Undervoltage protection circuit with a relay

[robsims]

Hi, here in my country we have a mains voltage of 127 volts. I want to make an undervoltage protection circuit with a relay. When the voltage is around 105 volts the relay must be switched off. The relay is 12 volts and has a coil resistance of 322 ohms. Coil current is 0.0031 Amps.    I used a transformer, bridge rectifier, capacitor but the relay disconnects at mains 40 volts. Made a circuit with Zener diodes, bridge rectifier, transistors and a voltage dropper capacitor, but the relay also disconnects at mains 40 volts. Anyone has any idea? At mains voltage of 105 volts the relay must be disconnected. Many thanks

[thm_w]

If you plan to use it for important applications, you can buy one off the shelf for $10-30 or so:
https://www.ia.omron.com/products/family/3381/
https://www.geya.net/product/single-phase-voltage-monitoring-relay-grv8-01/

If you want to build your own design, you'll need a way to measure the mains voltage (voltage divider) and compare it with a reference (zener, TL431, etc).
https://www.homemade-circuits.com/mains-high-low-voltage-protection/

[robsims]

Many thanks thm_w. I'm going to build one myself. I have to learn how opamps work. iI don't have LM358 and
 IC 4060. I'm going to buy them. Suggestions from others are welcome too

[james_s]

Can you find a UPS? That will do what you want, as well as providing battery backup whenever the input voltage is outside of an acceptable range.

If you build your own cutout relay you might consider making it so that it latches off and has to be manually reset in order to prevent it from repeatedly cycling off and on. You also will probably want a brief delay so it doesn't cut off every time a motor or other heavy load starts up.

[robsims]

Many thanks  james_s. I want to try to build one myself.  I want to make it so, that when i press the "on" button the relay latches and stays that way and when there is an undervoltage the relay shuts off. It has to have a beeper to let me know when the voltage is normal again. The on button will also act as a manual reset button. The relay has to have a NO contact to connect my refrigerator an a NC contact for the beeper

[Vovk_Z]

Working with mains voltage is not a good start how to learn OPamps. I would recommend to buy one off the shelf and to learn OPamps at galvanically isolated DC low-voltages.
I am sure that 99.9% skilled professionals would buy a voltage relay for his house off the shelf but not do it by yourself.

[robsims]

Working with mains voltage is not a good start how to learn OPamps. I would recommend to buy one off the shelf and to learn OPamps at galvanically isolated DC low-voltages.
I am sure that 99.9% skilled professionals would buy a voltage relay for his house off the shelf but not do it by yourself.

True, but i want to build one myself

[robsims]

If you plan to use it for important applications, you can buy one off the shelf for $10-30 or so:
https://www.ia.omron.com/products/family/3381/
https://www.geya.net/product/single-phase-voltage-monitoring-relay-grv8-01/

If you want to build your own design, you'll need a way to measure the mains voltage (voltage divider) and compare it with a reference (zener, TL431, etc).
https://www.homemade-circuits.com/mains-high-low-voltage-protection/

Can i use a LM324N also?

[thm_w]

yes

https://components101.com/article/difference-between-lm358-and-lm324

[james_s]

You can use almost any op-amp or comparator in this project, the requirements are nothing fancy.

[2N2222A]

The relay is 12 volts and has a coil resistance of 322 ohms. Coil current is 0.0031 Amps.

12/322 = 37mA not 3.1mA.

.037 * 120V = 4.4W. A darlington transistor or MOSFET in series with a 5W resistor could be used to switch this directly. Better yet, a 24V relay would draw half as much current and require slightly less than half of 5W of heat lost. With a 12V relay that needs 37mA a 3.1k 5W resistor would work. The transistor would need to dissipate up to 2.5W so it may not need a heat sink.

A full bridge rectifier can be used to power the relay and transistor circuit. The MOSFET gate needs a 15V zener and resistor like 47k in parallel to let it turn off and not get over volted. Then connect the MOSFET gate to a 47k resistor in series with a 4.7kish resistor in series with a 130V zener diode connected to the recifier +. In between the 47k and 4.7k resistors there should be a capacitor connected to ground. edit: There should be a diode in series with the 4.7k resistor to keep it from discharging the capacitor when the AC Voltage is no longer peaked.

The capacitor smooths out the spikes so the MOSFET isn't turning on and off with the line frequency. The 4.7k resistor prevents the 130V zener diode from getting too much current put through it. The diode datasheet will need to be checked to make sure the 4.7k resistor is large enough.

This is a total of 14 discreet components.

[robsims]

Many thanks guys, I already read some articles about opamps and how to use them. I will use it in the second circuit of the article thm_w posted. Other suggestions are welcome

[Terry Bites]

Here's a simple idea. A regulated supply powers the comparator (or opamp) and provides a fixed 5V reference voltge.
Comparing the rectified input with the ref trips the comparator. R5, R6 set the trip point.
I used a 741 here

[MrAl]

Hi, here in my country we have a mains voltage of 127 volts. I want to make an undervoltage protection circuit with a relay. When the voltage is around 105 volts the relay must be switched off. The relay is 12 volts and has a coil resistance of 322 ohms. Coil current is 0.0031 Amps.    I used a transformer, bridge rectifier, capacitor but the relay disconnects at mains 40 volts. Made a circuit with Zener diodes, bridge rectifier, transistors and a voltage dropper capacitor, but the relay also disconnects at mains 40 volts. Anyone has any idea? At mains voltage of 105 volts the relay must be disconnected. Many thanks

An op amp can perform a comparator operation and so can a comparator like LM339.

The main idea is that you provide for a filtered DC voltage to the op amp and reference, then compare the filtered line voltage to the reference voltage.  You also want to have a pot to adjust the trip point and maybe some hysteresis which is a resistor from output of the op amp to the non inverting input of the op amp.

If you have a DC relay then the op amp would drive a transistor which would the drive the relay.

In less important applications you can use a resistor in series with the coil of an AC relay to adjust the trip point, but it's not as reliable.  It's the simplest though all you need is a resistor and a relay.  It's also harder to get the hysteresis right.  You can do better with a DC relay, rectifier, filter cap, and series resistor but it's also not as reliable as an op amp or comparator circuit.

[Terry Bites]

You might want to stick with a transistor that switches a small DC relay and use it's contacts to operate a contactor's coil. If you did use a contactor like this, you get UVLO and emergency stop into the bargain.
This also lets you use a very small transformer, just enough VA's to pull in the relay reliably. The 431 can just be an other zener. The trip point isnt critical.
As is, the relay would see a wide range of voltages. It probably needs its own regulator. This could be used to refine my back of envelope sketch.

Its refined now

[Kleinstein]

If space and costs are not at a premium, I would stay away from the super small transformers below some 2 VA: there have horrible efficiency and the no load loss is usually lowest at some 3 VA and gets again worst for the smaller ones. They also have some voltage stabilization build in as the usually drive the core close to saturation under low load. This makes them less suitable to "measure" the actual voltage.

The comparator would need quite some hysteresis, as the load from the relay turned on would lower the transformer output voltage.