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Given is the load that cannot be turned on immediately after the power gone off. This is a High-pressure sodium lamp, which needs cooling down for 5-10 minutes before the next ignition, otherwise it will not ignite successfully, and ignition attempts can even harm the lamp.

Therefore, I need a time delay relay that keeps the circuit open for some time after the power is turned off. Even if the power is turned on within this delay period, the load should not be energized until the period ends.

According to the article https://www.allaboutcircuits.com/textbook/digital/chpt-5/time-delay-relays/, I need

a normally-closed, timed-closed (NCTC) contact

NCTC diagram

Mostly on the web and here you can find the opposite: normally-open, timed-open (NOTO) contact.

I want to build it myself with the simplest possible components (preferably without chips).

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  • \$\begingroup\$ So the circuit should remember even without power that the lamp has been on so it is known to be warm, so next time power is applied it must wait 5-10 minutes? But if the lamp has been off for 5-10 minutes already, or overnight, and it is known to be cold then it can be powered immediately when light switch ia flipped? Can you monitor lamp temperature? Or just a time delay? And no chips at all? Because chips would make this much easier as they are just "black boxes" that work, compared to designing same functionality with a handful of discrete transistors and delicately verify their operation. \$\endgroup\$
    – Justme
    Jun 19, 2022 at 9:06
  • \$\begingroup\$ You need a big capacitor that provides a stable supply voltage for at least 10 minutes. This capacitor feeds a timer/counter which inhibits the lamp turn on signal. \$\endgroup\$ Jun 19, 2022 at 9:19
  • \$\begingroup\$ Bimetallic thermal switch placed somewhere that tracks the temperature of the lamp? \$\endgroup\$
    – bobflux
    Jun 19, 2022 at 9:26
  • \$\begingroup\$ @Justme - your understanding is correct regarding the usage scenario. Monitoring lamp temperature is an idea, I guess I'll follow the suggestion to use bimetallic thermal switch. But still it would be interesting to get the circuit for the NCTC contact relay as an answer to this question. Chips can be acceptable if they simplify the circuit. \$\endgroup\$
    – AKd
    Jun 20, 2022 at 16:02

1 Answer 1

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An electronic 'ON' delay timer, like the one shown below, may be used.

enter image description here

Here's the timing diagram of an 'ON' delay timer.

enter image description here

The following schematic would meet your requirement.

enter image description here

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  • \$\begingroup\$ This is "normally-open, timed-closed (NOTC) contact". The diagram is quite different from the one I've explicitly included into the question. So as far as I understand, your answer does not solve the task. Moreover, I don't understand what kind of electronic component is in the schematic. \$\endgroup\$
    – AKd
    Jun 20, 2022 at 16:06
  • \$\begingroup\$ @AKd - Hi, Explanations duly added. \$\endgroup\$
    – vu2nan
    Jun 28, 2022 at 7:05
  • \$\begingroup\$ Thank you for explanations, but I cannot accept this answer. It does not meet the requirement "... relay that keeps the circuit open for some time after the power is turned off". Your suggestion will always delay, even when the power was off for a long time. That's unacceptable. Moreover, "I want to build it myself" (means not to buy an appliance). \$\endgroup\$
    – AKd
    Jun 28, 2022 at 8:13
  • \$\begingroup\$ @AKd - Hi, The schematic has been altered to meet your requirement. Your non-acceptance of my answer is not an issue! A simple on-delay timer circuit is to be incorporated. \$\endgroup\$
    – vu2nan
    Jun 28, 2022 at 17:39

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