### Author Topic: Series switch voltages (check on fundamentals)  (Read 371 times)

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#### JourneymanWizard

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##### Series switch voltages (check on fundamentals)
« on: June 12, 2018, 07:47:28 pm »
Fundamentals check on rating switches (and switch-like elements):
(in fundamentals land, assume ideal elements unless necessary to do otherwise)
(There is a picture attached)

Imagine two simple switches [S1 and S2] in series, with a resistor [R] between them.  Imagine that a potential is developed across the two opposite unconnected ends of the switch [V1-V2].

When both switches are closed, the voltage across each switch is 0V, the voltage across the resistor is [V1-V2], and the current through each switch (and the resistor) is [V1-V2] / R.  The switch has to carry that current.

When S1 is opened and S2 closed, current drops to zero.  The voltage across S1 is [V1-V2].  The voltage across R + S2 has to be zero, as no current is flowing through that branch.  The switch S1 has to withstand [V1-V2].

When S2 is opened and S1 closed, the situation above is repeated for S2: S2 has to withstand [V2-V1].

When S1 and S2 are both opened, no current flows through resistor R; therefore the voltage drop across R is 0.  The withstand voltage of S1 + S2 has to be [V1-V2].  If the switches are in all ways equal, the withstand voltage can be [V1-V2] / 2 per switch.

... in posting, I think I Rubber Duck'd my original question (what are the minimum voltage ratings for two open switches).  Please let me know if you disagree with any of the above.

#### james_s

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##### Re: Series switch voltages (check on fundamentals)
« Reply #1 on: June 12, 2018, 07:58:25 pm »
The minimum voltage rating of each switch has to be equal to the voltage into the circuit, because if one or the other switch is closed that is the voltage that will be present across the open switch.

#### JourneymanWizard

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##### Re: Series switch voltages (check on fundamentals)
« Reply #2 on: June 12, 2018, 08:28:56 pm »
Agreed, unless both switches are always opened/closed simultaneously.  I think in that case that the rating can be [V1-V2]/2.  That was the point I was puzzling over a bit.

EDIT: Found an application mentioned on Wikipedia involving stacking.  Will want to think about it, but gives some ideas
https://en.wikipedia.org/wiki/Cascode#Other_applications
« Last Edit: June 12, 2018, 10:15:17 pm by JourneymanWizard »

#### tpowell1830

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##### Re: Series switch voltages (check on fundamentals)
« Reply #3 on: June 12, 2018, 10:09:49 pm »
Agreed with other posters. However, the voltage rating should be about 20% higher than V1-V2 for normal applications, but if there is a coil involved, the rating should be even higher (get out the calculator to determine flyback, etc.).

Hope this helps...
PEACE===>T

Smf