Archive for July, 2011

PostHeaderIcon How to Inspect VCC Power Supply Short Circuit Problem

When we repair a PCB, if VCC power supply has short circuit issue could be a very complicated matter, since the number of components embed in parallel circuit network between VCC and GND is huge, chips, capacitors, transistors, even anyone of which has short circuit will bring problems to the whole PCB board, the tin solder points and copper of PCB board surface may also cause short circuit. Maintenance personnel will remove the components one by one, until the short circuit phenomenon being eliminated when that causing problem component removed, if bad luck, the root cause of short circuit could not be found even after the components spread PCB board was almost removed completely, not only can not find fault, but also will damage the PCB board.


Hereby we would like to introduce several practical method which can help you save more time and energy:
As for Plug-in capacitors on the PCB board we can use diagonal pliers to cut one foot (Please be noted you should cut from the middle, do not cut from each end  of two sides),  cut power supply VCC pin of plug-in IC, short circuit will disappear when cut one foot, then we can judge that one of capacitors or chips has short circuit issue. If the chip’s package is SMD, we can use solder iron to melt the solder tin on the IC power supply pin which will help to raise the chip away from the VCC power supply. After replacement of related short circuit components at the office or the tilt can be re-weld, and if those capacitors at both ends of the power supply VCC, you can use hot air welding machine to blow them one by one, after remove them, you can use multimeter to test them and got the value, until short-circuit issue being eliminated.


There is also a faster method, but to use a special instrument—milliohmmeter. We know that the PCB board copper foil is also a resistance, if the thickness of copper on the PCB board is 35um, printed circuit wire width is 1mm, then each 10mm long, the resistance value is around 5mΩ, such small resistance can’t  be measured by ordinary multimeter, but milliohmmeter can measure it. We assume one component is short circuit, then measured it with an ordinary multimeter, the result could be 0Ω, but if measured by a milliohmmeter could be tens to hundreds of milliohms, we just put pen table legs on two sides of the short-circuit component, the value of resistance must be minimal (if on the other components on both feet measured, the value of resistance also includes PCB board copper trace resistance), so through compare milliohmmeter resistance differences among different components, when measured to a component (if there is existence of tin solder points and copper trace short circuit will use the same way) which get the minimum value of resistance, it will embrace the biggest suspect. Through this way you can quickly find the point of failure.