The only electronic website that replies to every comment, and addresses all circuit related issues.

Mains High Low Voltage Protection Circuit with Delay Monitor

The post explains an upgraded version of my previous mains 220V/120V high-low voltage cut off protection circuit which now includes a delayed restoration of power for the load with 3 LED power status indicators.

The idea was requested by one of the dedicated members of this website.

The Request

Dear Swagatam, I just followed you're explanation and is it possible that you can help us with the following: To design a safety circuit that should provide for household appliances for over-and undervoltage protection. 

The protective circuit must immediately switch off upon detection of low-and high-voltage household appliance and upon detection of normal voltage switch on again after 3 minutes. 

The protective circuit must comply with the following: If the line voltage is within the normal range (100 to 130V ac), it will wait for the protective circuit 3 minutes before the output will be energized. During these 3 minutes there is an amber 

LED light. If the line voltage is outside the normal voltage, the output of the protective circuit will never be under tension. If the line voltage is less than 100VAC, the protection circuit "low voltage" must indicate by a red LED that lights up. 

If the line voltage is present, the protection circuit must pass a voltage greater than 105 Vac "normal tension" it will indicate by a green LED that lights up. 

Similarly, the line voltage protective circuit has to be higher than 130V ac "high voltage" will be indicated by a red LED that lights up. Only when a voltage is less than 125VAC, it must indicate the protection circuit "normal tension" by a green LED that lights up. 

Upon detection of over-and under voltage protection, the circuit should give a beep of 5 seconds. 

This should be constructed with an opamp oscillator circuit in this functionality.

Mains High Low Voltage Protection Circuit with Delay Monitor

The Design

The above shown mains high/low voltage cut off protection circuit is an enhanced version of my previously explained design which had similar high low cut off protection feature except the delay timer stage which has been added in the present design as per the  request.

The timer stage ensures a delayed power switch ON for the load each time the mains is cut off due to an abnormal fluctuating voltage so that the load is never subjected to an abrupt or random voltage switching situation .

The circuit also includes 4 distinct LEDs which indicate the corresponding mains voltage levels or status through their individual colors. The two red colors indicate high and low voltage situations respectively, the amber color LED indicates the intermediate delay counting status of the circuit, while the green LED informs the user regarding a healthy mains output condition.

The P3 preset or pot is used for setting up the delay time switch ON for the IC 4060 stage

How the Circuit Works:

We already know from our previous post that whenever the input voltage crosses the higher threshold, a logic high is created at the output of the upper opamp and when the voltage drops below the lower threshold the lower opamp generates a high logic at its output.

This implies that during both the conditions a high logic is generated at the cathode junction of the diodes connected with the opamp outputs.

We know that the timer IC 4060 is forced to reset in the presence of a positive trigger at its pin#12, and the IC stays disabled (output open) as long as a high is sustained at this pinout of the IC.

Therefore for so long the output from the opamps is held positive, pin#12 is kept high and subsequently the IC 4060 output pin#3 is held deactivated, which in turn keeps the relay switched OFF along with the mains load disconnected through the N/C contacts.

Now as soon as the mains voltage returns to its normal level, the high logic at pin#12 of IC 4060 is removed, so that the IC is allowed to commence its counting process.

The IC now begins counting as per the values set by C3/P3. Supposing the mains remains stable during the whole counting process, the IC counting finally elapses enabling a logic high at its pin#3, which triggers the relay and the load into action.

However suppose while the counting was in progress, the mains kept fluctuating, the IC would be forced to reset repeatedly and this would keep the output completely switched OFF making sure that the load was never allowed to face the unpredictable and fluctuating mains condition.

How to Set Up the Circuit.

Initially keep the power supply disconnected with the circuit.

Apply mains input to the power supply transformer and measure the DC output across the filter capacitor, and also measure the existing input mains level at the input of the transformer.

Let's say the mains voltage is found to be around 230V, which results in the production of a DC output of around 14V.

Using the above data now it may be possible to calculate the corresponding upper and lower cut off  thresholds, which may be used for setting up the respective presets .

Suppose we want 260V to be the upper cut off level, and 190V as the lower cut off, the corresponding DC levels could be calculated with the help of the following cross multiplication:

230/260 = 14/x

230/190 = 14/y

where x represents the corresponding upper cut-off DC level and y the lower cut-off DC level.

Once these values are calculated, using a variable DC power supply, feed the upper DC level to the circuit and adjust the upper preset such that the upper opamp LED just lights up.

Next, in a similar fashion apply the lower DC level and adjust the lower preset until the lower opamp LED just lights up.

That's it! The adjustments for the upper high, and lower under voltage cut-off setting up procedures are complete, and the system can be now be plugged-in with the mains for the actual test.

Parts List

R1, R2, R3, R4, R7 = 10K
R6, R8 = 4K7
R5 = 1M
P3 = 100K POT
C2 = 0.33uF
C3 = 1uF
C1 = 1000uF/25V
P1,P2 = 10K PRESET
Z1, Z2, Z3 = 4.7V/ 1/2 WATT
D1---D4, D8 = 1N4007
D5----D7 = 1N4148
IC1 = LM358
IC2 = IC 4060
T1 = BC547
T2 = BC557
RELAY = 12V/250 OHMS, 10 AMPS
L1----L4 = LEDS 20mA, 5mm
transformer = 0-12V/1 AMP

Please Share this Post:
  •  Facebook
  •  Twitter
  •  Google+
  •  Stumble


    1. The circuit will short mains through relay contact

    2. Hello, just seen your circuit of the over/under voltage protector. this is a very good circuit. My only concern is the mains transformer. If there is going to be a high voltage greater than 260v from the line this will definitly blow the transformer's primary winding. Any ideas, can one include a vdr to protect the transformer?

      1. Hi, thanks,

        yes, above a certain level the transformer itself might become vulnerable of getting burnt, a simpler and effective approach could be to go for a triac based crowbar circuit across the primary of the transformer.

        the crowbar could be set to trigger at around 260V using a 260V zener diode which would finally blow off the series connected fuse and cut the supply.


    Please Note: If you find any DEAD link starting with http//, please replace it with for making it visible and alive again.