Simulation eBook - Introduction to using a simulator
Introduction to using a simulator
Using a simulator is not quite the same as building a real circuit. There are many things that can catch out the newcomer to simulation because neither real world nor most simulator components are ideal. The departures from ideal are often different between the two and if these differences between real and simulated components in a circuit are not clearly understood, they can lead to confusion when the results of a even a simple simulation are different from those expected.
Part of the problem is that a user's expectations are informed by experience of real world measurements. Therefore, it is important to understand exactly what measurements are being made and how the simulation circuit will affect the results. In cases where there are differences between real and simulated results this may also require a deeper understanding of what is going on in those circuits and what assumptions are being made - often unconsciously - about them.
Learning to use a simulator means thinking more about what the real world really looks like, how it differs from the theoretical world of textbook problems and simple diagrammatic circuit representations and therefore what the results of measurements are likely to be.
Avoiding common mistakes
The following section describes and illustrates some of the most common mistakes, misunderstandings and causes for confusion.
All simulation circuits MUST have a ground node
A feature of the way simulators work is that they MUST have a ground node (also referred to as the 0 net) somewhere in the circuit. All voltages probed in the circuit, unless explicitly probed as voltage differences are then measured with respect to that ground node. The ground node can be placed anywhere in the circuit that is convenient for the purposes of the simulation but it must exist somewhere in that circuit.
SIMULATION SCHEMATICS THAT DO NOT HAVE A GROUND NODE WILL NOT RUN.
This is illustrated in the following two examples:
With no ground symbol, the simulation will not run:
Once any of the available ground symbols is added, the simulation will run:
The ground node MUST NOT have a Voltage Probe attached to it
The reason for this is explained later in the section on Probing Voltages.
All simulation circuits MUST have a power and/or signal source
In exactly the same way that a real circuit must have some sort of power supply - even if that power supply is actually the signal source itself (for example, a crystal set or a volume control potentiometer) or comes from a capacitor pre-charged to some voltage or an inductor pre-charged to some current prior switching the circuit on - a simulation schematic must have a power and/or a signal source or an initial condition such as a capacitor pre-charged to some voltage or an inductor pre-charged to some current prior to the start of the simulation.
Power supplies can be built from ideal zero series resistance voltage sources. Simple but more realistic models of battery and voltage regulator supplies can be built using voltage sources with some series resistance or current sources with a parallel resistance (i.e. Thevenin or Norton equivalent sources).
EasyEDA provides a wide range of signal sources. These will be covered in detail in their own section on Sources but the basic use of some of these sources to provide power to a circuit is illustrated below:
Every point in a simulation schematic MUST have a DC path to ground
Unlike real circuits, every point in a simulation schematic MUST have a DC path to ground (or 0 net). Attempting to run a simulation with a node that has no DC path to ground will fail with errors.
Before discussing the significance of the DC path to ground, however, it is essential to look at some of the basic components and sources that are used in almost all simulations and to understand how they affect these DC paths.
Values and DC paths of RLC components
This section is about the values of - and DC paths through - resistors, inductors and capacitors.
By default, inductors in LTspice have zero series resistance (i.e. ESR = 0).
- Therefore, inductors have a DC path through them.
By default, inductors in LTspice have no parasitic parallel capacitance or resistance.
By default, capacitors in LTspice have no parasitic parallel conductance (i.e. they have an infinite DC resistance).
- Therefore capacitors in LTspice do not have a DC path.
By default, capacitors in LTspice have zero series resistance (i.e. ESR = 0). By default, capacitors in LTspice have no parasitic series inductance (i.e. ESL = 0).
Inductors and capacitors in LTspice can be set to positive, negative or zero values. Beware that setting a component to a negative value may cause the circuit connected to them to exhibit instability or oscillation. This in turn may cause the simulation to fail.
Resistors obviously have a DC path through them.
Resistors in LTspice can be set to positive or negative values but cannot be set to exactly zero: they MUST have a non-zero value. Setting a resistance to zero will cause the simulation run to fail with an error. This is basically because any voltage difference across a zero resistance (such as may occur normally in a circuit or even just as a consequence of numerical "noise" in the simulation calculations) will generate an infinite current, which will obviously crash out of the top end of the simulators calculation dynamic range. This applies to all resistors in LTspice, including the Ron and Roff values in switches and resistances given in device models. Beware that setting a resistor to a negative value may cause the circuit connected to it to exhibit instability or oscillation. This in turn may cause the simulation to fail.
- A common cause of confusion in simulation is the apparently unexpected behaviour of circuits using open circuit switches.
Just like switches in the real world, switches in EasyEDA have non-zero ON resistances (Ron). They also have finite OFF resistances (Roff). If an EasyEDA switch is connected in series with a voltage source into an open circuit load (such as presented by a Voltage Probe) then the voltage at the output of the switch will be equal to the open circuit voltage of the voltage source whether the switch is open or closed. This is different from the real world experience because in a real circuit, the voltage would be measured with a voltmeter or an oscilloscope probe having a much lower resistance than the open circuit resistance of the switch.
To illustrate this, let’s take a simple example of a switch connected in series with an ideal voltage source set to 1V and then measure the V(Output) of the switch when open and closed.
With the switch closed the expected output is V(Output) = 1V.
And that’s what the simulation shows.
OK. With the switch open the expected output is V(Output) = 0V.
What the simulation actually shows is V(Output) = 1V.
What’s that all about? Surely the switch must not be simulating properly and is stuck closed?
Well, no actually; the switch and voltage are doing exactly what they should. It’s just that these results are unexpected because there's a misunderstanding about how the simulated circuit differs from a real circuit.
No real switch has an infinite OFF state resistance but similarly, there is no such thing as a real open circuit that presents an infinite load impedance.
The Voltage and Current Controlled Switches and the Static Switches in EasyEDA are not ideal: they all have a finite OFF resistance. For the voltage and current controlled switches, the OFF resistances are specified by the user editable R(off)(Ω) parameter in the right hand panel with a default value of 1GΩ. The Static Switches have a fixed 10GΩ OFF resistance. In either case these resistances are large but certainly not infinite.
However, the effective resistance to ground at the Output node using a Voltage Probe or plot expression, really is infinite. Therefore when operating into an infinite resistance load there would be no difference between the ON and OFF state voltages at the Output node.
Note that if a load resistance of 1GΩ were to be connected from the Output node to ground, then there would be a clear ON/OFF state difference.
This is illustrated in the following simulation:
Some further examples of the effects of finite switch resistances are illustrated in this next simulation:
Note that, in LTspice, the Ron and Roff values used in switches can be set such that Ron > Roff. This is a useful way to invert the logic sense of a switch.
At this point it is worth noting that if it is important that the simulation results are as close as possible to those expected to be observed when probing a real circuit using real test equipment, it is sometimes useful to place a realistic load on wires (nets) in the simulation schematic where voltage measurements are to be made in the real circuit. Similarly for current measurements, realistic ammeter insertion impedances should be connected in series with the wire. To avoid unnecessary loading of the simulated circuit however, only place such loads in the locations where external measurement devices are to be connected to the real circuit and only in the same numbers as there are measurement instruments being used at the same time. For example, if there are two oscilloscope probes connected to a circuit but one of them is moved around, only connect loads representing simulated oscilloscope probes to two places in the simulated circuit. If different places need to be probed then move the simulated probes and rerun the simulation.
DC paths through Voltage and Current Sources
This section is about the DC paths through Voltage and Current Sources.
Note that this section only discusses the default settings of the LTspice voltage and current sources. For much more detail about the options available for these highly versatile devices please refer to the relevant sections of the LTspice native Help pages or the online pages at:
http://ltwiki.org/LTspiceHelpXVII/LTspiceHelp.html
Voltage sources in LTspice (including V, B, E and H voltage sources) have, by default, zero source resistance and have no current limit. The output voltage will be constant for any load current. This is true when sourcing or sinking current. sinking. This is not the same behaviour as regulated and current limited bench power supplies set to give a constant voltage output. Except for specialised supplies such as Source Measure Units, bench supplies can usually only source currents for positive outputs or sink currents for negative output voltages, up to some current limit set by the user.
Default Voltage Sources in LTspice therefore have a DC path through them.
As a consequence of their zero source resistance, in the same way that damagingly high currents will flow if real batteries are connected in parallel without some resistance between them, voltage sources in simulation schematics cannot be connected in parallel without some resistance in series between them. This is true even if they are set to exactly the same voltage. Attempting to do so will cause the simulation run to fail with an error.
Similarly, in the same way that damagingly high currents will flow if an inductor is connected directly across a real power supply without some resistance between them, inductors in simulation schematics cannot be connected directly in parallel with voltage sources without some resistance in series between them. Attempting to do so will cause the simulation run to fail with an error.
Current sources in LTspice (including I, B, F and G sources) have, by default, an infinite source resistance and have no voltage limit. The output current will be constant for any load impedance and voltage across the current source. This is not the same behaviour as regulated and current limited bench power supplies when they are set to give a constant current output. Except for specialised supplies such as Source Measure Units, bench supplies can usually only source currents up to some maximum positive voltage at the output or sink currents down to some minimum negative voltage at the output, as set by the user.
Default Current Sources in LTspice therefore do not have a DC path through them.
As a consequence of their generating a constant current through an infinite source resistance, in the same way that damagingly high voltages can be generated if two real current sources are connected in series without some finite resistance across each source, current sources in simulation cannot be connected in series without some finite resistance connected in parallel with each source. Attempting to do so will cause the simulation run to fail with an error.
In simulation, connecting a capacitor in parallel with a current source generating a current, I, without also connecting a resistor, R, in parallel, will cause the voltage on the capacitor to ramp towards infinity. Even with the resistor in parallel, the voltage will ramp to I*R which may still be a large voltage. If such a circuit exists in a simulation schematic then, unless it is shorted out by a switch with some suitably low value resistance or set to some initial voltage, the voltage across the capacitor will already have ramped towards infinity at time t=0, i.e. as the simulation starts. This can lead to unexpectedly high voltages right from the start of a simulation.
The effects of adding DC paths
This section is about creating DC paths and some of the effects they can have.
- A favourite of elementary electrical engineering classes and an example of a very tricky problem to solve using a simulator is a circuit that has two capacitors in series. This must have a DC path to ground from the common point between the two caps but the path to ground does not have to be direct. It can be via other elements in a circuit. For example if one end of one of the capacitors is connected to ground with the other end of the other capacitor connected directly to a grounded voltage source or to a Thevenin Source (a voltage source in series with a resistance) or to a Norton Source (a current source in parallel with a resistance) then placing a single high value resistor in parallel with either of the capacitors or simply from the junction of the two capacitors to ground would be sufficient.
The following simple example will run but not converge because there is no DC path to ground from the common node B, between the two capacitors:
Because the top end of C1 already has a DC path to ground through the ammeter and voltage source and the bottom end of C2 is grounded, simply adding a resistor to ground from the junction of the two capacitors at node B will allow this simulation to run. It will however, show voltages across the capacitors that can be confusing if the effect of the DC path resistance is not clearly understood:
- Resistors placed across capacitors to provide a DC path can be scaled so that the RC time constant that they will create with the capacitors is very large compared to the time interval over which a Transient simulation is to be run. Another way to look at this for an AC Analysis is that the 1/(2piRC) frequency that they form is far outside the frequency range of interest.
The value of these resistances can be anywhere between milliΩ (1e-3 Ω) up to 1G Ω (1e9 Ω). In many cases the values can be smaller or larger than this range but in some circuits that can lead to simulations failing with errors. This is usually because the ratio of the largest to the smallest voltages or currents in the circuit is too high for the simulator to handle. As a general rule, keeping the ratio between the smallest and the largest component value for any given type of passive component to no more than 1e12 should help avoid this sort of simulation failure.
- It must be noted however that whilst a purely theoretical analysis of the voltages across capacitors in series due to a current flowing through them based on Q=It=CV, may yield sensible values, setting up both real and simulated circuits to demonstrate this can be quite challenging. Voltage measurements in real circuits can be difficult whilst the need for a DC path in simulations can present different but no less tricky problems.
Any attempt to try to measure the voltages across a pair of capacitors in series using DC sources will either end in a failed simulation, because there are no DC paths to ground, will run but give the wrong voltages across the capacitors, or will just yield the steady state DC voltages across the resistors used to create the necessary DC paths to ground.
There are a couple of ways to measure capacitor voltages in this sort of circuit. As illustrated below, the simplest is to replace the DC voltage source with a PULSE source configured to generate a fast edged step from 0 to 1V. As long as the RC time constant of the DC path resistance and the capacitors is large compared to the simulation stop time then the voltage across each capacitor will be equal to that given by a theoretical analysis and will be defined by the ratio of the capacitors.
Another possibility is to apply a linearly ramped DC voltage of 1V/s across the capacitors in order to generate a constant current through them. The voltages across the individual capacitors will then rise in inverse proportion to their capacitances. By dividing these voltage by the time, a voltage can be generated that is equal to those which would be observed in a purely theoretical analysis of a 1V DC source with two capacitors in series thrown across it. The following example illustrates a way to actually measure the capacitances. It also introduces some of the more advanced uses of the B source which will be covered in more detail later on.
- Similarly, a transformer coupled circuit where the primary side is driven from mains live and neutral and one side of the secondary circuit is connected to earth (which is probably where the ground symbol would be placed in the circuit diagram), must have a DC path back to this ground from one or both ends of the transformer primary (or from a centre tap if one is available).
Similarly to placing resistors across capacitors, DC path resistors used with inductors can be scaled so that the R/L time constant that they will create is very large compared to the time interval over which a Transient simulation is to be run. Another way to look at this for an AC Analysis is that the 1/(2piR*C) frequency that they form is far outside the frequency range of interest.
Another example of where very high value resistors would be added is in transformer coupled data-communications networks such a 100BaseTx or 1000BaseT Ethernet.
Here, the connections between the two transceivers are floating and so have to have DC paths to ground added to keep the simulator happy. Using resistances that are high in comparison to the network cable characteristic impedance and hence termination resistance is important in order not to introduce an impedance mismatch and so disrupt the signal integrity. In practice it is simplest therefore to add two resistors: one from each of the two signal wires to ground. This preserves the symmetry of the signalling on the wires and doubles the effective resistance between them.
In many transformer coupled circuits, the simplest solution is to ground one side of both the primary and the secondary sides of the circuit. This may look strange because the circuits are no longer galvanically isolated by the transformer but - as long as the reason for connecting the circuits this way is clearly indicated in the schematic (and removed or otherwise modified before passing into PCB layout!) - then it is a simple and very useful solution. Not only does it remove any problems with how large or small the DC path resistor has to be, it also refers the voltages on both the primary and the secondary sides to ground and so simplifies probing of many of the voltages on both sides of the transformer which might otherwise have to be probed differentially.
Some examples of this use of the ground return path resistor can be seen in the following collection of examples of transformers and coupled inductors, including a simple example of an open loop flyback converter:
To avoid confusion between passive schematics intended to be passed to PCB layout and simulation schematics that may have had DC path resistors added, it is a good idea to clearly identify any resistors added to a schematic soley to provide a DC path to ground. This can be done using resistor names such as RDCPATH1 or by labelling them for example as For simulation only.
It must be remembered that all voltages probed in a schematic are with respect to ground. This is particularly important to remember when probing signals that are floating, such as the transformer coupled examples discussed above. This is when
Diff_V_Probesymbols, B or E Sources can be used to probe two floating voltages and subtract them to simply generate the difference between them.
Common problems with DC paths
A DC path to ground is often provided by the rest of the circuit but here are some cases that are often overlooked:
All current sources (independent I and dependent B and F sources) are ideal: they have an infinite DC resistance (and AC impedance) and so do not have a DC path through them. Connecting a current source across a capacitor with one side grounded will throw an error even if the current source is set to zero (the capacitor voltage would ramp to infinity if a non-zero current were set and that would throw a different error!). So a resistor must be connected across the current source, to provide the DC path to ground to the other side of the current source / capacitor. A similar problem arises if two current sources are connected in series even if the two currents are identical (if they're not then the common node flies off to infinity again);
Switches have their own internal DC path between the switch terminals because they have finite OFF resistances. However, voltage controlled switches do not have a DC path between their control inputs.
Current controlled switches, Current Controlled Current Sources (CCCS) and Current Controlled Voltage Sources (CCVS) all have a zero resistance short circuit between their control inputs. At first sight it might appear that connecting the output of a current source to the input of a current controlled switch would not cause a problem because the switch control input places a short circuit across the current source output. There is, however, no DC path to ground from the these connections so a resistor must be placed from one of the current controlled switch input pins to ground. If the current source output/switch control input part of the circuit is not connected to anything else then the resistor can be replaced by a connection to ground.
This is illustrated by the placement of the RDC_PATH_TO_GROUND 1k resistor connected to output side of the CCCS, F1, and input side of the current controlled switch W1 in the example below:
CURRENT CONTROLLED SWITCHES ARE NOT YET IMPLEMENTED IN LTspice VERSION OF EasyEDA
Capacitors (unless using the more advanced options in LTspice) are ideal: they have no parasitic (leakage) DC resistance in parallel with them. This is why both ends of a capacitor must have a DC path to ground either through the external circuit or explicitly by adding a resistor;
Although the primary and the secondary or secondaries of transformers have DC paths, there is of course no DC path from primary to secondary. This must be added either by connecting one side of the primary and of the secondary to ground directly or through a resistor.
Voltage sources (including independent V and dependent B and E voltage sources) have the opposite problem. They are ideal so they have zero resistance. The same is true for simple inductors. If you connect voltage sources directly in parallel then LTspice will throw an error even if the voltage sources are set to the same value (if they're not then an infinite current would flow and that would then throw a different error). The same problem arises if you connect a voltage source directly in parallel with a simple ideal inductor because the voltage source tries to drive an infinite current through the inductor.
This problem often occurs when driving a transformer from a voltage source. Adding a small series resistance fixes this little gotcha (in a real inductor or transformer there will always be a finite winding resistance anyway!).
Components are connected by netnames
It is important to understand that although nodes (the pins on component symbols) in a schematic can be shown joined by wires or by netlabels, in a spice netlist in Sim mode (and the non-simulation schematic netlist in Std mode) they are joined purely by the net names given to them either automatically by EasyEDA or manually by the user placing Net Labels. This includes nets joined by any of the NetFlag GND ground symbols. It also includes nets joined by the Net Port, NetFlag VCC and NetFlag +5V symbols as illustrated below:
It is equally important to understand that when a wire between two components in a schematic is broken then - unless the wire on both sides of the break is explicitly given the same netname by manually placing netlabels with the same name - then the wires on each side of the break will have different netnames because even though one side may already have a manually assigned netname, the other side will automatically have a new and arbitrary netname assigned to it by EasyEDA.
The significance of manually assigning net names will become apparent a little later when looking at how to probe voltages and using the voltage on nets in expressions for B sources.
As just illustrated, breaking a wire creates two segments of wire that are no longer connected to each other. This is because EasyEDA automatically assigns different netlabels to each end of a broken wire.
To rejoin the connection, add netlabels with the same name to each segment of the broken wire. NetPorts and NetFlags can be placed anywhere on a net but take care that the little grey connection dot is placed onto the wire.
To avoid accidentally connecting nets together that are not intended to be connected when manually assigning netlabels, take care to ensure the netnames assigned are unique to each net. For instance, in the following example, using the name 'mid' instead of 'ammeterneg' would connect the negative side of A1 to the 'mid' net and would short out R1.