LTSpice - wtf? - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: April 10, 2014, 01:54:52 pm
content: I'm trying to simulate a relatively simple circuit: P-MOSFET switching a rectified, unfiltered voltage into a resistive load. For some reason adding even a little resistance to the gate causes the MOSFET to be constantly on. I'm pretty sure, that the circuit will work just fine, but I don't understand what makes the simulation go haywire. I have tried different solver settings and simulation settings. I have also tried to vary component values and use different MOSFETs, diodes etc, but It's still the same.Here's what works:Here's what doesn't:Any suggestions?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 10, 2014, 02:55:53 pm
content: Upload a zip file of the .asc file.What is node 007? If you label the schematic it will give the node the same label on the graph. V(n007) red, is out of phase between the two graphs.Current through the tiny resistor?The other thing that works is setting a node to an initial voltage. Like a directive. Forgot the LTSpice name of it though.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: April 10, 2014, 04:15:37 pm
content: Adding the resistor may also change where in the schematic is N007.

author: poorchava
author_rank: Super Contributor
timestamp: April 10, 2014, 04:42:26 pm
content: true, but it doesn't change the fact, that the mosfet becomes permanently on. The red signal being badly visible or out of phase doesn't mean anything. I didn't touch anything besides adding that single resistor.

author: muvideo
author_rank: Frequent Contributor
timestamp: April 10, 2014, 06:10:34 pm
content: Tried but it seem to work:

author: poorchava
author_rank: Super Contributor
timestamp: April 10, 2014, 07:02:21 pm
content: Hmm, thanks for the effort. I will re-draw the circuit tomorrow and see what happens... It's bizarre anyway

author: w2aew
author_rank: Super Contributor
timestamp: April 10, 2014, 07:34:59 pm
content: Quote from: poorchava on April 10, 2014, 07:02:21 pmYou have to be careful when connecting component terminals together directly without using wires. Sometimes, they look connected when they're really not. You're often better off drawing the schematic with more "white space", and connecting everything with wires instead of end-to-end. This might explain the weird results you initial got.

author: Fank1
author_rank: Regular Contributor
timestamp: April 10, 2014, 08:47:12 pm
content: You have a bad connection in your asc file.Make some room between the top mosfet gate and the gate resistor.I moved the ac voltage, bridge, and mosfet to the right a good ways and reconnected the resistor and supply.It worked.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2014, 09:46:19 pm
content: A good reason to always follow neat schematic layout rules. Even in such an ugly environment as LTSpice.Some ideas:http://webpages.charter.net/dawill/tmoranwms/Elec_Circuit_Rules.htmlTim

author: majki
author_rank: Contributor
timestamp: April 10, 2014, 10:09:13 pm
content: Just check the View / SPICE error log (Ctrl+L) for warnings about the unconnected pins.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 10, 2014, 10:53:33 pm
content: QuoteIt might if we knew what node it was, also it probably gives a big clue to the problem.