Electronic – Choosing components for a triac’s snubber

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.

Your simulation is far too pessimistic, because you are opening your switch at a zero-crossing of the source voltage. Because of the inductive nature of your load (the motor), this corresponds to nearly the peak current flow.

In actuality, your triac will conduct until the current zero-crossing, at which time, there will be very little energy stored in the inductance of the load.