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Topic: [RESOLVED] Will an RC snubber across relay contacts conduct to the load? (Read 218 times) previous topic - next topic

raschemmel

In a relay contact RC snubber application like the one shown , why doesn't the snubber conduct the ac through the resistor and capacitor ?
I realize I should already know this but truthfully I have never thought about it before and was wondering why the
load doesn't remain powered through the snubber when the relay disengages and the contacts open ?
Yes, it's a dumb question but I have to ask because somehow the answer isn't coming to me...
It says here there is no current flow through an RC snubber across the contacts when the switch is open but it
doesn't say why.
Quote
If the snubber is connected in parallel to the load, the current flow is also stopped through the switch and the snubber circuit becomes active. In contrast to the first method there is no current flow when the switch is open but there is a leakage current when the switch is closed.

PerryBebbington

Of course it does.

You are asking the wrong question. You should be asking if it conducts enough to matter.

raschemmel

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You should be asking if it conducts enough to matter ?
Does it conduct enough to matter ?

PerryBebbington

Well, I wanted you to go away and think about it. What do you think?

raschemmel

I think when the capacitor becomes fully charged it will stop conducting because even though it has 0V on it at
zero crossing , that is not the same as discharging a cap to ground because it is not being connected to ground so 
the initial state is the current conducts through the contacts, bypassing the higher resistance of the resistor, then
the second state is contacts open and current must find a path through the resistor which then charges the cap .
But when the cap is fully charged and the negative ac transition doesn't the negative voltage do the same thing
in reverse ? Also, it would seem obvious that only a non-polarize cap would work in this application. By Ohm's
Law the current through the resistor is a function of the resistor value and if , for example, the load is a 100W
incandescent lightbulb. If the resistor is large enough it might not conduct enough current to illuminate the bulb
or the bulb might be dim. I'm still not seeing the factor that prevents it from powering the load to some degree.
What am I missing ?

DVDdoug

Quote
I think when the capacitor becomes fully charged it will stop conducting because even though it has 0V 
You can't pass DC current through a capacitor but this is AC and the capacitive reactance (Ohms) depends on the capacitor value and frequency.

Quote
for example, the load is a 100W
incandescent lightbulb. If the resistor is large enough it might not conduct enough current to illuminate the bulb
or the bulb might be dim. I'm still not seeing the factor that prevents it from powering the load to some degree.
What am I missing ?
Probably less than one volt, but without a load you'll get the full voltage.

raschemmel

Then, for a 0.047uF cap at 60 hz , that would make
XC = 1/(2*Pi*60Hz*0.000000047 F)
    =  56,437.9 ohms
Xc = 56,437.9 ohms
P=I x V> I=P/V
I = 100W120V= 0.8333A

If my logic and math is correct, a capacitor with an
Xc of 56,437.9 would conduct 0.0000177A
I= V/R
 = 120V / 56,437.9 Ohms
 = 0.0000177A (17.7uA), which , conducting through the filament equivalent  resistance of (120V/[(100W/120V)]A)
[(120V/0.833A) = 144 ohms)]   V=I x R
                                                    = 0.0000177A x 144 Ohms 
                                                    = 0.0025V. (2.5mV)

I think what I forgot was that for AC the cap has reactance which allows the application of ohm' law to calculate
what , if any ac passes through the RC snubber.




PerryBebbington

Quote
Does it conduct enough to matter?
I think you answered your question.

arduarn

It says here there is no current flow through an RC snubber across the contacts when the switch is open but it
doesn't say why.
Quote
If the snubber is connected in parallel to the load, the current flow is also stopped through the switch and the snubber circuit becomes active. In contrast to the first method there is no current flow when the switch is open but there is a leakage current when the switch is closed.
I think you are simply misreading it. The lower diagram on page 1 of the application note applies.

The previous paragraph also states:

Quote
If the snubber is
connected in parallel to the switch, the current flows through the
protection circuit at switch-off and then decays. Because of the
connection in parallel to the switch, there is a constant current
flow through the snubber when the switch is open. To keep this
current flow to an acceptable level, a well designed circuit is
needed.

raschemmel

Quote
Because of the
connection in parallel to the switch, there is a constant current
flow through the snubber when the switch is open. To keep this
current flow to an acceptable level, a well designed circuit is
needed.
I think that means the capacitor value needs to be chosen for a sufficiently high XC to limit the current to an acceptable value. I am still not clear on what the design criteria is for the resistor value. I originally thought
it was a function of the load current but now I am not so sure.
I also understand that for a  properly designed RC snubber , the energy in joules (J) of the surge or spike to be
suppressed should be known, and then the cap value is
               2*E(J)
  C =   ---------------
           [VSWITCH(MAX)]2

raschemmel


Paul__B

In general, the current through the snubber is much less than the required operating current of the load, such as a solenoid or motor, so it does not move.

In terms of safety however, specifically personal safety, it is extremely dangerous.  :smiley-eek:

raschemmel

What is the method for choosing the resistor value?

Paul__B

I was about to answer that, but noted that your original circuit does not show an inductive load, so there is no actual purpose for a snubber!

PerryBebbington

Paul,
Probably worth answering for the benefit of others who might want to know (me for example).

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