# A smart on time delay unit. Help needed

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#### preetijoli

##### New Member

This circuit would be an intermediate between the power supply and the concerned device.

In order to avoid tripping in the device, as soon as the power supply is cut off from the device, the device should not function for 3 minutes even if the power suppy is on within 3 minutes of power off.

That is, as soon as power is off, a timer should start working ( without any power supply), ..such that
1. If power comes before 3 minutes, the timer should not let the power supply reach the device until 3 minutes (from the time when power went off) has passed.
2. If power comes after 3 minutes, there should be no delay in the switching on of the device.

I have few but I m not exactly able to implement them.

#### blueroomelectronics

##### Well-Known Member
What power supply? A computer power supply? What current / voltage?

#### preetijoli

##### New Member
power supply is 230v ac
concerned device is fridge/ac

#### blueroomelectronics

##### Well-Known Member
You simply need a large UPS that can handle an inductive load. Not cheap and probably only good for a few minutes with a fridge.
OR just leave the fridge closed when the power shuts off.

#### preetijoli

##### New Member
is it not possible to do this using ICs?

As soon as power goes off, an already charged capacitor starts discharging. when power comes back again, the voltage on discharging capacitor is compared to a reference voltage V ( The reference voltage will be the voltage on the capacitor after discharging foe three minutes).
If the capacitor voltage is higher than the reference voltage, the capacitor continues discharging until 3 minutes from power off have elapsed. And the power supply will be cut off from the device.
The moment the capacitor voltage becomes lower than reference voltage ( i.e it has discharged for 3 minutes), the device should get the power supply. And the same capacitor should start charging again.

Important point is that the comparator should not function as the capacitor starts charging and it is one of the problems I am facing in the designing of the circuit.

#### blueroomelectronics

##### Well-Known Member
I'd use a small microcontroller with a supercap to power it on AC fail. The real cost will be a relay or SSR that can handle the load. What's your budget and why does it matter the fridge comes on when the AC is restored?

#### preetijoli

##### New Member
yes I was wondering about the cost of the relay. What would that be?
And if without using microcontrollers it could be done, can the circuit i had thought of ,be corrected and realized using 555 or any other comparator ic.

#### tcmtech

##### Banned
What your looking for is a standard issue time on delay relay. They are about $25 -$100 on line at any electronics supply company.
They have multi function ones that are programmable for a number of different applications as well.

Every time the power is disconnected even for a fraction of a second they reset. then the power has to be supplied for the time set before it actually connects the load to the power source.

AC units, refrigerators and freezers have a built in protection circuit just for that purpose already. Its typically a thermal type self reseting circuit breaker. When the power blinks the compressor motor stalls and then the high input amps from it trying to restart under a load trip the thermal device which takes a minute or two to cool off and reset. By then the refrigerant gas pressure has dropped low enough for the motor to restart.

#### BrownOut

##### Banned
You can use a variety of IC's to accomplish that, but what's going to power them? If you can find power, then a 555 will do just fine. Be careful how you drive the inductive coil of the relay. I think industrial relays aren't every expensive, just find the one that will handle the switching requriements and call your local appliance parts supplier.

I had a boss once that was working on this same issue. This is a pretty ubiquitous problem.

#### preetijoli

##### New Member
thanks everyone.

the ICs wont work during power off. when power is off, the capacitor will start discharging, (it would hold the charge when power is on) (if anyone could explain how this can be done)

The IC would work and compare only when power is turned on and everything as i had posted earlier.

can get help with circuit diagrams?
main prob is resetting the comparator and charging the capacitor without comparator being on when the capacitor has discharged for 3 minutes.

This unit will be outside the device( the ac or refrigerator)

#### blueroomelectronics

##### Well-Known Member
A supercap will keep a small very low power circuit running for ages.

#### BrownOut

##### Banned
If the unit is going to be outside the appliance, then it could remain powered up. If you're not going to keep it powered, then I wouldn't use an IC. I don't have the patience to do schematics or search for one. I think we have several excellent references for timer and so forth. Use them to design your system and ask for help when you get stuck.

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