Sources and power rails
VUpdated v2.4.4At a glance
- A voltage source is a sheet entity, not a part — no footprint, no designator, never on the bill of materials
- Three kinds: fixed DC, AC with a waveform, and a variable DC supply you drive from a slider while it runs
- AC sources do sine, square, pulse and triangle, with amplitude, frequency, offset, duty and phase
- Drag the variable-supply slider and the traces bend as you drag — the capture is never wiped
- A power rail with a voltage in its name powers the net on its own, so a divider works with no source placed
- A rail steps aside the moment anything real drives that net — two ideal supplies on one node is a short
- A rail named VCC or VDD has no voltage in it, so that's reported rather than assumed to be 5 V
Press V for the Source tool. Three kinds, chosen as you place.
A source isn't a component
A voltage source has no footprint, never lands on a PCB and cannot be ordered — so it is not a library part. It's a sheet entity, the same class of thing as a power symbol: real on the drawing, absent from the board, never given a reference designator, never on the bill of materials.
A battery is the opposite case and stays a real part, because you can buy one and solder it in.
The three kinds
DC — a fixed supply. One number.
AC — a waveform generator: sine, square, pulse or triangle, with amplitude, frequency, DC offset, duty cycle and phase. This is your signal generator on the sheet.
Variable DC — a supply with a minimum and a maximum, driven by a slider in the simulation transport while the circuit runs. Dragging it patches the live circuit, so the traces bend as you drag instead of blanking and restarting. Putting the knob next to the scope is the entire point of a variable supply, and it's the one thing a properties dialog can never give you.
Double-click a source on the sheet to open its properties.
Power rails are supplies
A power symbol whose name encodes a voltage — 5V, +5V, -12V, +3.3V, 3V3, 1V8 —
contributes an ideal supply from that net to ground. So 5V ─ 10k ─ 10k ─ GND solves as a working
divider with no source part placed at all.
Only when nothing else drives that net. Put a virtual source, a battery or a regulator output on
+5V and no synthetic supply is added — two ideal voltage sources on one node is a source loop,
which any solver refuses outright. A rail supplies power exactly when nothing else does.
A rail is always plain DC at the voltage in its name. Anything else — a sine, a variable supply —
takes a placed source, because a rail labelled 5V that was secretly a 1 kHz sine would be a lie
printed on the drawing.
Two cases are reported rather than guessed at:
A rail named VCC, VDD or VBAT | No voltage in the name, so nothing drives it. Assuming 5 V would be silently wrong on every 3.3 V design |
| Nothing drives the circuit at all | Every voltage reads exactly 0, which looks identical to a broken solver — so it says so |
The order things get power in
Three things can drive a circuit, and they're applied in a fixed order, each one only supplying a node nothing earlier already drives:
- Placed parts — a battery, a regulator's output. Ordinary components with a simulation model.
- Placed sources — the ones above.
- Named power rails — the implicit supply behind
+5V.
Half-wired sources
A source with a terminal on no net is skipped and reported, never silently tied to ground — the same rule a half-wired part gets. Grounding it would invent a connection you never drew and produce a plausible-looking wrong answer, which is worse than an error message.
The rules check knows about sources too: wiring one straight into a chip's VCC — the most direct way there is to power a board — correctly doesn't raise "power not driven".