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Old 18th April 2013, 08:05 AM   #1
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Default Snuber question

theoretically rusty , just asking a question

In a DC circuit where a switch is switching a medium resistive or inductive load example: one switch drives a big relay to turn 6 headlights in a car or one switch that drives directly 2 small head lights in car

Is there a point or use , or anything wrong if a capacitor is used between the switch contacts to suppress the spark ?


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Sakis
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Old 18th April 2013, 09:40 AM   #2
effebi is offline effebi  Italy
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Hi Sakis!
Yes, I believe there is a point , a small capacitor will limit the switch arcing on turn off. Use a high voltage stuff, like a 400 or 1000 V poly stuff, since the back EMF can be quite high and otherwise perforate the dielectric. In fact the spark that you see is the air dielectric of that is perforated. A mains rated X or Y type should be ideal, since they are said to be auto-regenerating.
The counter effect should be only a small delay on turn off, that would be not noticeable if you keep the time constant small.

As a side note, filament lamps are a very nasty non linear load. At cold they are very low resistance, and the will reach their nominal one only once fully light up.
So your switch will have hard life on the turn on action as well, since it will see a current possibly several times its nominal one for a brief period.
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Old 18th April 2013, 10:05 AM   #3
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Effebi fair enough but your example is wrong in a filament application like a tube amplifier you are switching AC in the filaments or even if the filaments are DC you are still switching AC before rectification or regulation

My question has to do strictly with DC applications

If there was a practical use of such an application the #1 field to apply such a practice will be the car industry but still in no car application i have seen anything like that

I am missing something here ...

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Sakis
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Old 18th April 2013, 12:09 PM   #4
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To reduce a spark with DC switching, use a blocking diode, (reverse bias) with a 100nF capacitor across the contacts.
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Old 18th April 2013, 12:16 PM   #5
DF96 is offline DF96  England
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Yes, a cap or snubber across the switch will suppress sparks when opening the switch. The spark is caused by energy stored in the circuit inductance. In many cases the inductance in a DC circuit is sufficiently low that little energy is stored so any spark is very small. In some cases the circuit stray capacitance may be high enough to suppress the spark on its own.
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Old 18th April 2013, 12:24 PM   #6
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ok ...question remains ...

why this is not used in the automotive industry ?
either the spark is too small so nobody will bother
or there is no practical use of the application

Can any come up with an idea may be in switching DC but in higher voltage or power ?
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