All About Circuits

Shorts with a negative supply

R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
Is the DVM continuity check the best way to check SHORTS?

How do you measure Shorts with a negative supply?

Positive supply circuit:
When you set your DVM to continuity to measure shorts when the POWER IS OFF, so the circuit under test doesn't have power on its off, is this the best way to measure shorts?

1.) When i put the "power" to the circuit under test then the shorts appear

2.) Why does shorts only appear when their is power to the circuit but not when the circuit doesn't have power i don't read any shorts?

3.) If i measure with the "continuity check" on my DVM to find a short in the negative supply lines , its going to BEEP because the negative voltage is below ground making it act like a short when it isn't?

4.) When using the " continuity check " audio beep, how do you know if its a short?

Continuity check for shorts"
If i put my Black probe on the ground and then using the RED probe from node or PCB pad one at a time until i hear a "audio beep"?

But i found that the audio beeps with negative voltages so it fakes the DVM out , acting like a short when its not is this true?

Would reversing the RED and BLACK probe on continuity check when measuring negative supply help out to find out if its a short or not?

How do you guys check SHORTS on a PCB ? nodes and traces?

Because when you look at the PCB under a microscope i still can't see any shorts visuallly at the nodes or traces

Its weird how the SHORTS only appear when the voltage is applied but not when their is NO power

But i have found SHORTS also when NO power was on

Whats the differences between these types of shorts?
 
SgtWookie

SgtWookie

Joined Jul 17, 2007
22,230
You should put your red probe on the more positive side. In this case, since ground is more positive than the negative voltage supply, put your red probe on ground, and the black probe on the more negative side.

Try that, and then ask again if you're still confused.
 
Externet

Externet

Joined Nov 29, 2005
2,218
relicmarks said:
Is the DVM continuity check the best way to check SHORTS?

Define 'short' first, is it for you less than 1 Ω ; or whatever low resistance that causes voltage drop under specfications, or raise in temperature above a certain limit, or a discharged capacitor ?

How do you measure Shorts with a negative supply?

The same way you do it on a positive supply or with any 'short'

Positive supply circuit:
When you set your DVM to continuity to measure shorts when the POWER IS OFF, so the circuit under test doesn't have power on its off, is this the best way to measure shorts?

1.) When i put the "power" to the circuit under test then the shorts appear

A high resistance power supply can do that too. Define 'short'

2.) Why does shorts only appear when their is power to the circuit but not when the circuit doesn't have power i don't read any shorts?

Circuits can have characteristics as relays that do not engage until power is applied.

3.) If i measure with the "continuity check" on my DVM to find a short in the negative supply lines , its going to BEEP because the negative voltage is below ground making it act like a short when it isn't?

A continuity check meter can beep at the range of resistance it was designed to beep, can be from 0 to 200 Ω

4.) When using the " continuity check " audio beep, how do you know if its a short?

Define short

Continuity check for shorts"
If i put my Black probe on the ground and then using the RED probe from node or PCB pad one at a time until i hear a "audio beep"?

But i found that the audio beeps with negative voltages so it fakes the DVM out , acting like a short when its not is this true?

Would reversing the RED and BLACK probe on continuity check when measuring negative supply help out to find out if its a short or not?

No, reversing probes can sense voltage left by previous polarity and will not equally trigger the beep.

How do you guys check SHORTS on a PCB ? nodes and traces?

Isolating areas by cutting traces

Because when you look at the PCB under a microscope i still can't see any shorts visuallly at the nodes or traces

If there is a piece of clipped lead under a connector or a solder bridge under a component, or a ball grid array shorted, or a reversed diode, or.... never ends; you will hardly notice looking at the wrong places.

Its weird how the SHORTS only appear when the voltage is applied but not when their is NO power

What if a diode is reversed in the circuit ?

But i have found SHORTS also when NO power was on

Nothing weird there

Whats the differences between these types of shorts?
Click to expand...
Define short !


An effective but controversial way to find a 'short' is, as example, if your circuit works on minus 12V DC and you find it 'shorted', apply a 12V car battery to it with proper polarity. The 'short' would likely turn into smoke, and you had found it. Wear protective glasses and precautions.

Miguel
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
put your red probe on the more positive side. In this case, since ground is more positive than the negative voltage supply, put your red probe on ground, and the black probe on the more negative side.


1.) So even the "continuity check" on my DVM has a "potential polarity"?

With a oscilloscope its great for signal tracing , so the signal goes through each stage

But To find PCB board shorts or traces that are internally shorted from fabraction house problems you have to use a DVM meter for this

The power supply chips converts +12 volts to negative -12volts , so all the IC's are not getting -12volts only +12 volts, even if you change the +12 volt convert chip to -12 volts there is still PCB or traces that are internally shorted or a IC is shorted internally thats pulling the -12 to a short 0 zero volts

The -12 volts goes to many , many IC chips , transistors etc.

At this point to find these types of shorts a oscilloscope can't be used

So i turn OFF the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it doesn't appear anywhere

So i turn ON the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it does appear in multiple places

Why does the SHORTS only happen when the power is ON VS OFF?

THis is not a signal tracing short , this is not a passive short, this seem more like a power short, see the difference in different types of shorts

How do you guys find SHORTS using the DVM meter?
 
Externet

Externet

Joined Nov 29, 2005
2,218
You wrote above:
..."So i turn ON the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it does appear in multiple places"...

Again, if you did not read, what do YOU call a 'short' ? A beeping meter ???

If you are probing continuity with the circuit ON, the task is well over your capabilities, and your DVM may be the next thing you have to repair.

Miguel
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
if you did not read, what do YOU call a 'short' ? A beeping meter ???

Yes a beeping meter when my Black probe is to ground and my RED probe is to a node/solder pad

If my DVM meter beeps then its a SHORT right?

If you are probing continuity with the circuit ON, the task is well over your capabilities, and your DVM may be the next thing you have to repair.

Why can't i use "continuity check" when the circuit is ON or has power?

Can you give me how you test SHORTS or find SHORTS using a DVM meter?
 
JoeJester

JoeJester

Joined Apr 26, 2005
4,390
All that continuity beep means is the circuit under test has a low enough resistance to activate the beep.

Post your circuit diagram. If you built the circuit, post a picture of your work.

In toto, your inquiries have been hodge podge at best, coverning a multitude of issues that have not followed a logical sequence in the learning process.

Do you realize that there is a potential difference (voltage) that you can measure between your DMM probes when in the resistance (or continuity) mode? Every time you place your probes on a de-energized circuit, your applying that potential to the circuit where you placed the probes.

on edit ...

Attached is a diagram of a very basic analog ohmmeter. What do you think happens when you place the two probes on various parts of your circuit.
 

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beenthere

beenthere

Joined Apr 20, 2004
15,819
The diode check function is very similar to the ohms function, where the meter injects a small current through the device to test it. If you use either function on a powered circuit, something in the meter will probably get fried.

If the circuit has been functioning normally in the past, the problem is most likely to be associated with a component failure. A new circuit may have an inadvertent short circuit due to a mistake during construction.

Before frantically metering everything in sight, it's always a good idea to establish that the power supply is functioning and that the connectors are in good shape.

If the power supply has some poop, then the short is often self-identifying. Look for the smoke, find the discolored component, feel for the hot component. A meter is often the tool of last resort when dealing with a shorted component.

Always work from a schematic if you can. The relationship of components on the circuit board and their function in circuit is not always obvious. The components that handle the most power or are closest to where signals go off-board are the most like to fail.

Can you describe the circumstance that caused a short to disappear when power was removed?

Most accepted usage has the meaning of "short" as an accidental connection between traces of normally isolated wires. A bad component, like a shorted transistor, is just a bad component - "the output transistor was shorted. I put in a new one and it came up".
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
a potential difference (voltage) that you can measure between your DMM probes when in the resistance (or continuity) mode

So there is a potential difference then or " DVM Probe polarity" when using continuity mode

I didn't know that the "DVM probe polarity" made a difference in continuity mode

I thought a SHORT didn't have a polarity or it was a direct connection so the DVM probe polarity wouldn't make a difference

But when there is SHORTS in a negative power source/supply the DVM probe polarity does make a difference because now ground reference is the higher potential

So when you guys use a DVM how do you guys set it to measure SHORTS?

How do you know the difference if its a SHORT or just a low resistance to make the meter beep or just read 0 to 20 ohms

A IC chip uses + VCC and -VDD to power up the IC chips

How do you test +VCC shorts or -VDD shorts and how to elimate the odds of which IC chip would be the one pulling -VCC down to zero?

If a Circuit had 10 ten IC chips how would you know which IC chip is pulling the VCC or VDD down to zero? ( because all the ten IC chips -VDD pins are all going to read and measure the same which will be zero volts)

You would have to LIFT UP each -VDD pin to each chip one at a time until the - negative voltage raises up again

My main point is Power shorts have to different types
1.) positive voltage short (+VCC short)
2.) or a negative voltage short (-VDD short)
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
example:

PCB fabrication shorted traces problem

You get 100 boards made at a PCB fabrication house , the components are NOT shorted , the comonponets are not getting HOT.

There is 5 different shorts on each board out of the 100 boards

How would you find them? what approach would you guys try or do ?

These kinds of shorts on from the traces and how the boards are layout so the copper traces are shorting internally to the ground planes or the VIAS are shorting out somewhere

This is "short finding"

What are some "short finding" techniques please?

Instead of signal tracing i'm "short tracing or short finding"

What test equipment would you guys use and how would you use it please?
 
JoeJester

JoeJester

Joined Apr 26, 2005
4,390
Do you have a specific circuit your troubleshootng or what?

As far as a multiple chip board, if there were a short in the power [either from the supply or only when the power is plugged to the board], the voltages would be alot lower than normal. If you removed the supply from the board and measured between ground and the + or ground and the -, you should read some resistance. If the resistance is low ... depending on your definition of short .... you can desolder the + or - on each chip till the resistance goes back up. + or - pin depends on where your probes are connected.

If there is sufficient resistance on the + and -, indivdually measured to ground, the problem most likely lies in your power supply.

What are you troubleshooing?

I test for shorts by using a low resistance setting. I know the potential across the probes so I know if I accidentally can forward bias diode or transistor junctions.

I do not use the continuity function to test for shorts.
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
Mostly when i try to find a POWER SHORT because "power rail shorts" are hard to find because the + rail and - rail go to many IC chips and transistors etc. so it can be any of those components or it can be a PCB fabrication problem with the layouts of the traces or vias holes somehow causes a short to a ground plane

What i do is to start by LIFTING IC VCC or VDD pins one by one or remove components like resistors or capacitors that are connected to either VCC or VDD mostly load resistors or AC filter caps one by one until i can see the voltage raising up to the operating supply voltage on the schematic

Short finding list:
1.) So to find SHORT you don't have the power on , turn the power off
2.) Use RX1 scale on your volt meter
3.) I use a Analog ohm meter to find shorts i hate using digital DVM meters to find shorts , the analog "needle" lets me clearly know if its a open or short

When I'm short finding

I put my Analog ohm meter set at RX1 scale and but my Black probe on ground and Red probe going from each component or stage or node until i see the analog ohm meter's needle swing to a zero ohm short

With a DVM digital volt meter you can get alot of false short finding readings which i don't like

How do you guys approach "short finding"?
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
Example#3

When voltage VCC or VDD appears on IC pins where they aren't suppost to be

VCC is at +5volts and VDD is at -5 volts , on a normal IC it would be pin 8 VCC and pin 4 on VDD

for somereason your getting +5 volts on pin 6 and pin 2 also so there is a SHORT

First then you would think of is replace the IC chip , so when you replace the IC chip with a new on it still doesn't do anything

Next thing to do is remove the IC chip and then measure with your ohm meter from VCC to pin 2 but it ""does not"" read a SHORT, then you measure from VCC to pin 6 but it "does NOT" read a SHORT.

It doesn't make a different if you replace the iC chip 5 times its not the IC CHIP

Its only a SHORT when the power is ON not when the power is OFF, why is that??
 
beenthere

beenthere

Joined Apr 20, 2004
15,819
If you're getting poorly fabbed PCB's with via shorted to power/ground planes there are two steps to follow:

1. Throw all the boards away (unless your lawyer advises keeping them for the lawsuit).

2. Get another fab house.
 
Externet

Externet

Joined Nov 29, 2005
2,218
relicmarks said:
if you did not read, what do YOU call a 'short' ? A beeping meter ???

Yes a beeping meter when my Black probe is to ground and my RED probe is to a node/solder pad

If my DVM meter beeps then its a SHORT right? -----> very wrong.

If you are probing continuity with the circuit ON, the task is well over your capabilities, and your DVM may be the next thing you have to repair.

Why can't i use "continuity check" when the circuit is ON or has power? ----> At least read your DVM manual !
Can you give me how you test SHORTS or find SHORTS using a DVM meter?

----> Before you wrongly defined short as a meter beeping, the answer was already posted : isolating circuit areas by cutting traces.
Click to expand...
Grab a good 100µF or larger capacitor and test your beeper. Chances are it will show shorted to you. Then reverse the leads.

Miguel
 
R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
Grab a good 100µF or larger capacitor and test your beeper. Chances are it will show shorted to you. Then reverse the leads.

a capacitor is suppost to measure a OPEN not a short

If my DVM meter beeps then its a SHORT right? -----> very wrong.

Why is it wrong? because its just measure a very low resistance doesn't mean its a short right?

Before you wrongly defined short as a meter beeping

Explain why please?
 
SgtWookie

SgtWookie

Joined Jul 17, 2007
22,230
I beg to differ with you.

Initally, a large-value cap will appear as a SHORT. After a few moments, it will become charged by the meter, and appear as an OPEN.

This is much more apparent if you are using an ancient multimeter on the OHMS scale. The needle deflection will show the rate of charge of the cap. After awhile, you will know what's the appropriate rate of charge for a given cap specification - it's meter-dependent.

I learned with a Simpson 260 - that's about how much they cost (in dollars) in the early 1970's. By the late 1970's, they were around $350.

For steady signal levels, you can't beat a DMM for accuracy. But for varying signal levels, a decent plain old MM is a great tool to have in your box.

Since you have a 'scope, you REALLY should build an Octopus.
http://greynet.net/~gbarc/feedback/may75.htm (Scroll about 3/4 of the way down)
http://octopus.freeyellow.com/octopus.html (Decent page - check out the links)

Just build a simple Octopus. I suggest using a couple of cheap transformers in series, rather than trying to find a center-tapped transformer - you'll get much cleaner output.

Get to know what the waveforms mean. Experiment with some individual discrete components to understand the waveforms before you start attacking your "problem child". It will be time well spent.
 
Externet

Externet

Joined Nov 29, 2005
2,218
relicmarks said:
Grab a good 100µF or larger capacitor and test your beeper. Chances are it will show shorted to you. Then reverse the leads.

a capacitor is suppost to measure a OPEN not a short ----> DO IT and then come back !

If my DVM meter beeps then its a SHORT right? -----> very wrong.

Why is it wrong? because its just measure a very low resistance doesn't mean its a short right? ----> What if your beeper is made to beep at 5000 Ω ?

Before you wrongly defined short as a meter beeping

Explain why please?
Click to expand...

-----> Gladly; A meter beeping tells the resistance across its probes is within the range it was designed to beep.
Grab a ~50 Ω resistor (which you may agree it is NOT a 'short', it is a resistance) and connect your beeper DVM; then come back with results.


Miguel
 
JoeJester

JoeJester

Joined Apr 26, 2005
4,390
for somereason your getting +5 volts on pin 6 and pin 2 also so there is a SHORT.
Click to expand...
Not necessarily. First you never said what the device was, never told us the pinouts, never drew the schematic. All you did was measure the resistance to ground or the voltage on the pin. Would you say there was a short if both top and bottom of a resistor measured 5 volts?

You really need to quit being a random checker and develope a troubleshooting plan.

Attached are my lecture notes concerning the six step troubleshooting process.
 

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R

Thread Starter

relicmarks

Joined Oct 13, 2006
355
All you did was measure the resistance to ground

But i thought that a node with very low resistance or circuit resistance at that node is very low resistance to ground would be a short is this true?

the voltage on the IC pin

Well if the IC pin 2 and 6 were the same voltage as the supply VCC or VDD i would be worried why they have the same voltage when they were suppost too , so i would say there is a SHORT , even if you change the IC chip it still has the same supply voltages on multiple pins which is wierd

Would you say there was a short if both top and bottom of a resistor measured 5 volts?

YES I would , what would you call it then?
 
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