dimmer-with-triac.jpg

My circuit and the circuit in the link have differences. I have the following questions:

1-) Why is a diac used instead of a resistor?

2-) In my circuit I used R1 as 1k value. What is the best value for R1? How can I calculate it?

3-) The circuit in the link has a series 10k resistor next to the potentiometer. Why would that be needed?

4-) Here is another variant: http://www.electronicecircuits.com/electronic-circuits/filament-light-dimmer-circuit What is the job of C2 and R2 here? Suppressing what?

Answer 1

Your many questions:

1. The diac allows low power dissipation in the trigger circuit and makes it insensitive to trigger current asymmetry and variations.

2. Use the diac circuit, or a triac with a built-in diac.

3. To limit the current through the pot wiper so it is not destroyed when the pot is turned to near minimum.

4. R2 and C2 are a snubber circuit which limits dv/dt across the triac, preventing false triggering due to stray inductance and perhaps reducing EMI a bit.

Answer 2

The diac "opens" at certain defined level (quite precise) and the capacitor is discharged trough the diac forming a trigger pulse for triac. Once the triac start conducting the capacitor amd series resistor are bypassed, so no more capcitor charging. Without the diac, the trigger signal (not pulse anymore) would be more random angle, in simulation is good but in practice is useless.