# Snubber cap and R

Discussion in 'Power Electronics' started by Esq, May 24, 2019.

1. ### Esq

15
0
May 24, 2019
Hi Guys,
I am busy building a Triac switch to switch a contactor coil 240 Volts, 9VA holding power, 50 Hz and 507 ohms DC resistance. From the VA I calculated Amps 9/240 =0.0375 Amps.
Z = V/I= 6400 Ohms and Xl calculated 6380 ohms using the standard Z,R,XL formula.Z sq= Rsq +XLsq
Am I correct to say that the snubber cap should ideally have the same reactance as the coil, in this case 6380 Ohms? Roughly ).5 Micro farads for 6380 XC ohms?
Since the coil DC resistance is 507 Ohms, is there any sense in having a any series resistance other than the 507 Ohms DC resistance in the circuit and just a 0.5 micro Farad cap over the coil?
It makes sense to me to put the cap over the coil.
Cobus.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,214
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Jan 21, 2010
The resistor is typically placed in series with the capacitor, not the load.

3. ### Esq

15
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May 24, 2019
Hi Steve, the resistance of 507 ohms is the internal DC resistance of the coil.

4. ### Harald KappModeratorModerator

9,413
1,924
Nov 17, 2011
A snubber will dissipate additional power when operated with AC due to the impedance of the snubber circuit.
In my opinion a much better and energy saving solution to limiting the voltage across the coil at turn-off is a TVS diode. Bidirectional TVS diodes with breakdown voltages of ~400 V (for use with 240 V mains) are available with e.g. 1500 W peak power rating (e.g. 1.5SMC400CA). This diode can withstand up to 200 A surge current (single pulse - I assume you are not going to operate the contactor in a rapid sequence of more than 1 Hz, are you?) and should be up to the task for this application.

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5. ### Sunnysky

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Jul 15, 2016
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6. ### Sunnysky

465
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Jul 15, 2016
I love Falstad's ability to use your imagination. If you can scheme , you can simulate it.

Actually, your RC snubber looks best for EMI and OVP except you draw twice the power. No big deal. TVS is good for Triac protection and faster actuator release
(Tau) but that also generates EMI.

Below I simulated a Triac with ZCS turn off by controlling phase and I controlled turn on time with duty cycle . THen I put in ideal SPDT switch to compare 3 methods in one scope trace ( slow-mo)

Triac noise is on the bottom trace and the control voltage was on a logic control pulse with 90% duty cycle to a switch with 10 mohms (not 100m) So I clicked on each switch during a slow sweep to capture each trace to compare the results. I assume I have shown optimal config. But you can play with this too if you want the simulation link.
Bl;

To be clear, the TVS is better for conducted noise to grid. but worse for local radiation near Triac as it switches fast to the clamp voltage. But you can't put a cap like I did across pulsed Switch(Triac sim) as the coil would conduct when off.

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Last edited: May 24, 2019
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7. ### Esq

15
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May 24, 2019
Thank you, I will play with it and see what the outcome is. Appreciate your advice. Cobus.

8. ### Esq

15
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May 24, 2019
Wow, this is great. Thank you. Will you send me the simulation link please.
My contactor will not be switching much, switch on for about 60 seconds and off for 100 seconds in some cases, but not for fast switching at all. Cobus

9. ### Esq

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May 24, 2019
Hi Harald,
I am looking for this TVS diode to do some experiments with it to familiarise myself with it. Looked at datasheets and the forums ideas. I need to see if the noise radiation will affect my project where I will have at least 8 Triacs on one PCB. Thanks for the TVS reference.
Rgds Cobus

10. ### Sunnysky

465
115
Jul 15, 2016
If you examine my bottom two traces, the noise around the triac and the noise generated on the grid can be easily simulated with a loop of wire and a small relay coil with 50% V to the coil in series with the NC contacts. THis will radiate enough noise to see if your breadboard or protoboard will be immune to interference. The RC snubber is better around the Triac and the TVS with cap is almost as good and better on the grid but it must be a Y cap.

11. ### Esq

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May 24, 2019
Great, thank you! I will do that. What is a Y cap?

12. ### Harald KappModeratorModerator

9,413
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Nov 17, 2011
A Y capacitor is a capacitor rated to be used on mains from line to chassis specifically designed to fail in open circuit mode, not short circuit mode. See e.g. here.