What Is an Electrical Pulse?

An electrical pulse is a brief departure from a signal’s normal level, followed by a return to that level. A pulse can indicate that a product has passed a sensor, a shaft has moved through a defined angle, or a motion command has been issued. Pulses are widely used in PLCs, measurement and motion control. In Turkish technical usage, the English word pulse also appears as pals or puls.

Digital systems commonly use rectangular pulses, although pulses can have other shapes and can be positive or negative. A signal may briefly rise or fall from its baseline. High and low are interpreted using the receiving device’s electrical thresholds; they do not represent universal voltage values.

What Information Does a Pulse Carry?

Imagine a conveyor sensor producing one pulse whenever a box passes. Counting those pulses gives the number of boxes. Measuring the intervals between them provides information about the rate of passage. Pulse count represents a quantity, while pulse repetition rate describes how often events occur. The signal’s purpose matters as much as its shape.

Amplitude, Pulse Width, Frequency and Duty Cycle

Amplitude: For the two-level examples used here, this is the voltage difference between the high and low levels. A signal switching between 0 V and 5 V has a 5 V level difference. A 5 V signal and a 24 V signal are not interchangeable. Check the receiver’s voltage limits and logic thresholds before making a connection.

Pulse width: The time for which a pulse remains active, usually expressed in seconds, milliseconds or microseconds. For a positive pulse, this generally describes the high interval. Oscilloscopes commonly measure pulse width between the rising and falling crossings at 50% of the pulse amplitude.

Period and frequency: For a regularly repeating signal, the period (T) is the duration of one cycle. Frequency is the number of cycles per second, calculated as f = 1 / T. An isolated pulse does not have a defined repetition frequency in the same way as a periodic pulse train.

Duty cycle: For a positive pulse train, this is the high interval as a percentage of the period: D = (t_ON / T) × 100. Duty cycle can change while frequency stays constant. This distinction is central to understanding pulse width modulation (PWM).

Edges and transition times: A rising edge or falling edge is a transition between signal levels. Real signals take time to change. Rise time is commonly measured from 10% to 90% of the amplitude; fall time uses the reverse transition. The instrument’s selected measurement thresholds must also be considered.

Worked Example: 100 Hz at 40% Duty Cycle

Suppose a signal stays high for 4 ms, then low for 6 ms, and repeats. The time bar below shows how one period is divided.

T = 4 + 6 = 10 ms = 0.01 s, giving f = 100 Hz and D = 40%. Changing the high interval to 7 ms and the low interval to 3 ms keeps the frequency at 100 Hz, but increases duty cycle to 70%. A wider pulse does not necessarily mean a higher frequency.

Single Pulses, Pulse Trains and PWM

Single pulse: One pulse generated for an event, such as a trigger or reset command. Monostable describes the behaviour of a circuit or function; it is not another name for a single pulse.

Pulse train: A sequence of pulses. A regularly repeating train has a measurable frequency. An ideal square wave is the special case with equal high and low intervals, giving a 50% duty cycle.

PWM: Pulse width modulation. In common implementations, the period stays constant while the active interval changes. With a suitable power stage, it can control LED brightness or be used in motor control. The PWM control signal and the power stage supplying the load have different roles.

Calculating Shaft Speed from Encoder Pulses

Assume an encoder manufacturer defines 1000 PPR as 1000 complete signal cycles per revolution on one channel. At 900 RPM, the shaft turns 15 times per second. The frequency on that channel is therefore 1000 × 900 / 60 = 15,000 Hz, or 15 kHz.

With x4 quadrature decoding of channels A and B, the receiver obtains 4000 counts per revolution and 60,000 counts per second at this speed. The 15 kHz channel frequency and the total rate of 60,000 counts/s describe different quantities. Always check how the manufacturer defines PPR, CPR and resolution.

Why Can a PLC Miss Pulses?

When an input is read through the normal program cycle, a short pulse can start and finish between two input reads. Pulse duration, input filtering and scan time must be considered together. A suitable high-speed counter (HSC) can count fast signals using dedicated hardware on supported inputs, independently of the normal program scan.

An input filter helps reject noise, but an excessively long setting can also suppress valid short pulses. Check the input’s maximum frequency and minimum high and low times. Select the filter using the device documentation and actual signal conditions rather than automatically choosing the shortest available setting.

How Are Pulses Used as Motion Commands?

A suitable PLC high-speed pulse output can send a motion reference to a drive that accepts pulse commands. In a pulse-and-direction arrangement, pulse count determines commanded travel according to the configured scale, pulse frequency determines commanded speed, and the direction signal selects the direction. This method is used with stepper motors and compatible pulse-input servo motor systems.

PTO generates a pulse train for motion commands, while PWM commonly varies the active-time ratio. A servo drive’s electronic gearing or a stepper drive’s microstep setting changes the motion represented by each command pulse. Commanded motion must still be checked against actual motion. Mechanical relay outputs are unsuitable for generating high-speed pulse trains.

Electrical Compatibility Matters

Pulse describes how a signal behaves over time. NPN, PNP, open collector, push-pull and line driver describe electrical interfaces, such as encoder output types. Verify sinking and sourcing compatibility, supply voltage, current limits and the manufacturer’s wiring diagram together.

Suitable line driver outputs provide complementary signal pairs for a differential receiver. An arbitrary pulse input may not support that interface. Cable routing, shield connections, noise suppression and signal filtering become especially important with fast signals. Follow the device instructions for shielding and signal-reference connections.

How Do You Measure a Pulse Signal?

An oscilloscope reveals voltage levels, pulse width, transitions and noise. An intermediate voltage shown by a multimeter in DC mode may not represent the actual high level; the reading depends on the instrument’s measurement method. Frequency and duty-cycle readings are meaningful only within its input specifications. The measurement point and reference connection also affect the result.