---
title: "Short note: Op-amp in static relay"
subject: "Switchgear and Protection"
chapter: 8
key: "semester_6/switchgear_and_protection:short-note-op-amp-static-relay"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-op-amp-static-relay
site: BCT Engineering
---

# Short note: Op-amp in static relay

Switchgear and Protection · Chapter 8: Static and Digital Relays

- Asked in: 2073 Magh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-op-amp-static-relay
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8

## Question

Write a short note on use of operational amplifier in static relay.

## Short answer

Operational amplifiers in static relays provide signal amplification, level detection, precision rectification, filtering, and timing functions. They offer high accuracy, low burden on CTs/PTs, and compact design, but require a regulated DC supply and surge protection.

## Answer

An **operational amplifier (op-amp)** is a high-gain DC-coupled differential amplifier IC. Its high input impedance, low output impedance and easy gain control make it the basic building block of modern static relays.

**Uses of op-amps in static relays:**

| Function | Op-amp circuit |
|---|---|
| Amplifying CT/PT signals | Inverting/non-inverting amplifier |
| Level detection (pick-up) | Comparator / Schmitt trigger |
| Amplitude comparison | Difference amplifier on rectified S1, S2 |
| Mixing of V and I signals | Summing amplifier |
| Rectification of small signals | Precision rectifier (no diode drop) |
| Time delay | Integrator, RC timer with comparator |
| Phase shift / filtering | Active filters (harmonic restraint) |
| Isolation/buffering | Voltage follower |

```text
 V1 ─R─┐
       ├──(−)╲
 V2 ─R─┘     ▷── V_o = −(V1 + V2)·Rf/R
       ┌─(+)╱
      ─┴─ (Rf from output to (−))
```

**Advantages:** high accuracy and sensitivity, very low burden on CTs, compact, characteristics easily shaped, fewer components. **Limitations:** need a regulated DC supply (±15 V), sensitive to surges and temperature drift (offset), so protection circuits are needed.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which properties make the op-amp a basic building block of static relays?
   A. Low input impedance and high output impedance
   B. High input impedance, low output impedance and easy gain control
   C. Unity gain and no DC supply
   D. High burden on the CT
   Answer: B. High input and low output impedance with easy gain control suit signal processing and give very low burden on CTs. (Key point: Why op-amps)

2. Which op-amp circuit is used for level detection (pick-up) in a static relay?
   A. Summing amplifier
   B. Voltage follower
   C. Comparator or Schmitt trigger
   D. Active filter
   Answer: C. Level detection is done by an op-amp comparator or Schmitt trigger. (Key point: Circuit-function matching)

3. Why is a precision rectifier built with an op-amp used for small signals?
   A. It has no diode voltage drop
   B. It produces AC from DC
   C. It raises the burden on the CT
   D. It removes the need for a supply
   Answer: A. The op-amp precision rectifier avoids the diode drop that would swamp small signals. (Key point: Precision rectification)

4. Which is a limitation of op-amps in static relays?
   A. They need a regulated DC supply and are sensitive to surges and temperature drift
   B. They cannot amplify signals
   C. They have very high burden
   D. They cannot be used for time delays
   Answer: A. Op-amps need a regulated supply such as +-15 V and suffer offset drift and surge sensitivity, so protection circuits are needed. (Key point: Limitations)

## Related in Switchgear and Protection

- [Essential components of a static relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-relay-essential-components) — Switchgear and Protection, chapter 8, asked 5×
- [Static differential relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-relay) — Switchgear and Protection, chapter 8, asked 3×
- [Static distance relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-distance-relay) — Switchgear and Protection, chapter 8, asked 3×
- [Directional static overcurrent relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adirectional-static-overcurrent) — Switchgear and Protection, chapter 8, asked 2×
- [Static differential relay for two-winding transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-two-winding) — Switchgear and Protection, chapter 8
- [Static MHO relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-mho-relay) — Switchgear and Protection, chapter 8

## Related in other subjects

- [Op-amp properties and summing amplifier](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-summer-v1-2v2-3v3) — Basic Electronics Engineering, chapter 4
- [Summing amplifier using op-amp](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Asumming-amplifier-op-amp) — Fundamental of Electrical and Electronics Engineering, chapter 6
- [Op-amp properties and integrator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-integrator) — Basic Electronics Engineering, chapter 4
- [Ideal op-amp properties; non-inverting gain](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-noninverting-gain) — Basic Electronics Engineering, chapter 4, asked 3×