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Electronics Forum / CAD / November 2007PSpice and series of capacitors
PSpice get crazy to determine voltages when I have a series of capacitors: <http://img213.imageshack.us/img213/1521/ex231yg7.gif> After 'googled' a bit, I tried to fix the limitation of floating pin (seems that PSpice can't determine DC voltage in such situations) by adding two shunt resistors (R2 and R3) of huge value (1 TOhm). In option tab, I enabled GMIN (1E-12 S) too. Is there a possibility to use PSpice or I have to switch to LT SwitcherCAD III ? Still was wondering how a software like PSpice can fall down so poorly with a simple circuit like this. Thanks in advance and my apologies for my (surely poor) English ;-) > PSpice get crazy to determine voltages when I have a series of capacitors: > [quoted text clipped - 13 lines] > > Thanks in advance and my apologies for my (surely poor) English ;-) You are going to see the same problems in LTSpice, and just about any other spice there is. Every node needs to have a DC path to ground for the differential equations to make sense. And then, you blew it by putting in a ridiculous resistance like a teraohm. Usually, 100MEG is more than sufficient, occasionally a full 1G if you need really small shunt currents, but this is a simulator with a realisitic dynamic range. There aren't real 1T resistances out there, and they blow up the simulators when you have 1T resistances, and then really small resistances. Get real, get some experience, and then you can critize the simulators for real problems... Charlie Charlie Edmondson ha scritto: >Usually, 100MEG is more than sufficient, Also using 100MEG, PSpice puts out exotics voltages. > Charlie Edmondson ha scritto: > >> Usually, 100MEG is more than sufficient, > > Also using 100MEG, PSpice puts out exotics voltages. So, as the student, why is that? What is it about that arrangement of resistors and capacitors causes those voltages? Learn and understand... Charlie Charlie Edmondson ha scritto: > You are going to see the same problems in LTSpice, LTSPice simulate it correctly. >Charlie Edmondson ha scritto: > >> You are going to see the same problems in LTSpice, >LTSPice simulate it correctly. LTspice adds the R's that everyone has mentioned. Likewise series R's for inductors. "correctly"?... you are an IDIOT. ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave Jim, >>> You are going to see the same problems in LTSpice, >> LTSPice simulate it correctly. [quoted text clipped - 3 lines] > > "correctly"?... you are an IDIOT. I'm afraid you are probably the village I-word here, Jim. You might first check out what LTspice actually does there, instead of spinning rhetoric. LTspice is probably more accurate then without such matrix modifications if you are talking real world circuits. In any case, it's certainly more accurate then PSpice's addition of gmin to current sources, or PSpice's Gear integration, both of which have contributed to design and analysis errors of silicon, in one case, even by your good self. Regards, --Mike >Jim, > [quoted text clipped - 21 lines] > >--Mike I guess I incorrectly assumed you added shunt resistance around floating caps, since the OP said, "LTSPice simulate it correctly". Floating caps are floating caps... aren't they??? ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave >>Jim, >> [quoted text clipped - 28 lines] > > ...Jim Thompson Now that I drove to the grocery store and back. I don't quite understand your little outburst. HOW can you "simulate it correctly" if it's truly a floating node? You have to be doing at least some kind of surreptitious node setting, or adding R to ground. I will continue to call idiots as I see them ;-) ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave Jim Thompson ha scritto: > you are you are in my kill file >Jim Thompson ha scritto: >> you are >you are in my kill file You are ignorant beyond specification... and you will always be ;-) ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave > PSpice get crazy to determine voltages when I have a series of capacitors: > [quoted text clipped - 5 lines] > > In option tab, I enabled GMIN (1E-12 S) too. Ading resistors in parallel to the capacitors is OK. GMIN doesn't help anything in this case. > Is there a possibility to use PSpice or I have to switch to LT SwitcherCAD > III ? I don't know whether there is an option in PSPICE to do that automatically. LTspice adds a little bit of conductance to avoid this problem. > Still was wondering how a software like PSpice can fall down so poorly > with a simple circuit like this. > > Thanks in advance and my apologies for my (surely poor) English ;-) SPICE requires a DC-path to every node. This is not a limitation. It just remembers you about a possible weakness of your design. Even in a real circuit, you can't predict the DC-voltage on the capacitors. It's detrmined by the insulation resistance of the capacitors which may vary many decades over production lots. And last but not least it's bad design practice to connect capacitors in series without an additional resistor in parallel. Best regards, Helmut Helmut Sennewald ha scritto: > And last but not least it's bad design practice to > connect capacitors in series without an additional > resistor in parallel. It's just an exercise assigned as homework. >Helmut Sennewald ha scritto: > [quoted text clipped - 3 lines] > >It's just an exercise assigned as homework. Bad professor, bad professor ;-) Or maybe GOOD professor thinning the class down to a manageable size ;-) ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave >>Helmut Sennewald ha scritto: >> [quoted text clipped - 10 lines] > > ...Jim Thompson I say GOOD professor for beating in to the students heads about DC floating nodes and the proper way to deal with them in Spice and real life. One can learn from bad design examples. --- Mark >I say GOOD professor for beating in to the students heads about DC >floating nodes and the proper way to deal with them in Spice and real >life. One can learn from bad design examples. Especially if one learns something the hard way. Suppose you are putting 2 caps in series to get a higher voltage rating. What size resistors do you use to ensure a DC ballance?  SignatureThese are my opinions, not necessarily my employer's. I hate spam. I would rather use my brain than use a simulator. At the end of the day a good engineer doesnt need a less than perfect simulator. >I would rather use my brain than use a simulator. > >At the end of the day a good engineer doesnt need a less than perfect >simulator. You would never be a competitor anyway, so who gives a sh.t. You certainly don't qualify for the moniker "good engineer". ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave >I would rather use my brain than use a simulator. Well, your software looks like it comes out of the '70s or early '80s at best, and at that point in time you could arguably still be productive without a simulator. > At the end of the day a good engineer doesnt need a less than perfect > simulator. True, but many a good engineer does feel that he needs a job, and for many jobs today simulators are requisite. :-) This same discussion comes up every now and again on SED, Marra -- Google can dig them up and you can read how many jobs have clearly demonstrable productivity gains via the use of simulation. ---Joel |
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