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.