delay on circuit schematic

12V Time Delay Relay Circuit

Protect your equipments with this tiny 12V time delay relay circuit. The SMPS based power supply of these modern electronic devices is vulnerable to spikes in the mains line so it gives a time delay of one minute before applying power to the device. This prevents deleterious effects due to inrush current and spurious spikes at power on.

Inrush current at power on or power resumes after a power failure can cause unexpected damage in SMPS based power supply of electronic devices. The spurious spike in the power supply when power resumes is due to heavy magnetic flux in the distribution transformer in the mains network. If a short delay is provided, such damages can be avoided. The time delay relay circuit described here is intended for this purpose. It gives power to the device only after one to two minutes of delay after the power is switched on. The circuit is a zener controlled switch.

Capacitor C1 charges through R1 and VR. When the voltage in C1 rises above 3.1 volts, zener conducts to trigger T1. The relay connected to the collector of T1 energizes and power will be available through the common and Normally Open contacts of the relay. Relay remains latched as long as the voltage level in the mains is normal. Capacitor C2 keeps the base bias of T1 steady so that relay clicking can be avoided. Diode D1 prevents back EMF when T1 switches off. Red LED indicates the Relay On status. Delay time depends on the value of C1.

Time Delay Relay Circuit Schematic

time delay relay on circuit schematic

delay on circuit schematic

Power to the circuit can be derived from a standard 12 volt transformer with rectifier and smoothing capacitor. Use a socket to connect the TV as shown.

SMD, one of our top members has given us an alternative version of the circuit that seems to be working quite well.

SMD’s 12 volt time delay relay schematic and perfboard

Turn On Delay Video Presentation

Check out his Youtube page.


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  • SMD

    Reading your criticism again it dawns upon me you actually haven’t got a clue what you are on about. The wiring on the gates is PERFECT!!

    I suggest you at least build up a circuit (and do the optimising if that pleases you) first before you put up such nonsense!!

    These fets are not there FOR the delay circuit, these are only there to discharge C3 fast to re-engage the delay immediately at a fast turn off/on.

    And the website, do you get off on this stuff? Get a life!!

  • Colin Mitchell

    The delay circuit:

    is one of the worst designs I have seen.
    It does not work.

    See my discussion:

    • SMD

      I have a video here to show the design works. This was made here in my home of components I had in the drawer, off course not optimised to perfection yet. It’s been one day since it’s been put up on a breadboard. I’ve drawn the scematic exactly how it is sitting on the breadboard, with the component choice there present. I can think of a million alternative component values and qualities, the basic demands are met and the circuit by itself has proven itself to work.

      I’ve done NO math on this circuit so far, it’s just been pulled out the hat as is and with that in mind it works really well actually.

      The reason for NOT going high impedance is because that makes it far too vulnerable and prone to failure. These fets can take the full rail voltage on their gate, so obviously you are just too scared.

      Your criticism is misplaced, way too harsh and reeks of some form of jealousy or grudge. Probably still can’t stand I mocked you about not knowing ohms law all that well eh?


  • Colin Mitchell

    What does C2 do?

    • SMD

      Ever seen ‘Back to school’??

      Let us say ‘It keeps a charge’

      Now I shall watch global seismographic activity and I will probably know your exact location soon.

      Such a brain must make a racket!

  • Colin Mitchell

    Where is the 5 minute RC network?

    • SMD

      What do you mean, you don’t see it? It’s VR1 and C3. You’ve opened the link towards the schematic right?

      It is a perfectly linear working adjustable delay. If you give it 5K/5K on the trimmer you get two and a half minutes. Give it 10K/0K and you’ll get five minutes. Give it 8K/2K you get one minute. Off course these are ballpark values, but you will get the exact same delay time over and over.

      You can choose different super caps, the ones with low ESR will discharge the fastest. The ones of around 1F will give massive delay times of maybe half an hour, but they will discharge slower. You can also use a normal electrolytic cap to have a small delay time for softstarts for instance. Or use a 3.3V regulator and you can take even lower voltage supercaps…. It’s got a lot of potential and by using the regulator the circuit is completely voltage independant. Off course min/max voltage should be taken in to account! This circuit was tailored to work with ~10-15Vdc.

  • SMD

    Schematic of improved delay circuit. This one does zero to five minutes and reboots from zero no matter what. It is also not voltage dependent, so quite precise. Here tailored to work on 12V, but could easily be adapted to work on other voltages…

  • Nina

    Dear Mitchell
    We are the professional PCB suppliers, if you have any product of PCB,pls connect me.

  • Colin Mitchell

    You can increase the value of R1

  • Colin Mitchell

    This specific circuit is not covered on my website. However the circuit will work if the LED is removed.

    • shwetanklal

      Thanx a lot.
      One more question
      Can i adjust it to delay power on by 10 sec or less using the given 50 k var. resistor or i will have to change its value or that of capacitor.

  • shwetanklal

    Thank you Mr. Collin Mitchell for making us aware of mistakes of the circuit.
    kindly give me the exact link for the corrected circuit.
    I went to your website and found it full of projects.Since my connection is slow so its hard to roam around and search the desired contents as sometimes it takes forever to load the page.

    Thanking You

    • SMD

      This will work:


      I’ve tested it with 12 volts and it’s still pretty fresh, so you might want to tweak it a little here and there. With a 10mF capacitor you get a max. delay of about one minute.

      Still going to test with different transistors, that might extend the adjustability range further without extremely large capacitors.

  • Colin Mitchell

    How long do you want before the circuit turns ON?