Test points, voltmeters and ammeters
Updated v2.4.4At a glance
- Instruments live ON the sheet and read out live while the circuit runs — the numbers sit right where you put the meter
- Test point reads a net against a reference net you choose; voltmeter reads across its two terminals
- Ammeter wires in series and reads the current through it, with an arrow on the symbol saying which way is positive
- A measurement never changes the circuit — a test point and a voltmeter are ideal opens and stamp nothing at all
- Adding, moving or deleting one cannot change a single net, which is what makes them safe to leave on a drawing forever
- They print, because where you put them is part of the document — but their live readings never do
- Before a run and after a stop they show the net name, so a test point annotates a sheet you're still drawing
Three instruments, placed like any other symbol and wired in like any other part:
| Tool | Key | Terminals | Reads |
|---|---|---|---|
| Test Point | Shift+T | 1 | The voltage on its net, against a reference net you can set (default: the ground the circuit found) |
| Voltmeter | Shift+V | 2 | The difference across its terminals |
| Ammeter | Shift+I | 2 | The current through it, wired in series |
Like sources, these are sheet entities rather than library parts: no footprint, no reference designator, never on the bill of materials. They're instruments, not components.
The readout is on the sheet
While the circuit runs, each one shows its live value right where it sits, following the symbol as
you pan and zoom. Before a run and after a stop it shows the net name instead — which makes a
test point a genuinely useful annotation while you're still drawing, not just a simulation
accessory. Paused, it holds the last value. A terminal on nothing reads —.
A measurement must not change the circuit
This is the rule the whole design is built to keep, because every way of breaking it fails silently — a merged net still simulates, still passes the rules check and still exports a bill of materials.
- A test point and a voltmeter are ideal opens. They stamp nothing into the solve at all, and a divider solves to identical voltages with and without them. A voltmeter bridging two nets cannot merge them, because it never enters the connectivity pass in the first place.
- An ammeter behaves exactly as a wire does:
A —[A1]— Bderives the identical netlist to an unbrokenA — B. Inside the solver it becomes an ideal zero-volt element with its branch current observable, which is what an ideal ammeter is.
Adding, moving or deleting any of them cannot change the set of nets, any net's name, or any pin's net assignment. That's what makes them safe to leave on a sheet permanently — and it's what will keep PCB layout honest, since a board with instruments on its schematic must route exactly like the same board without them.
Signs and series
An ammeter's reading is signed, so the symbol carries a direction arrow: positive means current flowing the way the arrow points. Rotate it 180° and the reading negates, which is the only way a signed current means anything.
Wire an ammeter in parallel — dropped beside a run rather than inserted into it — and you're told so directly: "A1 is wired in parallel, not in series." The underlying failure would be a source loop, which is technically true and useless.
Why a test point has no current mode
A single node has no single current. It's a junction, and the currents into it sum to zero. Measuring a current takes a series element, which is exactly what the ammeter is. So the test point's properties have no current control at all, and say why in one line, rather than offering a disabled checkbox and leaving you to work it out.
On paper
Instruments print, because where you chose to put them is part of the document and a printed sheet is what someone takes to the bench. Their live readings never print — a number frozen onto paper would read as a specification rather than a measurement. See Printing a schematic.