A rectangular pulse generator function outputs a continuous stream of voltage waves with independently adjustable high and low durations, allowing you to switch power electronics on and off at precise, non-symmetrical intervals. In a real circuit or installation, this function changes your capability from static DC testing to dynamic AC-coupled analysis, timed energy delivery, and high-frequency switching that steady DC cannot achieve. Makers commonly confuse a rectangular pulse with a standard square wave (which is strictly locked to a 50% duty cycle) or a single-shot trigger pulse; remember that a rectangular pulse is a continuous periodic waveform where the pulse width (high time) is intentionally unequal to the space (low time).

Where You Meet This in Practice: Power & Energy Storage

When you move beyond basic resistive loads and start working with chemical energy storage or power conversion, the rectangular pulse function becomes a primary diagnostic and control tool. You will encounter it in three main areas on the bench:

  • Battery Internal Resistance (IR) and Impedance Testing: Injecting a high-frequency rectangular pulse into a battery allows you to measure the AC voltage drop across the terminals. Because batteries have complex internal impedance (a mix of ohmic resistance, capacitive double-layers, and inductive effects), a DC multimeter cannot reveal the true health of the cell. A 1 kHz rectangular pulse isolates the ohmic and charge-transfer resistance.
  • Pulse Desulfation for Lead-Acid Batteries: Sulfation on lead-acid plates can sometimes be broken down by applying high-current, low-duty-cycle rectangular pulses. The sharp leading edge of the pulse creates high-frequency harmonics that resonate with the sulfate crystals, while the long low-time (off-time) prevents the battery from overheating.
  • Gate Driving for Custom Inverters and UPS Systems: If you are prototyping a custom high-frequency inverter or a UPS topology, your microcontroller might not output a clean enough signal to drive heavy IGBTs or MOSFETs. A bench generator's rectangular pulse function provides a pristine, adjustable gate signal to test your power stage before committing to a final PCB layout.

Worked Numeric Example: 1kHz Pulse for LiFePO4 Impedance Measurement

Let us look at a concrete bench measurement. You have a 12V 100Ah LiFePO4 battery (such as an Ampere Time or SOK unit) and you want to measure its internal impedance at 1 kHz to establish a baseline for State of Health (SoH) tracking.

Target Parameters:
Frequency: 1,000 Hz
Duty Cycle: 10%
Pulse Current: 10A peak

First, we calculate the timing of the rectangular pulse. The period (T) is the inverse of the frequency:

T = 1 / 1000 Hz = 1 millisecond (ms)

With a 10% duty cycle, the pulse is HIGH for 10% of the period and LOW for the remaining 90%:

  • High Time (Pulse Width): 1 ms × 0.10 = 0.1 ms
  • Low Time (Space): 1 ms × 0.90 = 0.9 ms

You configure your function generator to output a 0V to 5V rectangular pulse with these exact timings, feeding it into a MOSFET gate driver that switches a 10A constant-current dummy load. Using an oscilloscope across the battery terminals, you measure the AC voltage drop during the 0.1 ms ON pulse. The scope shows a sharp voltage sag of 28 mV (0.028 V).

Using Ohm's law for the AC component:

Z = V / I = 0.028 V / 10 A = 0.0028 Ω (or 2.8 mΩ)

This 2.8 mΩ reading is your 1 kHz impedance baseline. If this number creeps up to 4.5 mΩ over the next year, you know the battery's internal chemistry is degrading, long before its overall capacity drops noticeably.

Real-World Scenario Walkthrough: The Desulfation Setup That Cooked a MOSFET

Theory is clean, but power electronics are unforgiving. Here is a real-world failure scenario involving a rectangular pulse setup that highlights common bench mistakes.

The Setup

A hobbyist was attempting to desulfate an old 12V flooded lead-acid battery. They used a bench function generator set to output a 50 Hz rectangular pulse at a 20% duty cycle, with a 5V peak amplitude. This 5V signal was wired directly to the gate of an IRFZ44N N-channel MOSFET. The MOSFET was switching a 15A load from a 14.5V bench power supply into the battery, intending to 'ring' the sulfate crystals off the plates.

The Numbers

  • Frequency: 50 Hz (20 ms period)
  • Duty Cycle: 20% (4 ms ON, 16 ms OFF)
  • Gate Drive Voltage: 5V
  • Peak Load Current: 15A

The Outcome

Within two minutes, the IRFZ44N MOSFET became too hot to touch. At the five-minute mark, it failed short-circuit, drawing maximum current continuously and blowing the bench power supply's 20A fuse.

What Went Wrong

There were two critical errors in how the rectangular pulse was applied to the physical circuit:

  1. Insufficient Gate Enhancement: The IRFZ44N has a gate threshold voltage ($V_{GS(th)}$) of roughly 2V to 4V. However, threshold voltage is merely the point where the MOSFET begins to conduct. To achieve the datasheet $R_{DS(on)}$ of 17 mΩ, the gate requires 10V. By driving the gate with a 5V rectangular pulse, the MOSFET was stuck in its linear (ohmic) region. Its internal resistance was likely over 100 mΩ, causing it to dissipate massive heat ($P = I^2 \times R$) during the 4 ms ON pulses.
  2. Inductive Kickback Without a Flyback Path: Batteries and heavy gauge wiring possess significant parasitic inductance. When the MOSFET switched OFF after 4 ms, the collapsing magnetic field tried to keep the 15A current flowing. Without a flyback diode, the voltage at the MOSFET drain spiked violently ($V = L \cdot di/dt$). This spike easily exceeded the IRFZ44N's 55V $V_{DSS}$ rating, punching through the silicon junction and causing the short-circuit failure.
The Fix: Never drive a power MOSFET gate directly from a 5V logic pulse when switching high currents. Use a dedicated gate driver IC (like the TC4420) to translate the 5V rectangular pulse into a robust 12V gate drive. Always place a fast-recovery or Schottky flyback diode (such as a 1N5822) across inductive loads to clamp voltage spikes.

Step-by-Step Configuration on a Bench Generator

If you are using a standard bench unit like the Rigol DG1000Z series or Siglent SDG1000X series, follow these steps to configure a precise rectangular pulse:

  1. Select the Waveform: Press the Pulse button on the front panel (do not select Square; Square locks the duty cycle to 50% on many basic generators).
  2. Set Frequency/Period: Enter your target frequency (e.g., 1 kHz). The screen will automatically calculate the 1 ms period.
  3. Define the Pulse Width: Instead of using duty cycle, it is often safer in power electronics to define the absolute Pulse Width (High Time). Enter 100 µs for a 0.1 ms ON time.
  4. Set Amplitude and Offset: Set the amplitude to 5V and the offset to 2.5V. This yields a pulse that swings cleanly from 0V to 5V. If you set amplitude to 5V with 0V offset, the wave will swing from -2.5V to +2.5V, which can destroy microcontroller inputs or MOSFET gates.
  5. Verify on Oscilloscope: Always connect the generator to a scope to verify the high and low voltages before connecting it to your power stage. Look for ringing on the leading edge, which indicates impedance mismatch in your test leads.

Frequently Asked Questions

Can I use a 555 timer instead of a function generator for rectangular pulses?

Yes, a 555 timer in astable mode generates rectangular pulses, but it is highly susceptible to power supply noise and temperature drift. For battery impedance testing where microsecond timing and exact duty cycles matter, a crystal-controlled bench function generator is mandatory. For simple desulfation where exact timing is less critical, a 555 circuit is acceptable and much cheaper.

Why does my oscilloscope show a negative voltage spike when the rectangular pulse goes low?

This is inductive ringing. The wiring between your pulse generator, the MOSFET gate, and the ground plane has parasitic inductance. When the pulse transitions from HIGH to LOW rapidly (high $dv/dt$), that inductance rings with the gate capacitance. You can fix this by adding a small gate stopper resistor (typically 10Ω to 47Ω) in series with the gate, close to the MOSFET pin, to dampen the resonance.

Is a rectangular pulse safe for lithium-ion charging?

Standard CC/CV (Constant Current / Constant Voltage) charging is the safest and most efficient method for Li-ion and LiFePO4 cells. While pulse charging is heavily researched, applying high-current rectangular pulses to lithium cells without a highly controlled BMS can cause localized lithium plating on the anode, leading to internal short circuits. Stick to steady DC for lithium charging, and reserve pulse functions for diagnostics and lead-acid maintenance.

For deeper reading on battery impedance measurement techniques, refer to the Analog Devices technical articles on battery impedance. For foundational knowledge on switching waveforms, the All About Circuits guide on pulse-width modulation provides excellent baseline theory.