# Double Switch LT Spice

Discussion in 'Homework Help' started by Bigcountry, Jul 29, 2014.

1. ### Bigcountry Thread Starter Active Member

Jul 4, 2008
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I having to use Lt spice to input the circuit below. There appears to be a double switch but I cannot find it in the library. However, I did try to use the single switch in a configuration but cannot get it to run correctly. I am thinking I am almost there.

I am not sure if you can use the switch I have selected in that configuration.

thanks for any input anyone can give me.

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Is the intent that each switch has a "make-before-break" form-C contact? If so, you need to account for a short period where node V(a) is momentarily grounded as the switch wiper moves from one end to the other.

Here is how to do that:

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3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Looks like each switch has three states.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Yes, but is it A, both open, B (break before make)
or is it A, AB, B (make before break)?

5. ### Bigcountry Thread Starter Active Member

Jul 4, 2008
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I think I see how you did it. I am assuming you set this up in a logic format. I never understood it until I wrote each condition out. Is that how you come about that solution?

6. ### MikeML AAC Fanatic!

Oct 2, 2009
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When modeling something, you first have to understand the reality. The question is, how does the switch work?

Here is the opposite case: break before make. A complication is that if you open-circuit a current-source, the voltage goes infinite. To get the simulation to run, I added a Zener across the current-source so that the voltage is limited to the break-down voltage of the Zener (100V) during the 20ms while the current-source would otherwise be open-circuited.

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