Jan 02, 2012 Trophy points. Activity points. I have done a control circuit that needs to operate two contactors (with 240v ac coil). I'm experiencing interference on the circuit, the LCD goes mad when installed. And after some research I found that maybe, using an RC Snubber circuit on each coil would resolve the problem. Jul 06, 2017 New online calculator: Snubber capacitor calculator by Mads Barnkob July 6, 2017 Calculate the needed snubber capacitance in order to protect your inverters IGBT/MOSFETs from too high swithcing transients, depending on your busbar layout and stray inductance. RC absorption circuit module used on protecting the relay or thyristor because it can avoid damage from inductive electromotive force generated by the inductive load when on or off. Preventing electromagnetic interference and increasing the anti-jamming capability of the microcontroller. For AC or DC 5 400V inductive load (less than 1000W.

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RC Snubber Calculator Spreadsheet

In power switching applications, a designer often has to contend with spurious oscillations. These are the result of parasitic capacitance and inductance on the board and behave like the step response of an RLC circuit. These transients can induced undesired noise in neighboring circuits and create over voltage spikes that can compromise long term component reliability.

Figure 1 - Typical RC Snubber Circuit

The most common way to mitigate the effects of these transients is by using an RC snubber circuit. There are many excellent sources on the web discussing this topic so I won't repeat the discussion here (see references at the end for more information). Instead, I'm offering an Excel-based calculator that simplifies the components selection should you be faced with this task.

The Spreadsheet

The spreadsheet can be download through the link below:

Rc Snubber Design

And here's an Excel 2003 compatible version as some users can't open the latest excel formats.

Ac Rc Snubber Calculator Download

The spreadsheet is quite simple to use (See Figure 2).

Figure 2 - The Spreadsheet Calculator 'Interface'

On the left side, colored in green, is the 'inputs' section. In order to precisely 'tune' the RC snubber, you should measure the frequency of the oscillations (without having the snubber) using an oscilloscope and enter it under fRING.

The next step is to add a know value test film capacitor in parallel with the MOSFET (CTEST) and measure the new oscillating frequency as a result. Finally, you must select the damping factor for the desired response depending on how much overshoot you can tolerate. There's a graph in this document illustrating the normalized effect of the damping factor on the response.

The 'results' section of the spreadsheet (colored orange) automatically calculate the recommended R and C values. The estimated parasitic capacitance CP and inductance LP are also displayed for reference. Power dissipated in the Rs resistor is also estimated using the maximum supply voltage (VS) and the maximum switching frequency (fS) so you can select a resistor with the appropriate power handling capability.

A table with standard E-series values is included for convenience.

Enjoy!

References

Ac Rc Snubber Calculator Free

Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com

Hoping someone can help me out on my first pcb project please. Just got started learning about Arduino and as in lockdown boredom got right into it, learned basic schematic and pcb design on EasyEDA and loads of other things right back to school and Ohms Law again. Been stumped a few times and found the solutions, but I cannot for the life of me find an answer to calculate rc snubber values for arc supressression on my 12v contactor. I think I could work it out if I knew the Henry value for my loads inductance, but all I know is it's a 60 w, 240 mains ac water pump (aquarium type). So I don't know the energy the capacitor needs to absorb. Is it feasible to ballpark this?

Concensus seems to be snubbers are better across the load than the contacts switch. MOV across load is also recommend. 'Standard' snubber combos for mains AC are around 100ohm and 0.1mf, not sure if thats just too much for my setup. I'd guess I could do without a snubber but I'd like to learn anyway since I've come this far and operate good practice. Simplified schematic below.

Ac Rc Snubber Calculator Free

Any advice (other than you don't know what your doing mate give up) appreciated thanks.