SHORTS, voltage across resistor converts to current

[CocaCola]

You could just try removing components from the board until the short goes away. That will work as long as the short is in a component and not in your wiring, power supply, board, etc...

And assuming he cleanly removes the parts and confirms the pads are not shorting, due to leftover excess solder...

Do you have a circuit diagram?

I'm going to assume we won't even get a picture or clue as he implied his work is non-disclosure work related in another thread...

 

[danny davis]

Can you maybe answer the questions as in general so i can get the concept

 

[CocaCola]

Can you maybe answer the questions as in general so i can get the concept

See this is what I touched upon earlier, you want (or expect) us to give you a black or white answer and there simply isn't one each design will be unique unto itself...

 

[CocaCola]

One (ok a few) question(s) I have to ask, have you diagnosed any defective units yourself? What are you seeing as the problem? Have you located any shorts? Where are those shorts? Are they Vcc to Gnd shorts or shorts between I/O pins on ICs?

 

[danny davis]

So these questions you can't answer in general so I get the concept

If all 10 stages of a circuit use 12 volts Vcc, All 10 stages would have zero volts?

If you check the continuity between Vcc and Gnd for each stage of the circuit, all of the stages will measure a SHORT zero ohms right?

Because if you have a short between Vcc and Ground, That short is a parallel path to all the stages of the circuit?

When there is a short between Vcc and ground

I have disconnected, lifted input and output pins of a IC chip or stage, and I still get +Vcc volts on both sides of the input and output of that stage, even tho i lifted the input and output pins , I will get Vcc on the input pad/node and Vcc on the output pad/node

What does that mean, and what would you do for the next step of the troubleshooting please?

 

[danny davis]

Mostly the shorts are
1.) Vcc to ground shorts
2.) I get Vcc voltage on components I'm not suppost to
3.) Current is to high

A Vcc to ground short is "Railed" to each stage of the circuit

What I mean by Rail, Is that The Vcc voltage is "bussed/railed" to each stage of the circuit

So if you continuity check Vcc to ground you will get a SHORT on your DVM meter reading at any stage of the circuit because the Vcc voltage is bussed and rail to each stage of the circuit

 

[CocaCola]

So these questions you can't answer in general so I get the concept

Not really...

If all 10 stages of a circuit use 12 volts Vcc, All 10 stages would have zero volts?

Not necessarily if the stages are isolated or protected from each other...

If you check the continuity between Vcc and Gnd for each stage of the circuit, all of the stages will measure a SHORT zero ohms right?

Not necessarily if the stages are isolated or protected from each other...

Because if you have a short between Vcc and Ground, That short is a parallel path to all the stages of the circuit?

Only if that short is common to all stages...

I have disconnected, lifted input and output pins of a IC chip or stage, and I still get +Vcc volts on both sides of the input and output of that stage, even tho i lifted the input and output pins , I will get Vcc on the input pad/node and Vcc on the output pad/node

What does that mean, and what would you do for the next step of the troubleshooting please?

That means there is another route for power to complete the circuit...

 

[(*steve*)]

Mostly the shorts are
1.) Vcc to ground shorts
2.) I get Vcc voltage on components I'm not suppost to
3.) Current is to high

1 and 2 are pretty much incompatible with each other.

If your shorts are Vcc to ground then likely your Vcc rail will be pulled down to ground and you won't be seeing Vcc *anywhere*

edit: ignoring isolated power supplies, etc.

 

[danny davis]

If your shorts are Vcc to ground then likely your Vcc rail will be pulled down to ground and you won't be seeing Vcc *anywhere*

So Vcc would be zero voltages?


Most of the SHORTS are
1.) Vcc voltage gets shorted to a component or IC pin
2.) Or Ground is shorts to a component or IC pin

How would u diagnose these types of Shorts please , what are some basic steps you would try or do?

 

[(*steve*)]

I would measure the voltages (maybe currents and resistances too, but probably voltages first), notice a discrepancy (based on my understanding of the circuit and the normal operating parameters) then use my analytical skills to determine a likely cause. I would then make observations in order to try to confirm my hypothesis. If confirmed I would try to resolve it.

 

[CocaCola]

So Vcc would be zero voltages?


Most of the SHORTS are
1.) Vcc voltage gets shorted to a component or IC pin
2.) Or Ground is shorts to a component or IC pin

How would u diagnose these types of Shorts please , what are some basic steps you would try or do?

With your eyes or testing experience on that circuit... Again there is no black and white answer...

Example a Vcc short to one side of a 100K resistor that then 'shorts' to ground (aka shorting though the resistor) is going to have a completely different effect then if it shorts to the other side of the resistor and thus is a direct short to ground...

 

[(*steve*)]

With your eyes

A very good point.

Danny, this bit comes before you take any measurements.

 

[danny davis]

You would take the Voltages
1.) Measure the voltages across the components , resistors , capacitors , etc.
2.) Measure the voltages reference to ground to each pad and node

The Circuit Boards at my work or past jobs, The circuit board will test good and pass QC tests

I will put my DVM meter across either resistors or capacitors to write down the voltage i measure across the component. I have found that alot of the components don't even have a voltage drop. The circuit is working too , So It doesn't make sense to me how there is resistors and capacitors that don't have a voltage drop , how can you not have a voltage drop but the circuit is working?

I would think you would get voltage drops for each component on a PCB board

 

[CocaCola]

A very good point.

And it's amazing what you can detect without even trying if you are familiar with the boards...

It wasn't my primary job at Motorola but every so often I would help at the end of the reflow oven that was spitting out panels of phone boards (100s an hour)... You would be amazed at how fast you can turn yourself into a visual machine and catch probably 90% of the defects without even trying or focusing... That is if you apply yourself...

 

[(*steve*)]

how can you not have a voltage drop but the circuit is working?

If the potential either side of a resistor is the same, then no current will flow through it and the voltage drop (pretty much by definition) will be zero.

Alternatively, if there is no current flow then the potential difference across the resistor will likewise be zero.

I think this may be the third or fourth time I've said this, but you need to have an understanding of the circuit. If you don't have one then you must develop one.

Sure, for a particular circuit you might have a series of tests (measure this, if X then do Y) but these will have been written by someone familiar with both the circuit and the faults.

 

[danny davis]

If the potential either side of a resistor is the same, then no current will flow through it and the voltage drop (pretty much by definition) will be zero.

Why even have a resistor or capacitor have no current flow through it and have no voltage drop?

What are they used for in a circuit?

Cause there is like 30 components , resistor and capacitors that have no voltage drop, Why would a designer use components to have the same potential on both sides?

 

[(*steve*)]

I think this may be the fourth or fifth time I've said this, but you need to have an understanding of the circuit. If you don't have one then you must develop one.

 

[(*steve*)]

You're just asking questions that we can't possibly give useful answers to.

You can't or won't provide us with information that would help us help you.

We are going in circles.

I will probably walk away from this thread soon.

 

[danny davis]

No, you're helping me out alot

I just don't understand about resistors and capacitors having the same potential difference on both sides and what they are used for

 

[(*steve*)]

OK, here's a simple example.

Imagine your circuit is a switch, a resistor, and a LED and your power source.

The circuit is: +ve end of power source to switch; other side of switch to resistor; other end of resistor to anode of LED; cathode of LED to -ve.

This is a working circuit, right? The switch turns the LED on and off.

What is the voltage across the resistor and the diode when the switch is open?