Power supplies and function generators

Most measurements need a source as well as a meter. The bench power supply provides the DC rails a circuit runs on, and the function generator provides the test signals: sine waves to measure a frequency response, square waves to see a step response, pulses to trigger logic, and ramps to sweep a sensor input. Both instruments look like simple boxes with knobs, and both have behaviours that surprise people the first time: a supply that suddenly drops its voltage, a generator whose output is twice what its display says. This lesson explains what the instruments actually do, so that the source is never the hidden cause of a strange result.

This week Leo powers up a fresh prototype controller board for the first time, then drives its microphone amplifier with test tones to check its gain. Amara watches the first power-up, because a first power-up is the moment a wiring or assembly fault is most likely to cost a board.

The bench power supply: constant voltage and constant current

A typical laboratory DC supply has two settings for each output: a voltage setpoint and a current limit. It regulates whichever one is reached first. With a light load, the output sits at the voltage setpoint and the current is whatever the load draws; this is constant-voltage (CV) mode. If the load tries to draw more than the current limit, the supply reduces its output voltage until the current equals the limit; this is constant-current (CC) mode. The front panel shows which mode is active, usually with a CV or CC indicator. The crossover happens at the load resistance R = V_set / I_limit.

The current limit is the most useful safety feature on the bench. Before connecting a new or untested board, set the voltage, then set the current limit to a little more than the board is expected to draw, and only then connect. If there is a short circuit or a reversed part, the supply drops into CC mode, the voltage collapses and the fault draws only the limited current, often too little to damage anything. A supply left at its maximum current of several amperes will happily burn a trace, a reversed capacitor or a regulator before you notice.

The fault in the example turned out to be the input protection diode, fitted backwards during assembly. Reversed, it conducted from the rail to ground. With the limit set at 3 A, it would have overheated and failed short or open within seconds, and the failure might have taken the input trace with it.

Sensing, ripple and output details

A supply regulates the voltage at its own terminals, but the board sees the voltage after the leads. With 1 A flowing through a metre of thin lead, there can be 0.1 V or more of drop in each direction. Many supplies have remote-sense terminals: a second pair of wires that measures the voltage at the load, so the supply raises its output to compensate for lead drop. For most bench work, short, thick leads and measuring at the board are enough. Other specifications to know are ripple and noise, the AC on the DC output, which is a few millivolts on a good linear supply and more on a switching supply; transient response, how quickly the output recovers when the load current steps; and whether the outputs are floating, so they can be stacked in series for higher or bipolar voltages.

Floating outputs make bipolar rails easy. To power an op-amp circuit from ±12 V, connect the positive terminal of one output to the negative terminal of the other and call that junction ground. The free positive terminal is then +12 V and the free negative terminal −12 V. A supply's chassis ground terminal, usually green, connects to the mains safety ground; whether you tie the circuit ground to it depends on the setup, and lesson 7 explains the consequences for oscilloscope measurements.

The function generator

A modern function generator is usually an arbitrary waveform generator built on direct digital synthesis. A phase accumulator steps through a waveform table at a rate set by the desired frequency, and a digital-to-analog converter and filter turn the samples into a smooth output. It can produce sine, square, triangle, ramp, pulse, noise and user-defined waveforms, with control of frequency, amplitude, DC offset, duty cycle and rise time, and often modulation, sweeps and bursts. Its specifications include frequency range and resolution, amplitude accuracy and flatness across frequency, harmonic distortion for sine waves, rise time for square waves, and jitter.

The 50 Ω output and the doubling surprise

Almost every function generator has a 50 Ω output impedance: an internal voltage source in series with 50 Ω. That matches the coaxial cables and terminations used in radio and high-speed work, where a 50 Ω source driving a 50 Ω cable into a 50 Ω load gives clean, reflection-free signals. The generator's display assumes that 50 Ω load by default. It sets its internal source to twice the displayed amplitude, so that the 50 Ω divider leaves the displayed value across the load. If you connect the output to a high-impedance input instead, such as an oscilloscope set to 1 MΩ or an op-amp input, there is almost no current, no drop across the internal 50 Ω, and the voltage you see is twice what the display says.

The fix is a menu setting: most generators let you declare the load as high-Z, and then the display shows the open-circuit voltage. Even so, the safest habit is never to trust a source's display for a measurement that matters. Measure the voltage actually arriving at the device under test with the oscilloscope or meter, and compute gain, attenuation or response from two measured values. That habit also catches the cases where the load is neither 50 Ω nor high impedance, such as an amplifier input of a few hundred ohms, where the true voltage is somewhere in between.

# Voltage actually delivered by a 50-ohm generator into various loads
def delivered(display_vpp, load_ohm, setting="50"):
    rs = 50.0
    source = 2 * display_vpp if setting == "50" else display_vpp   # internal open-circuit amplitude
    return source * load_ohm / (load_ohm + rs)

for load in (50, 600, 10e3, 1e6):
    v50 = delivered(0.020, load, "50")
    vhz = delivered(0.020, load, "highZ")
    print(f"load {load:>9.0f} ohm: display 20 mV pp -> {v50*1e3:6.2f} mV pp (50-ohm setting), "
          f"{vhz*1e3:6.2f} mV pp (high-Z setting)")

# Supply crossover: which mode does each load put the supply in?
vset, ilim = 24.0, 0.150
for rload in (1000, 200, 160, 50, 21):
    i_cv = vset / rload
    if i_cv <= ilim:
        print(f"R = {rload:5d} ohm: CV, {vset:.1f} V, {i_cv*1e3:6.1f} mA")
    else:
        print(f"R = {rload:5d} ohm: CC, {ilim*rload:.1f} V, {ilim*1e3:6.1f} mA")

load        50 ohm: display 20 mV pp ->  20.00 mV pp (50-ohm setting),  10.00 mV pp (high-Z setting)
load       600 ohm: display 20 mV pp ->  36.92 mV pp (50-ohm setting),  18.46 mV pp (high-Z setting)
load     10000 ohm: display 20 mV pp ->  39.80 mV pp (50-ohm setting),  19.90 mV pp (high-Z setting)
load   1000000 ohm: display 20 mV pp ->  40.00 mV pp (50-ohm setting),  20.00 mV pp (high-Z setting)
R =  1000 ohm: CV, 24.0 V,   24.0 mA
R =   200 ohm: CV, 24.0 V,  120.0 mA
R =   160 ohm: CV, 24.0 V,  150.0 mA
R =    50 ohm: CC, 7.5 V,  150.0 mA
R =    21 ohm: CC, 3.1 V,  150.0 mA

The output confirms that the high-Z setting gives the right number for the oscilloscope and the 10 kΩ amplifier input but is off by half for a true 50 Ω load, while the 50 Ω setting is right only for a 50 Ω load. Neither setting is right for a 600 Ω audio input. The supply section shows the crossover at 160 Ω for a 24 V, 150 mA setting, and the 21 Ω fault giving 3.15 V in CC mode, close to what the panel showed.

Other generator features worth knowing

A square wave from a generator is not perfectly square: its rise time, often a few nanoseconds, limits the fastest edge you can test with, and the 50 Ω source driving a long unterminated cable can show reflections and ringing on the edges. The DC offset control adds a fixed level, which is how you produce a 0-to-3.3 V logic signal instead of a ±1.65 V one; make sure the offset is zero when driving an AC-coupled amplifier input you do not want to bias. The sync or trigger output provides a clean logic-level pulse once per cycle, ideal for triggering an oscilloscope when the main signal is noisy or small. Many generators also turn their output off at power-up; forgetting to press the output button explains many silent first measurements.