Designing an RC Snubber for TRIAC motor control

I am designing a snubber to reduce voltage transients for a TRIAC load phase angle control circuit.

The snubber is composed by an RC network in parallel with the load. The load is a 230V 5A electric pump (no more information about it unfortunately) and the TRIAC will be controlled by an MCU which is driving an opto-triac to generate the required turn-on impulse. There is also a MOV in parallel to the snubber/TRIAC to protect from the peak voltage of the transient.

I have looked through different Application Notes regarding RC snubbers in power applications and I tried to follow the calculations used in AN1048/D.

Specifically, I used the TRIAC DESIGN PROCEDURE at page 12 with these data:

$$\V_{RMS} = 230\,V\$$(mains voltage to the load)

$$\I_{RMS} = 5\,A\;\$$(and I assumed a pure inductive load)

$$\\phi = 50\,Hz\;\$$(mains frequency to the load)

$$\{\frac{dV}{dt}}_{max}=5\cdot10^6\,V/s\;\$$(for the chosen TRIAC)

$$\\rho=0.6\;\$$(Figure 6.18 at page 9, this value seems to me a good compromise between $$\V_{pk}\approx 406\,V\$$ and the chosen $$\\frac{dV}{dt}\$$).

The results I have applying the equations in the AN give me a capacitor of $$\23\,nF\$$ and a resistor of $$\3\,k\Omega\$$. My problem is that these values seem far from those I normally see in similar circuits or datasheets (eg. $$\C=0.1\,\mu F\$$ and $$\R=33\,\Omega\$$) and I believe I am not sizing the components properly.

I have tested this circuit with these values for a couple of minutes while changing the phase angle and it does seem to work, however I did not have the chance to verify the peak voltage of the transients as I miss the proper equipment to do so.

I also believe I can reduce $$\R3\$$ to a $$\1/4\,W\$$ power rating but I am not sure about the rating of $$\R6\$$.

Any suggestion is really appreciated. Thank you.

• I do have an oscilloscope but I do not know how to deal with high transient voltages and which probes I would need for that. Mar 22, 2021 at 18:34
• Oh, that's easy then. Do you have x 100 probes? If you don't find some cheap ones off of Mouser or Digikey. You can get them third-party for like $50-$100 if I recall rather than $400 from first-party. Mar 22, 2021 at 18:34 • Unfortunately not, I should definitely buy one. Mar 22, 2021 at 18:35 • mouser.ca/ProductDetail/Cal-Test/… But if you are working off mains you need to be mindful of the fact it is not a differential probe so you can't just instrument across any component you want. Differential probes make things a lot easier but now we're talking about$400 for a dirt cheap one. Highly recommended, but understandable if you can't afford it. You should definitely have a x100 probe though, or a couple so you can simulate a differential probe via subtraction when you really need to. Mar 22, 2021 at 18:36
• Use the app. note from ST, also. 3k ohm makes this snubber useless, IMO. Mar 22, 2021 at 18:40