i want to stop the flow of current .....


I have the following situation--> i have two DC power sources one is 9v and other is 7.5v i want to combine these two power supplies such that when one power source is off then other is available....  for example if 9v is off 7.5v is flowing and counter wise also but i want to combine in such a manner that when both are on both should give the output.....and the problem is the adapter giving 7.5v has a capacitor which could charge above its limit and can cause failure of the adapter or burn i should say...

i have the connection diagram.....this works but when i connect a led to its output it glows normally...and when i turn off the supply the led turns off slowly  giving an indication that the capacitor is over charging....please help me to restrict the flow of current of both power supplies in each other.......

Picture of i want to stop the flow of current .....
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The 9V is only 1.5V about 7.5V. Any caps in the 7.5V supply must be rated to survive that much continously. The closest capacitor rating that is greater than or equal to 7.5 volts is 10 volts. I have never seen a cap with say, an 8 volt rating.

I wouldn't worry about subjecting the output caps in a 7.5V supply with 1.5V more than usual.

Anyway, your circuit looks like it should work fine. I don't see how the 9V supply would backfeed the 7.5V one. If your LED turns off slowly when the supply is switched off, it may be that it is just the remaining energy from the caps, which have to be dischared before the LED turns off all the way.

Also, I think you could remove the diodes in the negative "-" connections, and just connect them directly together. They would share a common ground, and the posistive connections would be "diode OR'd together".
anuk07 (author)  LargeMouthBass6 years ago
Ok,,,now If one supply is off i want it to degrade slowly what should i do for that???? suppose i want 9v to degrade very slowly then what type of connection do i need????
I guess I don't understand what you mean when you say you want the 9V to degrade slowly. I thought that the connections you were after were just needed to connecta 7.5v and a 9v supply to the power input at the same time safely.

By degrade slowly, do you mean that you want the power supply output to slowly decrease after it turns off?
anuk07 (author)  LargeMouthBass6 years ago
ya ya i just want that the power supply output to slowly decrease after it turns off.... i am srry abt my bad English....

but u got it right.....

suppose that both supplies are off but i want to keep the o/p voltage for at least
1-2 Secs.....

srry about making puzzling questions....
anuk07 (author)  LargeMouthBass6 years ago
that means if i connect more than 10vdc instead of 9v then only it will cause problems....

yup i will remove the negative diodes......

thanx.!! u helped me a lot............
verence6 years ago
Your solution with the diodes is fine. You could of course remove the diodes in the - line, they will just reduce the voltage unnecessarily.

Btw. even if both supplies had 9V each (or the 7.5V had all caps rated at 9V or higher) the outputs of power supplies should NEVER be connected directly. If one power supply is not powered, current of the other one will flow into the unpowered one and might destroy parts in it (even 7.5V might destroy a 9V supply here).

If any supply can power the device alone, use diodes as you did, if you combine supplies for their combined current, some load balancing resistors should be used - the diodes wouldn't hurt either.
That should work. There could be issues from using diodes with a too low rating.

What do you mean by combine the supplies? the most you can get out of that circuit is 9V.
anuk07 (author)  The Skinnerz6 years ago
from combing i just mean to "connect"...only srry for that....

which diodes u will suggest me for restricting current of 0.5amps......

and are u sure the capacitor of 7.5v dc adapter will not cause any damage after long time usage?
1N4001 would work fine.