External Power supply current limit set to MAX amps to find SHORTS on a PCB

[Ian Rogers]

You need a good quality resistance tester and a bit of experience... If you want to find problems whilst ther power is applied you use freezer spray to locate problematic components.. Most of the times a short is easy to find...

 

[large_ghostman]

On the other website forum he is asking how to "repair" a pcb that is burned into charcoal.

What would be quicker, repair burnt to charcoal PCB or make new PCB?
These wouldn't happen to be pcb's from a certain space shuttle would they? Did your company make parts for space shuttles??

 

[ronv]

When is a component or something a dead short? and how do you find a dead short when there is no voltage drop or heat? no voltage drop is like an OPEN or BREAK , cut trace

Cause an OPEN = Zero volts
Dead short = Zero volts

So what's the difference?

Cause an OPEN = Zero volts -- Zero Amps
Dead short = Zero volts -------- 5 Amps

 

[large_ghostman]

next question is gonna be how do I know its 5amps wich button on my shiny H Z logic probe do I press.
Why does 5A make my brown pcb go black?
How do I know its 5A and not 4.8A
My manager says 5A is the standard charcoaling International standard value. Is he correct?
The technician on the right of my left hand side, uses 150A to get charcoal black, is he wrong? would it be better to use 5A??

just a prediction

 

[kubeek]

Cause an OPEN = Zero volts -- Zero Amps
Dead short = Zero volts -------- 5 Amps

I´d say OPEN = ANY volts -- Zero Amps

 

[large_ghostman]

I´d say OPEN = ANY volts -- Zero Amps

Hmmm I had to think that through for a second, yes I guess it depends wich side of the open you are

 

[audioguru]

OPEN is a floating connection to nothing. Its voltage is anything until the resistance of your meter or 'scope probe grounds it.

 

[ronv]

I´d say OPEN = ANY volts -- Zero Amps

Yes, Much better. Since I think he said they used 10 volts, maybe low amps 10 volts?

 

[large_ghostman]

On the other website forum he is asking how to "repair" a pcb that is burned into charcoal.

In my last answer to this I was kind of dismissive, so I have had a think and come up with the following solution, if you find it hard to follow let me know and over the weekend I will do an INSTRUCTABLE with pictures and everything.

First you will need a angle grinder, for through hole components I recommend 9 inch and smd stuff 4 inch.
1. very very carefully place grinding disc (metal type not stone), on top of the highest component on the board, lock your arms tight and gently squeeze the grinder trigger. Soon as you hear full revs, press down very very hard! keeping the pressure to max move the grinder in increasing circles, do this until all sparks have stopped,and the only thing that is being produced is toxic dust.

2.) Carefully wipe clean with a rag and solvent, if there is any channels that have been cut into the pcb substrate, dont panic, these we can fill in later with poly filler or chewing gum.

3.)turn the board over and repeat steps 1 and 2.

4.) Fill any grooves in with polyfiller or chewing gum, if you cant find anything else use paper mache

5.) get a marker pen, crayon or in a pinch, a women's lipstick, now very carefully draw the circuit back on both sides, if you have trouble marking the dots to drill later do the following. Hold the pen/crayon/lipstick vertical (use a spirit level), just above the point you wish to mark, now gently lower untill the tip touches the now carbon free board, push down hard

6.) Have a rummage around in a skip or bin, find any old bit of cable. Cut the required lengths of track, using scissors or an old knife.

7.) Hot glue the lengths of wire into place on the board (every 8mm should be enough (for space or aircraft work use 5mm spacing, mil spec use 3mm), make sure you cut any spiky bits of glue that stick up!
now gently in a figure of 8 motion wipe over with the angle grinder.

8.) solder any components directly onto the wire traces, dont worry too much about values, just try and make sure the components you use are the same shape as the ones you removed (that way no one will know )

9.) Now we have to test the new board, hook up to a PSU. IMPORTANT Do NOT turn the psu ON!!!.

10.) While the PSU is off, probe all around the board with a Oscilloscope, be careful to avoid components and wire . What you are looking for is a high Z zero reading of anything remotely electrical with funny symbols.

11.) repeat step 10 with Logic Probe

12.) Repeat step 10 with DMM

13.) Repeat step 10 with Analogue meter

14.) Once fully tested with all available equipment, carefully unplug from switched off PSU.

15.) as this is a high spec customer board, package carefully and dont forget the bow!

Any problems pm me and I will do instructable in pictures (easier to follow)

I forgot safety advice!!

Due to the toxic nature of some of the fumes, DO NOT INHALE, while repairing the board, hold your breath untill finished (approx 90 mins should do it).

If your unsure which end of the soldering iron to use, watch one of the tech's that sit near you, failure to do this will lead to burnt fingers.

Remember to plug the hot glue gun in, otherwise squeezing the glue out is hard work.

DO NOT FORGET TO ADD A QC STICKER TO FINISHED PRODUCT!!!

 

[Mike odom]

No, an open = full volts on power supply, 0 volts on device, short = 0 volts on power supply and component(s).

 

[Billy Mayo]

LITTLE GHOSTMAN< PLEASE STAY OUT OF MY THREADS AND STOP SPAMMING MY THREADS I DON"T DO THIS TO YOUR THREADS

Cause an OPEN = Zero volts -- Zero Amps
Dead short = Zero volts -------- 5 Amps

How can you have zero volts and 5 amps , that doesn't make sense

Water analogy if you have no water in the tank how can you have water flow at a speed rate of 5

There is no water in the tank , so how can you have water flow?

OPEN is a floating connection to nothing. Its voltage is anything until the resistance of your meter or 'scope probe grounds it.

An OPEN is a break in the path

Floating is not a break in the path, its a floating voltage until you reference it to something

It's a big difference audioguru

 

[picbits]

LITTLE GHOSTMAN< PLEASE STAY OUT OF MY THREADS AND STOP SPAMMING MY THREADS I DON"T DO THIS TO YOUR THREADS

You don't appear to contribute to any other thread than your own !

 

[Diver300]

If you have a meter that will measure to millivolts, you can sometimes trace a short by measuring the voltage drop in the supply or ground rails. There will only be a voltage drop along tracks that have current flowing in them.

It might not be possible with a circuit with ground and power planes.

 

[cowboybob]

...How can you have zero volts and 5 amps , that doesn't make sense ...

Billy, work the math. Perhaps this will help you understand. This SIM was using Texas Instruments TINA.



ANY other simulator will give you identicle results.

We are trying to help you understand what you are doing and why you get the results that you do.

You can help us by, if nothing else, simply accepting the explanations you are given, even if you can't comprehend the why of the answer.

Some things is life have to be taken on faith if one is unable to grasp the basics...

 

[Billy Mayo]

If you have a meter that will measure to millivolts, you can sometimes trace a short by measuring the voltage drop in the supply or ground rails. There will only be a voltage drop along tracks that have current flowing in them

.

What kind of short are u talking about?
Why would it be in the millivolts?

Do you mean, measuring from VCC to ground on each IC chip and what component is connect to the VCC and ground rails?

There will only be a voltage drop along tracks that have current flowing in them

So how do u know where the short is? there is NO voltage drop from VCC to ground on that component?

 

[Billy Mayo]

Cowboy bob

I still don't understand the SHORT how it has 5 amps and zero volts

Water Analogy example:
Voltage = the level of the water , pressure of the water in the tank
Current= the flow of the water, the rate/speed of the flow of water
DEAD SHORT = a direct pipe bypassing or a parallel path

How can you have a flow of water at 5 amps , when the water level is at ZERO? that doesn't make sense

 

[kubeek]

froget water level and stay with a closed pipe system, where voltage relates to pressure difference. So if you have two tanks connected by a thin pipe and push water through it, there will be big difference in pressure between the two tanks. If the pipe is big (infinitely big) there will be no pressure difference and thus no voltage drop in equivalent electric system.

 

[audioguru]

A DEAD short is zero ohms. It conducts A LOT of current but it has 0V across it. It does not get hot.
The pcb traces feeding the current to the short might have a resistance of 1 ohm so the current is 5V/1 ohm= 5A and the pcb traces dissipate 5V x 5A= 25W and burn out.
OR
You have an IC with a partial short of 1 ohm. Then the 5V supply has a load of 1 ohm + 1 ohm= 2 ohms. The total current is 5V/2 ohms= 2.5A. The voltage is divided so the IC dissipates 2.5V x 2.5A= 6.25W and the pcb traces also dissipate 6.25W. Maybe the IC and the pcb traces burn out.

 

[Billy Mayo]

Oh Ok i get it know thanks guys

big difference in pressure between the two tanks

So you're saying there has to be a potential difference

Potential different = Pressure differences = difference in pressure between the two tanks

You have an IC with a partial short of 1 ohm. Then the 5V supply has a load of 1 ohm + 1 ohm= 2 ohms.

Yes if you measure an IC output of circuit from the VCC pin to the ground pin you will measure a resistance , plus the internally traces inside the IC chip

So when you have a partial short , these is resistances from the internal traces inside the IC chip

 

[Billy Mayo]

The voltage is divided so the IC dissipates 2.5V x 2.5A= 6.25W

So u divided the voltage when there is a SHORT? is this a rule of thumb?