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LTSpice, Isolated Circuit Simulation

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ACharnley

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Hi,

I have a 40V max AC circuit which is switching in a lot of MLCC's. At 50V these are very expensive and take up floorspace. I only need to switch them in up to 16V so I came up with the following to cut the power at a particular point then I can reduce the cost/size of these. The problem is without a resistor to ground the circuit will hang, but with it the graph is inaccurate as it's relative to ground. In other words, how to create a second ground and tell LTspice to use it for the isolated part of the circuit?

Regards, Andrew

PS) I haven't verified the op-amp yet, may have the +/- the wrong way round.
 

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alec_t

Well-Known Member
Most Helpful Member
The problem is without a resistor to ground the circuit will hang
Spice is happy if you use a ridiculously high value resistor to ground, e.g 1G, to maintain virtual isolation.
 

eTech

Active Member
I tried the circuit without R1....it didn't hang.

It might be the opamp model you are using. I used an LM393 since you didn't include the opamp model.

eT
 

crutschow

Well-Known Member
Most Helpful Member
Spice can only have one ground reference.
But you can separate the circuits with a high value resistor as Alec suggested.
In the real world there's always some finite (albeit very large) resistance between isolated points anyway, unless it's floating in space.
 

ACharnley

Member
I made a few spurious changes and got it working.

I hadn't seen this method of creating an isolated fet driver using a pwm and capacitors - it's working well!
 

unclejed613

Well-Known Member
Most Helpful Member
you could also create a node with a label named "gnd2", connect it to gnd through a 1G resistor. instead of using a ground symbol, you would use a label flag (i usually use the "bidirectional" label). that way you actually have a "floating" ground. to connect other things, similar to what you did with "ac2" and "ac2x"
 
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