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Looking for some help with calculating the proper resistance and capacitance for an RC snubber for arc suppression and protecting relay contacts. voltage across the contacts is about 25-35 volts and upto about 750mA of current. Thank you for your time.
Snubbers are used to protect the contacts (or transistor) switching the coil of a relay (inductive load, and where energy is stored in the moving armature). There, the components are chosen on the basis of the inductance, switching speed, and energy to be shunted.
If you know something about the ultimate load that a relay is switching, then it is that load that determines what (if anything) needs to be done to protect the relay contacts.
Some decent estimate of the inductance of the load is needed. R = 2√(L/C). C=4L/R².
R and/or C are often determined empirically by the application and the other is calculated. Sometimes R is chosen as the characteristic impedance of a longer run of two 16 gauge wires. Sometimes C is chosen to limit the power dissipation to a 1 Watt resistor. Sometimes the DC resistance of the load is enough to reduce the size or value of resistor required.
All in all, you haven't been specific about your load and how often it is being switched, so it is difficult to make any specific recommendations for snubbing that load.
i was a little misleading, it is the contacts on a momentary switch. i attached a pic of the circuit and specs. im putting together a selenoid motor(youtube it). basically a rotating cam will activate the switch once per revolution. just trying to prolong the life of the contacts with this snubber. at this point i can only estimate the speed. i would guess between 1 HZ and upto 15 to 20 hz.
I tuned the RC snubber to match your coil. Bad news is the size of the cap. It has to be an AC capacitor (non polarized), so it will be huge. I'd go with the diode...
I tuned the RC snubber to match your coil. Bad news is the size of the cap. It has to be an AC capacitor (non polarized), so it will be huge. I'd go with the diode...
I tuned the RC snubber to match your coil. Bad news is the size of the cap. It has to be an AC capacitor (non polarized), so it will be huge. I'd go with the diode...
The simulator is LTspice. It is free, and is very good. Many forum members here and on other sites use it.
Attached is a sim of my suggestion to add a resistor in series with the snubber diode. Note that the inductor current decays much faster with the resistor, meaning the solenoid will "relax" much faster. The tradeoff is voltage across the open switch. I doubt that 100V will hurt it - possibly you could go somewhat higher.
Sorry. You need to put a revered biased diode across the solenoid and get rid of the snubber across the switch. You could place a varistor or ZNR across the switch, but more important is the diode.
Snubbers are for AC and at times, noise suppression. One also needs to model the Joules absorbed by the resistor with each switching event and multiply that by the frequency to figure out how many Watts it needs to dissipate. The resulting snubber, assuming a 100V bipolar electrolytic is used, would be huge compared to the diode and less effective too.
As MrRB points out, all you need is a diode. It has to be connected with the right polarity, but that's it.
The reason this works is because you only need to clamp the voltage when the relay is turned off. That's when you see a very high voltage rise due to the coil back EMF. Using a diode provides a path for the current which greatly reduces the back EMF, so it protects the switch. And the switch does not need protection for turn on, because that's when the coil acts as an inductor and limits the initial current. The current then builds up to maximum over a short time period.
The diode should be rated for at least the current in the coil during normal operation. A 1 amp diode would probably be ok here, like the 1N4004 or similar.
The only time a diode becomes questionable is when you have to do super fast switching. Then you may have to use a zener or something else to allow the coil to discharge more rapidly.
An RC snubber is only needed when there is more complicated switching going on where you can not allow reverse conduction by a diode. Then you might also use a diode but the diode there would be for a different purpose.
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