A level detector in a static relay gives an output (logic 1) when:

The input exceeds a fixed reference level

What does positive feedback (Schmitt trigger) provide in a level detector?

Different pick-up and drop-off levels, avoiding output chatter

A higher gain at the reference level only

How is the relay setting (pick-up level) fixed in a level detector?

By the reference voltage from a Zener diode or potentiometer

By the output relay coil resistance

2073 Magh · 4 marks

Short note: Level detectors

Short answer

A level detector is a comparator circuit that outputs a logic high when an input signal exceeds a set reference level. It uses a Schmitt trigger for hysteresis to prevent chatter, and serves as the pick-up element in static relays for overcurrent, voltage, and frequency protection.

Write a short note on level detectors.

A level detector is a static relay circuit that compares an input signal with a fixed reference level and gives an output (logic 1) only when the input exceeds that level. It acts as the pick-up element of a static relay.

            +Vcc
             │
 V_in ──►(+)╲
            │ ▷──── V_out → next stage
 V_ref ─►(−)╱   (high when V_in > V_ref)
             │
       (positive feedback R for
        hysteresis – Schmitt trigger)

Construction: an op-amp (or transistor) comparator; V_ref set by a Zener diode or potentiometer, which fixes the relay setting.
Hysteresis: positive feedback (Schmitt trigger) gives different pick-up and drop-off levels, so the output does not chatter when the input is near the threshold.
Input: rectified and smoothed CT/PT signal, or the output of a comparator/timer.
Uses: pick-up of overcurrent, over/under-voltage and frequency relays; detecting when a timer capacitor voltage reaches its set value; conditioning comparator outputs.
Requirements: accurate, stable threshold with temperature; high reset ratio (≈ 0.95); fast response.

✓ Answer checked by Claude Opus 5.5 (high effort)

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