# RC snubber calculation ??

Discussion in 'General Electronics Chat' started by capacitor1, Jun 3, 2013.

1. ### capacitor1 Thread Starter New Member

Jan 12, 2013
10
0
Hi everyone

I found on the internet this information about snubber RC circuit :
there's 2 rules here:

for a inrush current of 7A and coil 220V/50HZ

we found that C=5uF
and R=2.5;

no problem here
but the Power of resistor should be 220^2/2.5=88 watts !!!!?

I don't know where's the error in my calculation and can i use this rule for AC coil ??

thanx

Last edited: Jun 3, 2013
2. ### capacitor1 Thread Starter New Member

Jan 12, 2013
10
0

or I put my thread in the wrong forum (general electronics chat)??

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
No it shouldn't!!!?

4. ### capacitor1 Thread Starter New Member

Jan 12, 2013
10
0
ok but how , how i can calculate that ?? i found other answer that it's enough to put 1W resistor !??why?

thank u

5. ### crutschow Expert

Mar 14, 2008
21,437
6,141
The peak power may be 88W but it's only for a very short time. The maximum energy dissipated in the resistor is no more than the energy stored in the inductor. Since this is typically small, the resistor dissipation will also be small.

If you want to get technical you would need to know the surge energy (or peak transient power) that the resistor can tolerate. Otherwise you just make an estimate and go from there.

6. ### capacitor1 Thread Starter New Member

Jan 12, 2013
10
0
Mr Crutschow

but the energy stored in coil isn't big and equal to L*I^2/2??

for me , Inrush current 7A , and L= 0.55H

E=13.475 J

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Hello Capacitor1,

You need to explain your numbers carefully - they make no sense & possibly for several reasons.

Your main mistake seems to be in failing to understand or consider the transient nature of the response of this circuit.

Present your calculations in a more logical sequence and it might be possible to offer you some guidance.

8. ### crutschow Expert

Mar 14, 2008
21,437
6,141
13.475 J equals 13.475 watt-seconds. So the resistor would have to dissipate, at most, the equivalent of 13.475 watts for 1 second. Likely a 1 watt resistor could handle that if it didn't have to do it very often. What's the rep rate on the transient?

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
I think the OP's problem is that they want to design a snubber for an AC [220V / 50 Hz] solenoid and they are using design information for a DC situation. As they indicated in the first post this is their uncertainty.

With 220V 50 Hz the notional steady state [zero offset] current in a 0.55H solenoid would be ~1.3A RMS or 1.8A peak. So the peak energy storage in the inductor would notionally be ~0.9 Joules.

The situation would be somewhat worse when the inductor is energized from the AC supply such that there is a steady state offset in the AC current - albeit an impractical situation. In any event the energy would be at most ~1.8 Joules were the switch opened at the highest energy condition.

10. ### capacitor1 Thread Starter New Member

Jan 12, 2013
10
0
Hi Eng T_n_K

1- when the switch is closed the current is AC , and all rules will be on AC current.
2-when the switch is opened the kickback current is DC ,and all rules will be on DC current.

thanks

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
You are basically correct.

The AC analysis will tell you the circuit conditions from the time when the switch is closed until the switch is opened.

Once the switch is opened you take the "initials conditions" to be ...
* Inductor Current
* Capacitor Voltage

You then perform a transient analysis to determine what happens as the stored energy in the RLC loop is dissipated in the snubber resistor.

The chief concern of the design process would be to protect the switch from excessive over-voltage. Using your given circuit parameters the peak switch voltage could potentially exceed 850V.

Strictly speaking both the AC and DC analyses might need to be a transient solution since there is no indication from you as to the time frame in which the switch is closed and then opened. One would need to know the particular application you have in mind. It may well be that the switching device is semiconductor in type and in addition to the over-voltage stress issue one would have to ensure the snubber design allowed for the device peak current ratings.

Last edited: Jun 5, 2013
12. ### Pedant Engineer Member

May 27, 2018
129
40
Hey OP, I don't know if this will be of any interest to you now but I do hope you were smart enough to ignore most of the replies you got. Very few of them appear to realise that you are using a 50Hz AC source and therefore the switch operation is likely to be maybe a few times a day or at most 50Hz if the switch is actually a thyristor. I'm guessing it is not operating at 50Hz and therefore you do not need to consider the energy in the coil in quite the way you have. Your main concern should be the continuous power in the resistor because of the series connected capacitor. You seem to be ok with the mathematics so I'll assume you can make those calculations. As for the rest of it, when the switch opens the coil and cap will form a second order circuit and the resistor should be chosen to provide a damping factor of maybe 0.7 for a small overshoot or 0.86 for zero overshoot. Depends on other circuit elements etc. If this thread is not totally dead and you still want the answers, make a reply and I'll check back later.

eddie0x7c3 likes this.
13. ### Wendy Moderator

Mar 24, 2008
21,531
2,975
You do realize very few people will care about a 4 year old thread, We call this necroposting While I have approved it this time it is discouraged.

14. ### Pedant Engineer Member

May 27, 2018
129
40
I understand. I had mistaken the dates on the left hand side to be 'posting' dates when they were something else that has a small calendar as an icon. I have only just joined so not used to the layout. If my reply really is useless, happy to see it deleted, but bear in mind, I found it on the web in an unrelated search so the thread isn't so dead to google anyway ;-)
Regards,

15. ### Wendy Moderator

Mar 24, 2008
21,531
2,975
Naw it will likely just be ignored. But hey, I've been wrong lots of times,

16. ### hoppyest New Member

Nov 20, 2018
2
1
I'm interested in the topic discussed here. This is a question/problem/issue that will never go away... physics, electronics and mathematics vary very little over time as far as the fundamentals go. Coils will still give a kick when they are switched off, noise pulses will still be an issue, people will still confuse AC and DC analyses and steady-state conditions are not the same as transient ones. So, many thanks for not closing the thread, the issue is still very much alive for me. Someone switched off an electric fan running on 250V 50Hz and there was one hell of a bang in my rather costly speakers, so it is very much appreciated!