Hi,

This is my problem:
I have device which startup voltage is 9V, it operating voltage is 6,4V. After about 10s after startup devide should enter normal operating mode (6,4V).
Power source is 12V DC. I know device electrical characteristics so I can connect some resistor Rstart, and supply 9V to my device. I also can connect resistor Roper and make my device receive 6,4V.

How to replace Rstart with Roper automatically, after some time of operation (for example 10s) in my circuit ?

thanks
Vilius

 

If the current is low enough, you should be able to use an LM317 and a 555. The 317 would be set for 9 volts out, and the 555 would start timing at power on. After the 10 sec, the 555 would thurn on a FET like a 2N7000, and make a parallel path around the fixed resistor on the LM317. It would be adjusted to reduce the voltage from 9 to 6.4.

 

Resistance of device is about 40 ohms, so I guess corrent is not very low.

 

It is less than 1.5Amps, so a LM317 can handle it and you can use the solution Beenthere suggested. Another solution, if you have already a power supply with two different voltages of 9V and 6.4V you can use a relay and a 555 or just a relay with a capacitor (if you need just a few secs of delay) to make the switching.