Problem with voltage drop

[jakin]

Hi, All:
I have problem with a big voltage drop on my PCB. When I connect my PCB with 1.8V from power supply, I saw a significant voltage drop and the current rises up 0.05A. Can anyone tell me what this problem is ? My assumption is a short loop in the PCB. I do not know I am right or wrong. Please give me some advice. Thanks for advanced.

 

[A.Anand Srinivasan]

if the current peaks up all of a sudden then there must be some short circuit... better check out if there is any solder bridges.... they could be a big problem because solder is a very low resistance material.....

 

[kevpat]

Can you tell me what the voltage drop you are seeing is and what your supply source is. I don't think you have a short on your PCB as you would see a much larger current increase than 50mA if that was the case, simple calculations show that the impedance for the increase of 50mA would have to be 36 ohms, hardly a short. This impedance is most likely caused by leakage current in totem pole outputs of IC's. If your voltage drop is only a 100mv or so then you probably don't have a problem you just need a better regulated supply. As a simple check measure the impedance between vcc and gnd on an unpowered pcb.

 

[jakin]

Hi, kevpat:
Thanks for your help. The voltage drop on the supply source is dropping from 1.8 V to 0.96V. Is this about impedance or something else? Thanks for advances.

 

[kevpat]

Hi Jakin,

This is definitely too high a volt drop and it would appear you do have a problem on the board. I would suggest

1) With power applied quickly touch all the IC's, transistors etc on the board one after the other checking to see if any are getting excessively hot, simple but effective.
2) Checking all components between VCC and Gnd to make sure the correct values have been fitted or wrong component fitted i.e. a inductor instead of a cap, suprisingly easy to do if it is small surface mount components.
3) measuring the impedance between VCC and Gnd to see what you read. If there is a low reading then if you have a sensitive enough meter you can try to track down the faulty component by placing one probe on a gnd connection and then probing at various points on your VCC line as you get closer to the faulty componenet you should see a very small drop in impedance.

Another way to track down the fault is to isolate the supply to successive IC's then power the board, at the point that when power is supplied and the voltage stays constant the last IC you isolated would be the faulty one.

Hope this helps, keep me informed how you get on tracking this down.

 

[suresh_kvja]

Hi Mr.jakin,

The suggestion which Mr.kevpat gave seems to be too close to your problem.

Tried the explanation or not??