I have asked about the circuit which will, when plugged on 12V DC, turn on the relay, and after 0.5 seconds turn it off, although 12V is still on. You suggested me a parallel connection of capacitor 470 uF and 5kOhms resistor before the relay. It worked. But now I need it to take 30-60 seconds before turn off the relay. If I am right I would need very strong (also very big) capacitor, so can it be done in some other, smarter way?

 

I have asked about the circuit which will, when plugged on 12V DC, turn on the relay, and after 0.5 seconds turn it off, although 12V is still on. You suggested me a parallel connection of capacitor 470 uF and 5kOhms resistor before the relay. It worked. But now I need it to take 30-60 seconds before turn off the relay. If I am right I would need very strong (also very big) capacitor, so can it be done in some other, smarter way?

You can use the 555 timer as a monostable to make your delays or you can use a smaller capacitor in combination with a transistor to make a simple delay. The transistor draws a small current on its base so you need a smaller capacitor than driving the relay directly with a capacitor. If you want you can add a diode in series with the transistor's base to increase your threshold voltage (the voltage the transistor will turn off after the capacitor has been discharged).

 

When I used a transistor, I put 1kOhm resistor to the base, and +12V. What is the minimal impendance with which transistor can survive 12V? When I put the capacitor on the base, what impendance resistor should I put? Thanks!

 

When I used a transistor, I put 1kOhm resistor to the base, and +12V. What is the minimal impendance with which transistor can survive 12V? When I put the capacitor on the base, what impendance resistor should I put? Thanks!

Which transistor are you using?

It would be nice if you post a schematic of what have you done.

 

I made this circuit:



and it never turned off the relay. So I put the potentiometter instead of 2.5kOhms resistor, and from 0 to 22 kOhms nothing changed, it holds the relay on permanently.

For example this circuit works:



It holds the relay on for 0.5 seconds, and then turns it off. The capacitor discharges almost instantly, so I can turn it on again.

What should I change in first circuit to make it works?

 

Have a look at the attached schematic.

I'd posted a slightly different version a bit ago, but then realized that C1 would never discharge. The item marked "V1" represents a slow on-off voltage; it switches from 12.6 to zero volts. Your circuit will not be quite like that; the diode D1 will instead discharge C1 through whatever else the load is.

 

I made this circuit:



and it never turned off the relay. So I put the potentiometter instead of 2.5kOhms resistor, and from 0 to 22 kOhms nothing changed, it holds the relay on permanently.

For example this circuit works:



It holds the relay on for 0.5 seconds, and then turns it off. The capacitor discharges almost instantly, so I can turn it on again.

What should I change in first circuit to make it works?

You have to connect the left side of the 2.5K resistor to ground and also disconnect it from the capacitor.

 

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