Choosing TRIAC snubber resistor for multi-purpose switching

I'm designing a circuit to switch a 240V AC load and haven't done a lot with AC power control before. I'm planning to use a Fairchild MC3043-M optically coupled TRIAC driver along with a BT138-600 NXP BT138-600 TRIAC. Referring to the following diagram from the datasheet:

The comment is made that for highly inductive loads (power factor < 0.5), change this value to 360R. One load I'm switching is an AC fan (0.8A) which is obviously inductive although I have no idea of the power factor likely, and the other is a router using a 20W switching supply.

My question is to make the circuit universal considering it's not for a commercial product design and I may use for other purposes in the future is there any disadvantage to always using a 360R (well guess I'll use 390R) other than needing a higher power rating for the resistor? Also any hints on calculating the power dissipation through the resistor assuming a 5A load which is what I'm planning to use as a fuse value?

Answer

TRIACs switch off at (near) zero current. It is common for switches that switch passively at zero current to experience a voltage step that will cause circuit parasitic inductance and capacitance to ring. There are 2 problems:

• The peak voltage of the ringing can exceed the rating of the TRIAC.


• TRIACs also have a maximum rated $\frac {\text {dV}} {\text {dt}}$ that if exceed will cause the TRIAC to spontaneously trigger.

A snubber, like in your Figure 13 would be used to dampen energy in the parasitic elements. The inductance will be in the load ($L_L$), since it is common to use TRIACs for motor control which are inductive. The parasitic capacitance is the capacitance of the TRIAC $C_T$. The snubber works by providing an impedance match to the $L_L$ $C_T$ resonance. Snubber resistance $R_s$ is added to the load resistance $R_L$ to match characteristic impedance Zo = $\sqrt{\frac{L_L}{C_T}}$. They tell you to use a higher value for $R_s$ for loads with higher inductance because Zo increases with increasing $L_L$.

Typically you will want to use a value for $C_s$ that is 10 times $C_T$. For a medium sized TRIAC (one that handles about 10A) $C_T$ is often about 100pF. I didn't see a spec for $C_T$ in the datasheet for the NXP BT138 TRIAC. The best value for the $R_s$ is Zo-$R_L$.

Here is a link to an app-note that provides more detail.