# Help with project

Discussion in 'Electronics Homework Help' started by nevernight, Jun 21, 2013.

1. ### nevernight

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Jun 21, 2013
I have been given a project to design a delay circuit applied to a household appliance, and has to work in this way:

When the power goes out, the circuit is to provide a block to the AC for 3 minutes. If power comes before 3 minutes, it is to be blocked, until the 3 minutes are over.

To do this, I tried to use relays, which will be activated by a charged capacitor(which gets charged when the power is there) for 3 minutes. But, I learnt that capacitors cannot drive relays. My next step was to activate the relays using power from the rectifier circuit(which charges the capacitor) to activate the relays. And the capacitor to block it for some time. I trie to use a transistor for that purpose, but the answer is just eluding me. I'll be glad of any guidance regarding this matter. Thank you.

2. ### Harald KappModeratorModerator

11,522
2,654
Nov 17, 2011
Build a low power timer (e.g.using a 555 chip). Power the timer from a supercapacitor which is charged from the mains while available. Use the timer to control a transistor which in turn can drive the relay, but use the auxiliary power (which is derived from mains) to power the relay. Thus whenever the timer allows the relay to be energized, you will have power from mains to drive the relay. If you dn't have power, you don't have mains and no need to drive the relay.

3. ### Laplace

1,252
184
Apr 4, 2010
If the power has been off for longer than 3 minutes, then the relay must turn ON as soon as the power returns. But if the power has been off for a very long time, then the supercapacitor will have discharged. How long will it take to charge the supercapacitor before the circuit will turn the relay ON? Also, on power-up (after a long power-off) will the timer circuit initial state be to drive the relay ON?

4. ### nevernight

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Jun 21, 2013
That is all good. But, a timer using a 555 ic would require a push trigger, but the circuit should be automatic.

5. ### nevernight

3
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Jun 21, 2013
Also, any blocking circuit connected with the capacitor will also be connected to the rectifier(which charges the capacitor). Since all of them will be in parallel. Also, when power comes back before 3 minutes, the capacitor will go from discharging to charging, since the voltage of the rectifier would then be higher than the capacitor.

6. ### Harald KappModeratorModerator

11,522
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Nov 17, 2011
No, it doesn't. You can trigger the 555 e.g. from mains via a voltage divider.