Author Topic: Modeling a PNP circuit in LTspice, is this a short?  (Read 769 times)

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Offline YaminTopic starter

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Modeling a PNP circuit in LTspice, is this a short?
« on: April 30, 2020, 11:26:06 pm »
Hi guys,
I was just messing about modeling a PNP circuit in LTspice. I made a mistake with the first circuit where I connected the second voltage supply directly to ground. I measured the current through the voltage source and the plot shows the result. Isn't the voltage source connected directly to ground? Shouldn't it show a dead short/infinite current draw?
The correct circuit is on the right.
Thanks in advance for the help.
 

Offline Jay_Diddy_B

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Re: Modeling a PNP circuit in LTspice, is this a short?
« Reply #1 on: April 30, 2020, 11:50:34 pm »
Hi,

In this circuit the voltage across the 250 Ohm resister is

12V + 5V - Vbe = 16.3

The current is

16.3V / 250 \$\Omega\$ = 65mA

This is very close to what LTspice is giving you.

Regards,
Jay_Diddy_B
 
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Offline YaminTopic starter

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Re: Modeling a PNP circuit in LTspice, is this a short?
« Reply #2 on: May 01, 2020, 12:11:17 am »
Hi,

In this circuit the voltage across the 250 Ohm resister is

12V + 5V - Vbe = 16.3

The current is

16.3V / 250 \$\Omega\$ = 65mA

This is very close to what LTspice is giving you.

Regards,
Jay_Diddy_B

Thanks Jay,
I'm sorry I am finding it a bit hard to follow your calculations. Are you doing it for the loop of the circuit on the right? Hope you would elaborate this for me.
How is connecting the positive terminal directly to ground not shorting it?

Thanks so much again.
 

Online langwadt

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Re: Modeling a PNP circuit in LTspice, is this a short?
« Reply #3 on: May 01, 2020, 12:24:09 am »
Hi,

In this circuit the voltage across the 250 Ohm resister is

12V + 5V - Vbe = 16.3

The current is

16.3V / 250 \$\Omega\$ = 65mA

This is very close to what LTspice is giving you.

Regards,
Jay_Diddy_B

Thanks Jay,
I'm sorry I am finding it a bit hard to follow your calculations. Are you doing it for the loop of the circuit on the right? Hope you would elaborate this for me.
How is connecting the positive terminal directly to ground not shorting it?

Thanks so much again.

to get a short you need to connect both terminals to the same thing, you have one side to ground the other to a resistor

you can flip it around and set the voltage to -5V it is the same
 

Offline Jay_Diddy_B

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Re: Modeling a PNP circuit in LTspice, is this a short?
« Reply #4 on: May 01, 2020, 12:27:51 am »
Maybe this will help:




There is 17V between A and B

There is 17V - VBe = 16.3 between C and B

Does that help?

Jay_Diddy_B
 
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Offline YaminTopic starter

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Re: Modeling a PNP circuit in LTspice, is this a short?
« Reply #5 on: May 01, 2020, 09:49:18 am »
Thanks so much it really helps, really do appreciate it :D
 


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