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gnucap:manual:examples:things_that_can_go_wrong [2009/05/16 01:50] aldavis created |
gnucap:manual:examples:things_that_can_go_wrong [2015/12/11 15:39] (current) |
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| + | ====== Things that can go wrong ====== | ||
| + | |||
| + | ===== Voltage sources in parallel ===== | ||
| + | |||
| + | eg4.ckt | ||
| + | |||
| + | <code> | ||
| + | VOLTAGE SOURCES IN PARALLEL | ||
| + | |||
| + | V1 1 0 10.0 | ||
| + | V2 1 0 10.2 | ||
| + | .print dc v(1) i(V1) i(V2) | ||
| + | .dc | ||
| + | .end | ||
| + | </code> | ||
| + | |||
| + | |||
| + | Here we have V1 and V2 both driving the same node at about 10 volts. | ||
| + | Actually, V2 is very slightly higher than 10 volts so there will be | ||
| + | some argument between V1 and V2 as to exactly what the final voltage | ||
| + | at node 1 really is. You should see that huge currents are flowing | ||
| + | through the supplies (10,000 amps) just due to this small voltage | ||
| + | difference. Also note that gnucap does not throw in the towel and give | ||
| + | up, nor does it fail to converge... the answer that it gets for v(1) | ||
| + | is a compromise, halfway between the two sources. | ||
| + | |||
| + | What is does is introduce a slight imperfection in the voltage sources | ||
| + | so that they do have a small internal series resistor. This allows it | ||
| + | to make the best guess that it can in a difficult situation. How much is | ||
| + | this resistance? You can find out like so: | ||
| + | |||
| + | acs | ||
| + | options | ||
| + | |||
| + | Look at the value of the option called "short" (near the middle of | ||
| + | the block of options), this is the value (in ohms) of the internal | ||
| + | resistance of a voltage source. The "u" character means "micro" | ||
| + | or 1e-6 so the default value of a short circuit is 1e-5 ohms. You | ||
| + | might decide that a different short circuit value is more appropriate | ||
| + | for running the above circuit so you can type (from the prompt): | ||
| + | |||
| + | options short=0.5 | ||
| + | get eg4.ckt | ||
| + | dc | ||
| + | exit | ||
| + | |||
| + | Which should show you the same voltage (10.1) but now the current | ||
| + | has reduced to only one fifth of an Amp (still not small but a lot | ||
| + | more reasonable if you were building this with real supplies). | ||
| + | Other option values can be altered in much the same way and input | ||
| + | files can contain ".options" command lines in order to set these | ||
| + | options whenever the circuit is loaded. | ||
| + | |||
| + | ===== Current sources in series ===== | ||
| + | |||
| + | eg5.ckt | ||
| + | |||
| + | <code> | ||
| + | CURRENT SOURCES IN SERIES | ||
| + | |||
| + | I1 0 1 2.0001 | ||
| + | I2 1 0 2.0 | ||
| + | .print dc v(1) i(I1) i(I2) | ||
| + | .dc | ||
| + | .end | ||
| + | </code> | ||
| + | |||
| + | The case of putting two current sources in series is much the same | ||
| + | concept as two voltage sources in parallel. However notice that gnucap | ||
| + | copes with it in a different manner. It cannot find a compromise current | ||
| + | that is partway between the two sources and it always gives a huge | ||
| + | value for the voltage at node 1. At least it doesn't crash and it does | ||
| + | give results that give some suggestion as to where the problem might | ||
| + | be. There is an option "gmin" that introduces resistance into a current | ||
| + | source, or you can explicitly add these resistors if you like by putting | ||
| + | the resistor in parallel with the current source. | ||
| + | |||
| + | What if you had a big, complex circuit, you messed up by putting | ||
| + | two current sources in series but you never thought about checking | ||
| + | the strange node? How would you ever know that the circuit was broken? | ||
| + | Try this exercise: | ||
| + | |||
| + | acs | ||
| + | get eg5.ckt | ||
| + | alarm dc v(*)(-1e3,1e3) | ||
| + | dc | ||
| + | quit | ||
| + | |||
| + | Now you get a warning whenever any component gets more that 1000 volts | ||
| + | across it. This can be used to test component breakdown if you know | ||
| + | that you are using components that cannot tolerate high voltages. It can | ||
| + | also be used to ensure that your simulated circuit stays within what you | ||
| + | might expect to be the absolute maximum values. | ||
| + | |||