# how can I determine the Resistanc and capacitance value in a snubber circuit

#### ashokraj

Joined Feb 1, 2018
109
Hi, In the below circuit I would like to know how to determine the the resistance(R9) and capacitance(C6) value ? This circuit is a IOT based fan controller. Here we are using 240VAC, Even though they have shown the values I am interested in knowing how to design a RC snubber circuit for a fan with Triac.

Thank you,
Ashok

#### Ian0

Joined Aug 7, 2020
2,235
That’s a very useful link for anyone wanting to design snubbers for switched-mode power supplies, rather less useful for designing snubbers for triac circuits in AC.
When a triac’s gate signal is removed, it will switch off when the current between MT1 and MT2 reaches zero. On an inductive load this does not correspond to the voltage being zero, the triac enters its blocking state and the voltage across it rises abruptly to the mains voltage, which could, on a very inductive load equal mains peak. If the rate of rise is too high, above the triac’s dV/dt limit, the internal capacitance can retrigger the triac. Triacs have rather better dV/dt limits than they used to. Sometimes they will not need a snubber at all.
Generally, resistances of 47Ω to 100Ω, and capacitances between 47nF and 100nF do the job in the vast majority of cases.
The usual problem is that they let a small amount of current through, which will allow very small loads to continue to operate.
If that is the case, then a similar RC circuit can be places across the load to make it less inductive. If R is the same as the series resistance of the load and C =L/(R^2) the load looks completely resistive, but that may lead to a lot of dissipation in R.

#### Ian0

Joined Aug 7, 2020
2,235
You also seem to have a very peculiar way of driving your onto-triac. The 555 has enough output current to drive the opto directly, and as you want the opto on when the 555 output is low, connect it between pins 3 and 8 with a suitable current limit resistor. Some of the CMOS 555s will work at 3.3V

#### ericgibbs

Joined Jan 29, 2010
12,928
hi Ash,
For the 555 checkout this PDF.
Note the drive capabilities.
E

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#### ashokraj

Joined Feb 1, 2018
109
That’s a very useful link for anyone wanting to design snubbers for switched-mode power supplies, rather less useful for designing snubbers for triac circuits in AC.
When a triac’s gate signal is removed, it will switch off when the current between MT1 and MT2 reaches zero. On an inductive load this does not correspond to the voltage being zero, the triac enters its blocking state and the voltage across it rises abruptly to the mains voltage, which could, on a very inductive load equal mains peak. If the rate of rise is too high, above the triac’s dV/dt limit, the internal capacitance can retrigger the triac. Triacs have rather better dV/dt limits than they used to. Sometimes they will not need a snubber at all.
Generally, resistances of 47Ω to 100Ω, and capacitances between 47nF and 100nF do the job in the vast majority of cases.
The usual problem is that they let a small amount of current through, which will allow very small loads to continue to operate.
If that is the case, then a similar RC circuit can be places across the load to make it less inductive. If R is the same as the series resistance of the load and C =L/(R^2) the load looks completely resistive, but that may lead to a lot of dissipation in R.
Hi Ian,
How did you deduct the value of 47 to 100, and 47nf to 100nf ? I am interested in knowing that formula

#### Ian0

Joined Aug 7, 2020
2,235
SGS-Thomson is the ultimate authority in SCRs and triacs, so read the app note that @ericgibbs posted above.
The values I used were from a product where the load was unknown (customer could connect anything to it), so I had a choose a value that would be effective in most circumstances, and 100Ω/100nF seems to do it, along with a triac with good dV/dt characteristics such as BTA16-600BW - there’s a reason that they are known as “snubberless” triacs!