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
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.
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.
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.