Symbols, Define and Expert Mode

This is the part that is not SPICE. Any value may be a symbol, and the answer is an expression rather than a number.

A divider with no values at all: a source and two resistors in series, every value left as a symbol. We name the symbols vs, ra and rb, and the elements e, r1 and r2:

e,1,0,vs
r1,1,2,ra
r2,2,0,rb

Run in DC, the voltage across the second resistor comes back as

The divider rule, derived rather than recalled. No values were given and none were needed.

Define

Define gives values to symbols without editing the description. One per line, name = value:

ra = 4'k
rb = 6'k
vs = 10

Run the same circuit again and the same answer now reads

The description still says ra, so changing a value is one line and a re-run, not an edit to the circuit.

A symbol left undefined stays symbolic. You can define some and not others, and get a partly symbolic answer — which is usually what you want when one component is the unknown.

Evaluate

Evaluate takes expressions over the answers, one per line, after the solve:

vr1/ir1
pr1 + pr2

Conditions constrain the evaluation without changing the circuit — ra = rb, say, to see what the divider does when the two are equal.

Solve

Solve goes the other way. Give it an equation over the answers and an unknown, and it finds the value that satisfies it.

Suppose we want the resistor that puts 7 V across r2. We take rb out of Define, so that it stays a symbol and the answers come back in terms of it, leave ra and vs defined, and run again. Then, in the Solve card, the equation and the unknown:

vr2 = 7

rb

Press Solve equations, and the card returns rb = 9333.33 Ω: the resistance that puts 7 V across r2 when ra is 4 kΩ and vs is 10 V. A value the equation needs but the circuit does not know can also be given in the card's Conditions box, ra = 4'k on a line of its own, instead of in Define.

Real solutions only is a tick on the same card. Leave it on unless a complex root is meaningful in your problem.

Expert Mode

Expert Mode adds to the system the solver assembles, rather than working on its answers afterwards. Three boxes:

• equations — extra relations, one per line or separated by and
• unknowns — extra symbols to solve for
• conditions — constraints applied to the result

Use it when the condition has to be part of the solve itself, so that every answer comes back with it applied: the whole circuit at the component value that meets the condition, not the component alone. Put the condition in equations and the component in unknowns, and the circuit is solved and the condition satisfied in one system. For the component's value on its own, the Solve card above is the shorter route, and it is the one this documentation's worked examples take.

Values in these boxes are parsed exactly as circuit values are, so 4.7'k works and a bare 4.7k does not.