Bench power supply

Nought to thirty volts, a current limit, and the two lamps that are the whole point of the instrument.

A bench supply is not a battery with a bigger number on it. It has two control loops fighting over one output (one holding the voltage you set, one holding the current you allowed), and which of them is winning is what CV and CC are telling you. The moment that becomes useful is the first time you power something you are not sure about with the limit set to what it should draw: a short, or a part in backwards, costs you a lamp on the front panel instead of the part.

Pins

Pin	What it does
V+	The positive terminal.
V-	The return. It is an ordinary node, not an earth: tie it to the circuit's ground yourself.

Properties

voltage: 0 to 30 V, 5 V by default. What the voltage knob is set to, which is what the terminals hold while the supply is in constant voltage and not otherwise.

limit: 0 to 3 A, 0.5 A by default. The most current the supply will deliver.

output: the output button, on by default. Off, the terminals are a bleeder resistor and nothing else, which is not the same as the supply being set to zero.

All three have buttons on the face, and those buttons poke the running circuit as well as saving the new setting, so a supply can be turned up or down while you watch what it is doing.

What the model gets right

Constant voltage is a stiff source behind 10 milliohms; constant current is the same two elements with the resistor turned up to a megohm. A current source with a resistor across it is a voltage source behind that resistor (Norton and Thevenin are the same two numbers written differently), so the mode changes by restamping two values and the matrix stays linear. The part picks the mode from the terminal voltage after each solve, the way the op-amp's rail clamp does: in constant voltage it is delivering (voltage − v) / 0.01, and when that is more than the limit the voltage loop has lost; in constant current the terminal voltage is whatever the load makes of the limit, and when that reaches the setting the load is light enough again. The two tests are not each other's negation, so a load sitting exactly on the knee stays in whichever mode it arrived in instead of chattering between them.

Twelve volts set, a 1 A limit and ten ohms across the terminals reads 10.0 V, 1.00 A, CC, because twelve volts across ten ohms would be 1.2 A and it is not allowed. A short reads the limit current and almost no volts. An LED straight across it with no series resistor reads the limit and the LED's own forward drop, which is the other thing a bench supply is for.

There is a 10 µF output capacitor across the terminals, because a real supply has one and because it is what stops the crossover being instantaneous: short the output of a supply sitting in CV and the voltage does not jump to zero, the capacitor empties into the short first and the supply arrives in CC a moment later.

The supply is an analog source, so the nets it feeds are solved in the matrix. That is the difference between this part and the vcc symbol, which is a digital rail and the lighter thing for powering a board.

What it does not model

No ripple, no noise and no line regulation, because there is nothing upstream of it. No transient response beyond the output capacitor: the control loop itself is instantaneous, so a load step has no overshoot and no recovery time. Nothing gets hot, so there is no thermal fold-back, no over-temperature shutdown and no fan; the supply will sit in constant current for ever. The limit is a limit and not a trip, so it does not latch off the way a lab supply in OCP mode would.

The readout is the solver's own answer, with no meter accuracy specification, no calibration and no more resolution than the digits on the face. There is no earth terminal, no remote sense, and no series or parallel tracking with a second supply. The output is not floating: V- is a node like any other.

Common mistakes

Reading the voltage knob instead of the display. In constant current the setting is not what is on the terminals, and the CC lamp is the only thing that tells you, which is exactly the lesson The current limit is built around.

Leaving the limit wound up to maximum. It is the one setting that protects the thing you are powering, and it costs nothing to set.

Using it as a power rail for a board. It works, but it drags every net it feeds into the analog matrix; vcc is the part for a supply you do not want to think about.

See it in action

The current limit is twelve volts into ten ohms with a 1 A limit, and every button on the face has something to do on it. See the bench instruments for the whole set together.