The Multimeter

The multimeter is a bench digital meter you put beside the board and touch to the circuit with two probes. It is modelled on a real meter, the Brymen BM235: the same dial, the same jacks, the same input resistance and fuses — so it reads what a real one would, and goes wrong the way a real one does when it is used wrongly. It reads under Spice Lite only (Settings ▸ Spice Lite); with the logic engine the toolbar does not offer it and a meter already on the desk stays dark.

The multimeter button on the toolbar (beside the Analyzer) takes it out onto the desk — at the right of the view — and puts it away again; the button is lit while it is out. Putting it away and taking it out are ordinary edits (⌘Z undoes them), and both work while the circuit runs. It is drawn at its real size — about as tall as a full breadboard is wide.

The meter is not part of the circuit: it lies ON the desk, over the boards and everything on them. Drag it by its body anywhere — onto a board too, and while the circuit runs. It is never selected or highlighted, and whatever is under it cannot be reached until it is moved out of the way; only its own probes and leads lie on top of it.

The probes

Each probe trails a lead back to its jack: black from COM, red from whichever jack its plug is in. Drag a probe by its body onto any hole — an empty one, or one a chip's pin or a wire's end is already in (a probe touches the lead there; it takes nothing). A probe snaps to the nearest hole in reach, as a wire end does; dropped clear of every hole it lies on the desk where you let it go, touching nothing. A probe can also rest on a power supply's terminal.

Probes, the dial and the buttons all work while the circuit runs — a meter is used on a live circuit. Hold Option/Alt over a probe to reach the lead in the hole under it. Moving a probe, turning the dial and moving the plug are ordinary edits: ⌘Z undoes them.

The dial

Click a position's legend to turn the dial there. SELECT steps through what that position offers:

The display ranges itself (AUTO). RANGE holds the range it is on; each press after that steps to the next; hold it for a second to go back to auto-ranging. OL means the reading is past the range — or, on ohms, an open circuit.

The buttons act when they are let go, and a button held for a second does something else, as on the real meter:

• REC keeps the largest, the smallest and the average of the readings the display has shown since it started, and beeps at each new MAX or MIN (with Sound on). The display still ranges itself.
• Hz works on V, mV and the current positions. It counts the signal's rising crossings through a trigger band about its average, so a logic signal reads whatever level it sits at; on V it triggers on the range the display was on, and RANGE steps the trigger range (6 V is the most sensitive, about 0.1 V RMS). Below 2 Hz it reads 0. A fast oscillator the simulation draws slowed (one past 1 kHz) reads its TRUE rate.
• REC, Relative Zero and Hz are not available on AutoV, and turning the dial ends them.

The display updates five times a second, and shows the AVERAGE over that time, as a real meter does: a 50 % PWM on DC volts reads half its swing, and an AC range reads the true RMS of the waveform.

It is not an ideal meter

Whatever the dial selects is a real element between the probes, and the circuit sees it:

• Volts puts 10 MΩ across what it measures. A divider of two 1 MΩ resistors off 5 V reads 2.381 V, not 2.500 — exactly what a bench meter shows. A net nothing drives reads 0.000 V: the meter's own 10 MΩ ties it to the black probe. (A TTL input on that same floating net reads HIGH — the meter and the chip are asking different questions.)
• AutoV loads it with 2.1 kΩ, to drain the "ghost" voltages a high impedance meter shows. On a 10 kΩ divider that pulls the middle down to under a volt, and the display shows ----.
• Ohms, continuity, diode test and capacitance push their own small test current through what they measure.
• Current ranges put a small resistance in series — 100 Ω on µA, 1.9 Ω on mA, 0.04 Ω on 10A — and the circuit loses the voltage across it.

Measuring current

A meter measures current by being IN the circuit: break the circuit, plug the red lead into µA mA (or 10A for more than 600 mA), turn the dial to µA or mA A, and put the probes on the two sides of the break. The meter closes the gap and reads what flows through it. Put both probes on one connected net and it reads nothing — no current has any reason to go through it.

On a current range the meter is very nearly a wire. Put it across a supply and it shorts it: the µA mA input's 0.4 A fuse blows (F1), the display reads nothing, and the circuit it was in is open. The 10A input has an 11 A fuse (F2); a 1 A supply's current limit usually saves that one. A blown fuse is marked with the red (i) on the meter and a warning, and is replaced when the circuit stops — like a part's smoke, it is a mistake on the bench, not part of the design.

A red lead left in a current jack with the dial on anything else shows InEr and beeps — the real meter's warning. The 10A jack's shunt is wired to COM whatever the dial says, so "measuring volts" across a supply with the lead still in 10A shorts it.

When the circuit is stopped

Stopped, the bench is switched off: volts and current read nothing. Ohms, continuity, the diode test and capacitance still read — the meter's test source is the only thing driving the circuit, as on a real unpowered board. And it drives it in time, from the moment the probes touch (or the desk last changed): across a capacitor the ohms reading climbs as the test current charges it, until it reads open; an LED on the diode test glows faintly, lit by the meter's fraction of a milliamp.

On a live circuit these give false readings, as a real meter does — the circuit's own voltages are added to the meter's test voltage. Measure resistance and capacitance with the circuit stopped (running, capacitance reads OL).

Measuring capacitance

Turn the dial to →| and press SELECT: the display reads in nF, µF or mF. The meter charges what is between its probes one way and then the other, between two small voltages, and times the swing — so it shows ---- for as long as that takes: a moment for nanofarads, a few seconds for a thousand microfarads. A capacitor on its own reads its value exactly. A resistor or another part across it changes the timing and so the reading, as it does on a real meter — take one leg out to measure it. Past 10 mF it reads OL.

Sound

The continuity beep, the InEr beep and REC's beeps are silent unless the meter's Properties ▸ Sound is on; the display's backlight flashes for continuity either way.

In 3D

The 3D view shows the meter with its probes on the holes they touch and their leads running round the meter to its jacks.

What it does not do

The BM235's temperature, VFD, non-contact voltage and auto power-off are not modelled. Its readings are exact: its accuracy figures are not added. AutoV's input stays at 2.1 kΩ at every voltage (the real one rises above 50 V).

The meter is test equipment: it is left out of the schematic, the BOM, the build steps and the AI builder, and the KiCad and Digital exports leave it out — saying so when it was closing a gap on a current range, which the exported circuit then has open.

See also

• Spice Lite — the simulation the meter reads.
• Probing & Net Names — the probe tool, which reads a net's voltage and the current through a lead without a meter.
• Power & Clock Sources — supplies and their current limits.