LTspice - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: October 22, 2019, 04:41:24 pm
content: Hi everyone.I am trying to learn some basics on TLspice and I cannot figure out what I am doing wrong. I am using a P-channel Mosfet, a voltage source and a load. When the voltage source (ig battery) is reversed the Mosfet should be OFF because of Vgs>0 isn't it?Why I am measuring a negative voltage after the Mosfet and current passing through the load??Thanks in advance

author: StillTrying
author_rank: Super Contributor
timestamp: October 22, 2019, 04:53:45 pm
content: I1 is probably producing its own voltage, replace I1 with a 60R.

author: Ian.M
author_rank: Super Contributor
timestamp: October 22, 2019, 04:54:48 pm
content: StillTrying beat me to it.Don't use a current source as a load!!!As he said, replace I1 with a 60 ohm resistor and it will all make sense.I would also suggest putting V1 back the right way round then running a DC sweep with.dc V1 -12 12instead of .tran 1ms, which will show you when the MOSFET's channel has enough gate voltage to take over from its body diode. (Not that simmed gate thresholds are to be trusted - there's far too much variation between batches of the same part.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 22, 2019, 05:06:29 pm
content: To use a current source as a proper load in LTSpice, you can just tick the "active load" parameter in the current source parameters' dialog. It will act as you would expect. Very handy.

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2019, 06:50:15 pm
content: Quote from: SiliconWizard on October 22, 2019, 05:06:29 pmYes, right click the component and click advanced to get the active load parameter.

author: Nikos A.
author_rank: Regular Contributor
timestamp: October 22, 2019, 07:22:10 pm
content: Thank you guys!! I am going to run the simulation tomorrow and return with a feedback

author: Simon
author_rank: Unknown
timestamp: October 22, 2019, 08:45:11 pm
content: If you don't mind readng there is "the book":https://simonselectronics.co.uk/product/the-ltspice-iv-simulator-wurth-elektronik/

author: Nikos A.
author_rank: Regular Contributor
timestamp: October 22, 2019, 08:49:12 pm
content: Quote from: Simon on October 22, 2019, 08:45:11 pmThank you Simon, I didn't know about that!!

author: ogden
author_rank: Super Contributor
timestamp: October 22, 2019, 09:10:48 pm
content: Quote from: Simon on October 22, 2019, 08:45:11 pmSorry Simon, but there'sbetter dealon AmazonSome university classes are done using LTspice. By aiming search queries accordingly you can get some course handouts that most likely is not so comprehensive as book, but may still be helpful enough. Just randominternet search hit.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 23, 2019, 02:06:23 am
content: Hi,Let us explore the effect of checking the load box, that was proposed by Zero999.Here is a model:There is a standard current source I1.A current source with the load box checked. Note that the current is zero for zero volts and goes to the set point at 0.4V This is handy in some applications.There is also a behavioral current source that is 1A if the voltage is greater than 0.And the results:Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 23, 2019, 02:18:00 am
content: Nikos A,Try this:Regards,Jay_Diddy_B

author: StillTrying
author_rank: Super Contributor
timestamp: October 23, 2019, 11:12:00 am
content: Quote from: Simon on October 22, 2019, 08:45:11 pmI think you've overdone the 000s, 1.6 billion LT users?1.6 million ..perhaps.

author: Simon
author_rank: Unknown
timestamp: October 26, 2019, 07:57:42 pm
content: Quote from: ogden on October 22, 2019, 09:10:48 pmTrue, I'm one of themjust wait until I see that asshole from wurth that sold them to me.