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    This time delay switch circuit is useful to switch on an AC load such as lamps after the delay of three minutes. This protects the load from inrush current and transients at power on which may be detrimental to the device.

    The time delay switch is directly connected to the AC lines to derive power to the components. Resistor R1 drops AC voltage and D1 rectifies the same to give low volt DC. The high value capacitor C1 act as a smoothing capacitor as well as the time delay device. That is the SCR conducts only after C1 is fully charged. It takes around 2 minutes. When C1 charges fully gate of SCR gets firing pulse and it conducts. This charges C2 which also give another one minute to fire the Triac trough R3. When the triac conducts, AC load gets electrical continuity and turns on. Capacitor C3 keeps the voltage level at the gate of triac and eliminates the back lash effect.

    Time Delay Switch Circuit Schematic

    Time Delay Circuit diagram

    Note:Take extreme care while handling the circuit, since it is at mains lethal potential. Do not touch any points when it is connected to mains. It should be constructed only by an experienced person.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    54 Responses to "Time Delay Switch with Triac"

    1. can you please tell me what type (or specifications) of the lamp (or load) should be used?

      • D.Mohankumar says: on November 17, 2010 at 1:04 am

        Triac BT 136 is rated for 4 Amps load.You can use an AC load with maximum 200 watts.If the load is high, use a heat sink for the triac.

      • Hi,
        can this circuit handle home appliances like fridge or AC?

        if not, how to change it so that it can handle large load?

        thanks much.

      • Safdar Abbas says: on August 2, 2012 at 11:54 am

        Put relay on its output

      • Madhura Parasanna says: on July 15, 2012 at 12:54 pm


        I want to fix my air condition one hour on one hour off time delay circuit. can u dicained 230v control circuit.
        Please send me a above circuit diagrame.

        Thank you.

      • Sir

        I am doing mobile jammer as my project work.I have seen the schematic in ELECTROSCHEMATICS .could you suggest me the changes i need to make in it such that it works in INDIA.

        With regards
        Ravi kumar

      • mark rebreb says: on April 1, 2014 at 5:01 am


      • mark rebreb says: on April 1, 2014 at 6:37 am

        hi ang circuit na ito ay maaari bang gamitin sa 1000 watts na amplifier? para sa proteksyon ng amplifier

      • Bro, pwede gamitin yan sa 1000 watts Amplifier. Kasi mababa lang naman ang load ng amplifier. Ang 1000 watts Amp refers to power output ng Audio circuit (RMS or Peak to peak) power hindi ng power supply input.

      • s narayanan says: on January 14, 2015 at 5:14 pm

        sir, r u having stabilizer circuit.everest make

    2. thank you very much.

      By the way just for clarification. What does the P and N stands for in the 230V AC supply? Can we determine the positive and negative sides of an AC supply? Also in the R3 resistor, does it really have a 50V specification?

      • D.Mohankumar says: on November 17, 2010 at 5:26 pm

        P stands for Phase ( Hot side) and N for neutral ( Cold side) of AC lines.Using a tester you can check phase side. Usually RED wire is for phase, black for neutral and green for earth ground.Regarding R3, it is 1/4 watt resistor not 50 V. it is a mistake.

      • thank you… I was questioning that because our AC Supply here has only two sockets (one for Hot and one for Ground) and so we don’t really know which one is the Phase or Neutral since it is an AC. Is this circuit also applicable for that kind of AC Supply?

      • Safdar Abbas says: on August 2, 2012 at 12:00 pm

        No problem , you can use any condition. But its better if you use phase wire with triac.

        Normally Black is used for neutral.

    3. Is this also applicable on a 220V 60Hz AC supply?

    4. Hello. I just tried this circuit but there was no time delay. As I plugged it into an AC source, the lamp/load just flickered for about 3 seconds and then lighted up as normal. There was no time delay. I believe I had done the right connections.

      Can you help me?

      • D.Mohankumar says: on November 22, 2010 at 2:43 am

        Hello Vincent

        The working of the circuit is simple.At power on SCR has no gate current.It conducts only when the capacitor C1 fully charges.The time for the charging is the delay time.Your problem may be
        1.Capacitor is leaky. 2.SCR is faulty .Check all the capacitors. Check the pin connection of SCR. Voltage rating of the capacitors must be 50-100 volts

      • hello sir,

        I am vijay kumar . i need a temprature indicator /com/ controller using pT100RTDs,THERMOCOUPLEs, 7sigment displys, mc,Relay,…..

        which mc is suffiset to desigen this….

        how to make it can you give me any idaa???

    5. is it okay to use 2P5M scr instead of 2P4M?….what is the difference between the two? please include also the procedure on how to test the scr..i will appreciate your reply..

    6. hi,i need this time delay with a setting of 1 minute, 3 minutes and 5 minutes.. if so, what are the values of the components that will be changed so that the delay time will will be 1 minute, 3 minutes and 5 minutes? can i use it for appliances such as refrigerators? im waiting for your reply.

    7. here as soon as D1 rectifies the AC , the gate current for scr is available no?. then why do you want capacitor C1.

    8. can we do the simulation for this……. please suggest me anyone

    9. how can we increase the load limit from 4 Amps to 7-8 Amps……..

    10. i connect everything right. check all the pins and components. still it didnt delay the time of operation of the lamp(7w and 100w) i try the two but nothing happens. as i connect it to ac source it flickers and lights the bulb.

    11. Would this work in the US with 115 volts? If not what mods will make it work?

    12. The way a cap works is at the beginning the capacitor is a straight short. Which means it has zero volts on both sides of the capacitor, as it charges the positive side of the cap will develop a positive voltage compared to the negative side which is tied to earth.

      When this happens a voltage drop will be created across R2, when the voltage gets high enough it will produce the current required to turn on SCR. The time it takes to charge the Capacitor to develop the voltage across R2 is the first component in the time delay. The second part of the time delay is how long it takes to charge C2 once the SCR is turned on and conducting current to charge C2.

      To increase the power to the load, you will need to buy a triac capable of providing the current needed by the load. This may or may not change the value of R3 depending on how much current is required to turn on the different triac you will need.

    13. Malcom, no you will need to adjust the resistors and capacitors for the lower voltage. More then likely, R1 will need to be lowered to create a smaller voltage drop to maintain the 20V the rest of the circuit requires.

    14. The C1 capacitor will charge up gradually through the R1 resistor until the voltage across the capacitor reaches the threshold voltage of the thyristor (Vc1=0.7V). The voltage across the capacitor, Vc1 at any instant in time during the charging period is given as:
      Vc1 = Vs[1 – e^(-t/RC) ]
      Vs= the peak supply voltage
      If we use 230 Vac, the peak voltage is therefore 230* 1.41, which is about 325 V. Do the math and obtain 70ms time delay.

      Later the triac turn on for Vc2> Vgt +Igt*R3 ~ 4V
      Second time delay ~ 270 ms
      Total time delay = 70+270= 340ms= 0.34s??

    15. I have already tested this circcuit. Results here:

      To improve this circuit need to use a DIAC, see the details here:

      Components used for:
      Triac = TIC206M/TIC216M -sensitive gate triacs

      Thyristor = C106M
      DIAC = DB3

    16. sir D. Mohankumar i read all your suggestion in this type circuit it helps me a lot to my project

    17. Thank you sir

    18. am a new person for now, i will like to try it fist.

      • Safdar Abbas says: on August 2, 2012 at 12:05 pm

        Be carefull don’t touch any component when powered. Its takes about 2 mins to lose all charge from capacitors.

    19. can we use a normal scr inted of scr 2p4m

    20. Respected Sir,

      I want to Make A Timer For On Delay . How much is The Discharging time of this circuit … i need Discharging too Fast . Could U Suggest me A timer for 10 seconds For 220 V AC .

      Thanks & Regards

    21. replaced C1 with 10uf 100V

    22. Respected sir,
      I have seen the schematic of cell phone jammer circuit.could you please send me the necessary changes i need to make in it such that it works in india.Please tel me where can i get those components

    23. R/SIR
      I am done with the connections of the circuit but im not getting the delay ….
      plsss do reply soon

    24. hello sir..

      how much voltage/current required to turn on the triac and the scr

    25. I want to turn off the power after few minutes.Can the triac be modified to such a way?I want to cut off the transformer power after a two minutes delay

      • Jim Keith says: on December 17, 2012 at 8:26 pm

        How is the turn-off delay to be initiated?
        NO pushbutton contact closure?
        NC pushbutton contact opening?
        Application of AC signal voltage?
        Removal of AC signal voltage?

      • Sir Jim, This circuit does not delay the turn OFF. This circuit assumes that there is a power failure or interruption and it will delay the turn ON of the load connected(Appliance or lamps)once the power resumes.

    26. sir i want to ask how analyze different phases like(red blue green), if i dont confirm the source exactly.
      because if i check by tester then tester will bright on all phases.

    27. hi sir can i change charging and discharging time using veriable resistor plz suggest me how and which value is needed?

    28. Sir…

      Please can u tell me about delay off light about 1-2 minute .

      meaning .. after switch off , light should off after 1-2 minute.

    29. why do you use scr to trigger the triac instead of diac?

    30. Can you please help me create a circuit for the following purpose.
      my projector lamp is burning off as I switch off the power supply before cooling fan shuts off.so I need to create an 5min off delay timer which works on 230v.I want to fit it inside a switch box and when I turn the switch “off”.there should be a 5min delay in cutting power off to my projector.pls help

    31. Sir..

      This is delay on circuit .

      Pls. Can u tell me the delay off circuit as same.

    32. Hi sir , that’s a great work ,,, am looking for circuit to keep my 1500W of load is ON at least for “15 second” after disconnecting 220V-AC power source , and i need that without Relay.
      please help me ..

    33. sir,
      how can i use this circuit as a notification device, when there is light (220VAC) it give an audible alarm for 1 minutes then the alarm stops . Just to announce there is power…….


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