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Lt spice-floating transformer

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boro3

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Hello to all,

is it possible to simulate a transformer (floating) without connection to ground with ltspice?
in this example
http://www.analog.com/media/en/tech...mers-mikeengelhardt.pdf?domain=www.linear.com

if i connect the out of secondary side of the transformer with a resistor to ground (yes ,the transformer in the example is connected to gnd on the secondary side), then there is current flowing to ground, which schould not be happen, because the transformer schould be isolated.
i hope you understand what i mean :D

Thanks
 

ronsimpson

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Most Helpful Member
SPICE does not know how to deal with a voltage that is not referenced to ground. (0)
I will use "left" and "right" ground in this example.
Current flows V1 though L1, GND-left back to V1.
Current flows L2, OUT, GND-right, back to L2.
No current flows GND-lift to GND-right.
When I make a isolated power supply in SPICE; I make the left side and pick a point and call it ground. Then I make the right side and pick a point and call it ground. The two grounds are connected. No current flows just like in the included example. In real life I do not connect them.
upload_2018-4-5_11-33-36.png
 

alec_t

Well-Known Member
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You can also connect a point to ground via a resistor with an arbitrarily high value (e.g 1e9) to keep Spice happy.
 

ronsimpson

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You can also connect a point to ground via a resistor with an arbitrarily high value (e.g 1e9) to keep Spice happy.
When you do that; remember SPICE measures voltages from ground. (you can measure V1-V2 but by default V1-ground)
You can also connect two grounds with a arbitrarily low value (0.01) to keep SPICE happy.
 

boro3

Member
thx for the answers,
ok, but how to deal with this:
what happens if i have a resistive divider as a load on the secondary side, and want to measure the voltage of a resistance with an opamp (differential amplifier) ,
for the op amp, i have to connect a feedback resistor at the non inverting input to ground,
this makes a current flow from the transformer secondary to the feedback resistor of the op amp, which usualy would not happe, because the secondary side is floating...
 

ronsimpson

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Most Helpful Member
Not certain of this is your circuit.
I added more to the blue lines. The signal makes a circle. Transformer, resistors, go ground, back to the output of the amp, resistors and back to the transformer. This circle may include the (+) or (-) 5V supplies. This circle will not effect what is happening on other circuits on other ground points.
upload_2018-4-5_14-55-23.png
 

MikeMl

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boro3 ,

Since you have a floating secondary winding on the transformer, why do you think you need a "differential" amplifer? This can be done much more simply.
 

ronsimpson

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Most Helpful Member
Top two circuits do not effect each other.
Bottom left is the same as bottom right.
If all 6 batteries are on the same board the do not effect each other.
upload_2018-4-6_8-21-18.png
For electrons to flow there must be two wires. Only one wire and no flow. Ground is one wire.
 
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