I'm designing a circuit for a power-on delay that powers a centrifugal water pump. When the power is switched on, it needs to be delayed by 60s before starting up the electric pump. The power source used is essentially from a wall plug: 300V max, 50Hz. The pumps themselves will typically operate between 1500 - 2000W.

I've taken multiple approaches but to no avail. I am by trade a computer-electronic engineer, so this is somewhat out of my comfort zone.

What I did so far:

  • Invasive Hall effect sensor combined with a toroid to measure the current and power an IC that controls a relay. However, I realised that the AC I'm trying to delay is the AC that is needed to create the magnetic field, which I have to measure, but I can't have that because I need to delay it from ... etc ..
  • Using two separate toroids, one before the relay is closed, and the other after the relay is closed. I can then generate the magnetic field while the relay is open, 60s delay, then close the relay. I can also use the second relay to monitor when the power is cut. But this approach may be too convoluted and I will still need a battery.
  • Arduino nano that measures the current over a shunt-resistor/hall-effect sensor, which in turn operates the relay. I also need AC to measure this current, so again a no-go.

To 'verify' my designs I bought an Off-the-shelf Adjustable Voltage Protector (AVP) (Sinotimer SVP-912). The big realisation here is the AVP not needing another power source to operate. It only needs the AC source and gives you the 'correct' output.

There is an abundance of material for light-current delay circuits. However, I'm having a hard time finding tutorials, diagrams, material, etc for this use case.

Just as a note: I cannot use off-the-shelf AVP's, so this needs to be printable on a custom circuit board, therefore, relays, IC's, passive components, etc...

Thanks all!

  • \$\begingroup\$ There’s a variety of off the shelf solutions. Why do you want to re-invent the wheel? Timer relays come to mind. In the old days a hot wire and bimetallic switch would’ve been a solution. A simple resistor, capacitor, transistor and relay should suffice if you want an electronic solution. \$\endgroup\$
    – Kartman
    Jun 9, 2021 at 14:28
  • \$\begingroup\$ @Kartman I completely agree with what you're saying. The client asked for a design that they can manufacture themselves which they can easily change to fit the dimensions of their products. \$\endgroup\$
    – JBM
    Jun 9, 2021 at 14:43
  • \$\begingroup\$ 6A pumps can draw >30A if switched on at peak voltage to startup, so using. Zero crossing controlled Triac switch is better with opto isolation. 60 second timers are trivial with CMOS CD4060 or 74HC4060 with timer frequency selected at 2^n/f= 60 s, opto ZCS Triac and 15A power triac. Then add thermal protection as needed to reset timer. Offline supplies for LV and low current are simple. But memory for interruptions vs control on/off need to be defined etc. \$\endgroup\$ Jun 9, 2021 at 15:38
  • \$\begingroup\$ You can purchase on or off delay timers that will switch your load for not to much money. \$\endgroup\$
    – Gil
    Jun 13, 2021 at 3:47

1 Answer 1


Plan to use a timer IC or microcontroller to implement the delay and a solid state relay to turn on the pump. It seems like you're looking for a design that uses a power switch at high voltage and non-contact sensing; instead think about having the power switch on a low voltage DC circuit or powering the low-voltage DC circuit from the power you've just turned on.

Another note: Most electronic switches (relays & SSRs) for these voltages don't come PCB mount, plan to have your PCB just be the low-voltage circuit. You almost certainly don't want to have your high voltage relay on your circuit board.

Here's a design:

  • Your input voltage is directly connected to an AC-DC converter (e.g. Mornsun PN LD03-23B05WR2 or RECOM RAC05-05SK/277/W) and one leg to an SSR (e.g. Autonics SR1-1440-N or Crouzet PN 84137450N).
  • The DC circuit includes a microcontroller that implements your delay, and a low-voltage switch that's your "power switch" and any other features you might want to control or monitor.
  • The microcontroller can activate/deactivate the SSR to connect the pump.

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