How to identify voltage leaks in a PCB

[czabhinav]

Hello all,

I have a PCB where I am using SIM900 and Microcontroller. As Microcontroller runs on 5v so I am using 7805 to convert 12v to 5v. For Sim900, I am using LM2576 to provide it 4v and upto 2amps. Now I want to individually test the SIM900, so for the time being I have disabled the 7805 5v so microcontroller and rest of the circuitry(power led,etc) which needs 5v are turned Off and only LM2576 is working. So Vbat pins of SIM900 has 4v. Now there is switch which is used for PWRKEY. Whenever I am pressing the switch to turn on the SIM900, power led which is connected to 5v gets turned on. I checked the voltage at the led, it is 2v. Not only led is on, infact most of the circuit which should run on 5v has 2v. So I think there is voltage leak problem because as I press the switch rest of the circuit gets 2v and when I release the switch they have 0v. I have double checked the schematic and board layout for any shorting but it seems like everything is fine.

Is there any other way to know the actual cause of this problem.
What steps should I take to identify this problem.

 

[FvM]

The question can't be well answered without a schematic. Most likely the SIM900 digital output pins are driving the 5V supply through processor input pins and clamp diodes to VDD. Unless you intend to operate SIM900 with 5V powered down (don't see why this would be useful), there's no actual problem, just a bad considered test setup.

 

[schmitt trigger]

You are experiencing what is called "sneak paths"; which is current flowing thru semiconductor junctions of non-powered devices.

Agree with FvM, an incorrect test setup.