RC snubber calculation ??

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 ??

help me to understand please

thanx

No answer ??!

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

capacitor1 said:

but the Power of resistor should be 220^2/2.5=88 watts !!!!?

Click to expand...

No it shouldn't!!!?

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

please help

thank u

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.

Mr Crutschow

thanx for your reply

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

what about this number ??


please help me to know

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.

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?

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.

Hi Eng T_n_K

I can understand from your reply that:

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.

Please help me to understand

thanks

capacitor1 said:

Hi Eng T_n_K

I can understand from your reply that:

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.

Please help me to understand

thanks

Click to expand...

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.

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.

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.

Wendy said:

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.

Click to expand...

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,

Naw it will likely just be ignored. But hey, I've been wrong lots of times,

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!