i made a power suppy of +15v and -15v fixed power supply using 7815 and 7915 using 24v 1amp transformer for powering my opamp. After using few times i saw sag in output +15 changed to +11v and -15v changes to -17 volt i checked circuit many times but cant find anything wrong. Can anyone help me out?..

 

Yes, post your circuit and pictures of the ready-made psu please...

 

I checked circuit many times but problem continues. Also please suggest a easy circuit for fix dual 15 v power supply to power my opamp if you have a better than this.


Thanking you.

 

Your schematic show two 12 Volt regulators, not 15 volt. but I assume you are using the basic circuit only with 24 Volt transformer and 15 Volt regulators. what current are you trying to draw?? are the regulators on heatsinks? (are they over heating) and what is the current rating of the transformer you are using. post a picture.
To supply more than a low current, they need to be mounted on a good ventilated heatsink, remember that they have internal thermal and over current protection.
Take a look at the spec sheet for more info.
https://www.fairchildsemi.com/datasheets/LM/LM7815.pdf

 

What transformer are you using, is it a 15-0-15v or a 12-0-12v?

 

i am sorry for the wrong circuit diagram i am using 12 v transformer with 1 amp current i have not used any heat sinks on 7815 or 7915. It was working fine but after few times using i faced this problem.

 

You may be having a problem with regulator drop out due to insufficient voltage supplied to the regulators. With only around 17 volts in, there is not enough for the regulator to operate correctly. You really need 20-25 volts in. As soon as you start drawing current, the transformer voltage is likely to drop and the rectified output will be insufficient for the 15 volt regulators.
At 1 Amp, you really need good heatsinks on the regulators and I suspect that unless you use a good 15-0-15 or preferably 18-0-18 volt transformer, you will continue to have problems. Remember that different manufacturers sometimes state the RMS voltage of which the P/P is 1.414 times this, and sometimes they state the expected rectified DC voltage at the current rating. You need to know which.
Also, some of the made in China transformers exhibit terrible regulation, maybe the open circuit voltage is say, 15 Volts, but under load drops off very quickly.
A centre tapped transformer will only be able to supply half the current from each winding. a point missed by many when using them for split supplies.

 

Is the sag under all conditions no load to full load, only under load, only under load after some delay?

The plus side sounds like the regulator current limiting or overheating and turning itself down.

I have no idea why the negative regulator over voltages. That makes suspicious the chip is bad.

 

i am sorry for the wrong circuit diagram i am using 12 v transformer with 1 amp current i have not used any heat sinks on 7815 or 7915. It was working fine but after few times using i faced this problem.

Is the current of the load equal on both positive and negative side ?

 

i replaced the 7815 with new one it started working fine as i checked voltage drop at input side of ic which was less on 7815 compared to 7915. still 10 v output was coming out of 7815. I question myself do ic damage partially i mean it is a voltage regulator is it regulating more ?.

Is the current of the load equal on both positive and negative side ?

i didnt checked the current as i checked voltage was not equal.

You may be having a problem with regulator drop out due to insufficient voltage supplied to the regulators. With only around 17 volts in, there is not enough for the regulator to operate correctly. You really need 20-25 volts in. As soon as you start drawing current, the transformer voltage is likely to drop and the rectified output will be insufficient for the 15 volt regulators.
At 1 Amp, you really need good heatsinks on the regulators and I suspect that unless you use a good 15-0-15 or preferably 18-0-18 volt transformer, you will continue to have problems. Remember that different manufacturers sometimes state the RMS voltage of which the P/P is 1.414 times this, and sometimes they state the expected rectified DC voltage at the current rating. You need to know which.
Also, some of the made in China transformers exhibit terrible regulation, maybe the open circuit voltage is say, 15 Volts, but under load drops off very quickly.
A centre tapped transformer will only be able to supply half the current from each winding. a point missed by many when using them for split supplies.

while checking transformer output with my DMM after setting it on ac voltage. Am i seeing the rms value or ac voltage p2p

 

Do you have the instructions that came with your DMM? They will tell you if the reading is true rms. I doubt very much if it is peak to peak and I think you really meant peak. an oscilloscope would show that. Without looking at the specs of your meter I cannot say. If you don't have them, post the make and model number on here and I'm sure someone else will have one and be able to let you know.
You could try google-ing the meter instructions too.

I have seen some very cheap DMM's that give readings that are neither rms or peak Voltage.