Power supply with output change timer

[Fady_18]

Hello guys,

I am trying to make a power supply which could change the output after a few seconds. Let's say for 2 seconds the output would be 12v then after it would drop to 3v constant. I have tried to use a double output power supply with a change over relay and timer but the problem is the time. I need the voltage to drop without the time difference caused by the relay changeover time. Does anyone have an idea how could that be achieved with the PCB itself ? I would really appreciate any idea and input

Cheers
Fady

 

[danadak]

One possibility, not knowing your power / current levels using simple mosfet switch
change the R value in the reference leg of an adjustable regulator -

So change R2 to two R's in series, and ground one of them to change V.

This is a 1 A kind of part.



Regards, Dana.

 

[crutschow]

Below is the simulation of an example circuit using the voltage regulator suggested by Dana.
C1-R4, plus M1 and M2 generate a delay to connect voltage control resistor R5 to ground after about 2 seconds from power on, to lower the output voltage from 12V to 3V.

 

[Tony Stewart]

You cannot design anything without specs.
What supplies are available? 12V? What is the load ?
WHat is the load at 3V?
Anything else important? Slew RATE? Variable control?

When all your specs with tolerances are firm, the solution is easy. OTherwise its often a waste of time with trial and error.