A current-sensing relay is a device that is used to detect abnormal current conditions in electrical circuits. It monitors the current flowing through a circuit and actuates when the current goes above or below preset thresholds.
In this article, we will look at the circuit diagram of a current sensing relay for overcurrent protection.
What Does a Current Sensing Relay Do?
A current sensing relay measures the current in a monitored circuit using a current transformer which converts the primary current to a proportional secondary current. This is applied to a relay with an adjustable threshold and hysteresis.
When the secondary current exceeds the preset threshold, after a time delay the sensor contacts change state to disconnect the load circuit, providing overcurrent protection.
Circuit Diagram of a Current Sensing Relay Circuit
The circuit diagram of a simple current sensing relay for overcurrent detection is shown below. It consists of:
- Current Transformer (CT1)
- Rectifier Diode (D1)
- Filter Capacitor (C1)
- Adjustable Resistor (VR1)
- Comparator (IC1)
- Reference Zener Diode (ZD1)
- Output Relay (K1)
- Calibration Resistor (R2)
- Ripple Counter (IC2)
Fig – Circuit Diagram of a Current Sensing Relay.
Let’s analyze the function of each component in detail:
Current Transformer (CT1)
CT1 senses the primary current in the monitored circuit via a single turn through its window. It converts the primary current (Ip) into a small secondary current (Is) using the electromagnetic induction principle.
CT1 electrically isolates the high-voltage power circuit from the low-voltage control circuit.
Rectifier Diode (D1)
Diode D1 rectifies the alternating secondary current from CT1 into a pulsating DC waveform. Only the positive half cycles of the AC current pass through the diode. This produces a corresponding pulsating DC voltage across capacitor C1.
Filter Capacitor (C1)
Capacitor C1 filters the pulsating DC voltage from the rectifier diode. It charges to the peak voltage of each pulse and discharges minimally between pulses. This produces a steady DC voltage proportional to the peak AC voltage, which corresponds to the RMS value of AC current.
Voltage Divider (R1 and VR1)
The potentiometer VR1 and resistor R1 divide the supply voltage to generate an adjustable reference voltage Vref.
Where Vin is the supply voltage. Varying VR1 varies Vref to set the pickup threshold.
Zener Diode (ZD1)
ZD1 provides a regulated voltage Vz to IC1.
Vz = Zener voltage (constant)
This allows stable operation of the comparator over a wide supply range.
IC1 compares the rectified voltage Vdc to the reference Vref.
If Vdc > Vref, output Vout is high which triggers the output relay.
If Vdc < Vref, Vout is low.
Output Relay (K1)
Relay K1 isolates the low-voltage control circuit from the high-voltage power circuit. The contacts energize on receiving a high signal from comparator IC1. K1 contacts control the coil of magnetic contactor M, isolating the load on overcurrent.
Calibration Resistor (R2)
Resistor R2 calibrates the pickup threshold by passing a known current Ical to set the reference voltage Vref to actuate the relay.
The sensing circuit quantifies the primary current for comparison with thresholds. The output relay isolates it from the trip circuit.
Ripple Counter IC2 (CD4060)
IC2 contains a 14-stage binary ripple counter configured as a divider to provide timing pulses. The internal oscillator frequency is set by R5, R6, and C1 to the required value.
Output Q9 provides a pulse after 512 oscillator cycles, set by the divide-by-512 counter. This Q9 pulse creates the time delay for current sensing.
Q9 pulses a piezo buzzer through a transistor circuit to give an audible alarm. The ripple counter IC2 thus acts as an adjustable timer and oscillator using simple RC components.
FAQs – Frequently Asked Questions and Answers
1. How Are Microprocessors Used in Digital Current Relays?
Answer: A microprocessor does digital sampling of the current sensor output, runs relay logic programs, and drives output contacts.
2. What Communication Interfaces Are Provided in Modern Current Relays?
Answer: Modern relays provide communication interfaces like Ethernet, Serial, USB, and wireless for integration with SCADA and automation systems.
3. How Are Self-Diagnostics and Testing Implemented in Smart Current Relays?
Answer: Smart current relays have built-in testing circuits to continuously check the status of components and circuit operation.
A current sensing relay is an important device in electrical and industrial applications. Understanding this circuit will help engineers and technicians to design and troubleshoot electrical systems more effectively. Hopefully, this article will help you in this regard.
Subscribe to our newsletter
& plug into
the world of circuits