ESP Soft Start Circuit

[Alcaid]

Hi, have anyone build the soft start circuit on the bottom of this page: http://sound.westhost.com/project39.htm

Are the component values correct for approx 300ms delay? I'm trying to simulate it in Multisim 7 but don't get it to work. (noob to simulating). I get errors or oscillations and can't get it to work correct.

Schematic:

[paulb]

You don't need to simulate. It is how long it takes the capacitor C2 to charge up to half the supply rail, when the op-amps input voltages are equal. It's reasonably close to R1 * C2, about 100 ms.

[Alcaid]

Originally posted by paulb
You don't need to simulate. It is how long it takes the capacitor C2 to charge up to half the supply rail, when the op-amps input voltages are equal. It's reasonably close to R1 * C2, about 100 ms.

Ok, I just don't understand how the circuit works. (I'm not an EE). However Rod claims approx 300ms delay in the article. Care to explain me how it works?

I see the opamp have half the supply voltage on the NI input from the voltage divider (R2-R3). Don't understand how the opamp/transistor circuit work....

[SY]

I'm not an EE, either, but the operation is straightforward. At turnon, the inverting input of the 741 is at ground, the NI input is at half the rail, something like 6 or 7 volts. So the output of the 741 is pinned to near the positive rail (the 741 is really used as a comparator). So Q1 is turned off. As C2 charges through R1, the inverting input eventually has a voltage equal to (and then more positive than) the NI input. So the 741's output slams down to near ground, turning on Q1, firing the relay.

R4 makes the 741 look more like a comparator, and the diode gives the 10u cap a discharge path at power down. Because of the wide tolerance of C2, the delay time here is going to be approximate.

[Alcaid]

Thank you SY for the great explanation. Understand it now.