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I have a few questions:

1) Could we use an AC rated Contactor for DC switching? I have heard that typically the switching voltage a AC contactor can handle at DC for the same current is 10 times less. How do we arrive at this scaling factor of 10 times? Is it a practical figure or a properly calculated number?
Both of them are DC to DC converters. Both switching regulators and switching controllers can be obtained/configured in either buck (output voltage < input voltage), boost (output voltage > input voltage), or both topologies.
Does it mean that a contactor rated to switch 690V AC, 20A can handle 69V DC, 20A?

2) What is the fundamental limitation on switching DC vs AC. Is it just the longer sustained arc? If so, in case I use proper snubbers/ arc suppression circuits could I make use of AC rated contactors for DC?
A switching regulator works by taking small chunks of energy, bit by bit, from the input voltage source, and moving them to the output. This is accomplished with the help of an electrical switch and a controller which regulates the rate at which energy is transferred to the output (hence the term “switching regulator”).

The energy losses involved in moving chunks of energy around in this way are relatively small, and the result is that a switching regulator can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they can power useful loads from higher voltage sources.

Switch-mode regulators are used in devices like portable phones, video game platforms, robots, digital cameras, and your computer.

Switching regulators are complex circuits to design, and as a result they aren’t very popular with hobbyists. However Dimension Engineering creates switching regulators that are even easier to use than linear regulators, because they use the same 3 pin form factor, but don’t require any external capacitors.
3) Most relays and contactors are rated for resistive and moderately inductive loads. In case I am switching capacitive loads the switching becomes very bad and leads to welding of contacts. What is the best recommended approach to overcome this?

4) What is the fundamental difference between force guided relay and a contactor. Let's say both are at the same current rating.

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