SHORTS, voltage across resistor converts to current

[walters]

the percentage of total power supply current going through the short depends on the particular circuit design

Wouldn't the SHORT be 98% of the total power supply current?

you can be assured that KVL and KCL must stand true

How can KCL stay true when there is a SHORT somewhere?

The percentage of the total power supply current is divided up and the SHORT will have 98% of the current , right?

I just don't under stand KCL with there is a short applied to that theory

Because KCL is saying all the currents in every stage and network must equal the total power supply current

If you measure the current of the First stage of the circuit and its 10uA and the total power supply current is 100mA, Where is the rest of the current going to? how do you find the rest of the current? the hunting of the resting of the current is the problem I have

 

[godfreyl]

The percentage of the total power supply current is divided up and the SHORT will have 98% of the current , right?

Not always. It depends where in the circuit the short is. Look at the simple amplifier circuit below.

If there is a short between the base and emitter of the transistor, then the total current through the circuit will only be about 1% of what it should be, but all of that current is flowing through the short.

If the short is between the base and collector of the transistor, then the total current drawn by the circuit is nearly double what it should be, but less than 1% of that current is flowing through the short.

~~~~~~~~~~~~~~~~~~~~~~~~

If you want to find the problem in a circuit that is not working, you first need to understand how the circuit works. Then you can measure voltages at various points in the circuit to see if they are as you expect.

When you find the problem, it may be a short circuit or an open circuit or a faulty component.

I think you are going about this the wrong way. You ask how to find a short circuit, but what makes you think there is a short circuit in the first place?

It is only when you find the problem that you discover whether it is caused by a short circuit or something else.

 

[walters]

If there is a short between the base and emitter of the transistor, then the total current through the circuit will only be about 1% of what it should be, but all of that current is flowing through the short.

1.)The Total power supply current is 100mA
2.) then the total current through the circuit will only be about 1% of what it should be? 1% of 100mA?
3.) but all of that current is flowing through the short? the 99% of 100mA right?

If the short is between the base and collector of the transistor, then the total current drawn by the circuit is nearly double what it should be, but less than 1% of that current is flowing through the short.

1.) If the Total power supply current is 100mA
2.) then the total current drawn by the circuit is nearly double what it should be?
3.) but less than 1% of that current is flowing through the short?

 

[godfreyl]

The circuit I showed normally draws about 2mA from the battery.

If there is a short between the base and emitter, then the circuit only draws about 20uA from the battery, and 99.9999% of that goes through the short.

If there is a short between the base and collector, then the circuit draws about 4mA from the battery, but only about 20uA flows through the short.

 

[walters]

The circuit I showed normally draws about 2mA from the battery.

If there is a short between the base and emitter, then the circuit only draws about 20uA from the battery, and 99.9999% of that goes through the short.

1.) 20mA - 20uA = the 99% shorted current?

If there is a short between the base and collector, then the circuit draws about 4mA from the battery, but only about 20uA flows through the short.

This gets me so confused about shorts, because a short can go either way 99% or 1% of the total power supply current

The Voltage across the short would be if it's 99%? would the voltage be 99% of Vcc?

The Voltage across the short would be if it's 1%? would the voltage be 1% of Vcc?

The circuit I showed normally draws about 2mA from the battery.

Can a short draw more current that 2ma? what kind of short would draw more current than the total power supply current?

 

[godfreyl]

The points I was trying to make were:

• When there is a short circuit, the total current may go up, go down, or stay exactly the same.
• When there is a short circuit, the current through the short may be very high, very low, or anywhere in between.
• If something is wrong, do not assume there is a short circuit. The problem may be caused by a short circuit, an open circuit, a faulty part, or something else.
  
  When a piece of equipment that was working suddenly stops working or develops a fault, the problem is most likely not cause by a short circuit.

---------- Post added at 18:20 ---------- Previous post was at 18:05 ----------

1.) 20mA - 20uA = the 99% shorted current?

No. The total current is 20uA. All of that current goes through the short, so the current through the short is 20uA.

This gets me so confused about shorts, because a short can go either way 99% or 1% of the total power supply current

Yes.

The Voltage across the short would be if it's 99%? would the voltage be 99% of Vcc?

No! A short has no resistance so there is never any voltage across it.

The Voltage across the short would be if it's 1%? would the voltage be 1% of Vcc?

No. Same reason.

Can a short draw more current that 2ma? what kind of short would draw more current than the total power supply current?

In this circuit, a short between collector and emitter would draw about 4mA.

I will try to explain better with more pictures a little later.

 

[walters]

When there is a short circuit, the total current may go up, go down, or stay exactly the same.

The Total power supply current can go Up, down or stay the same
Yes I have notice this in circuits

When there is a short circuit, the current through the short may be very high, very low, or anywhere in between.

Yes I have noticed this in circuits

If something is wrong, do not assume there is a short circuit. The problem may be caused by a short circuit, an open circuit, a faulty part, or something else.

When a piece of equipment that was working suddenly stops working or develops a fault, the problem is most likely not cause by a short circuit.

Well I might not be a short circuit, But it doesn't change the CURRENT in all the branches and stages I have noticed

Mostly the Faulty stage or network will either be 99% of the total power supply current or 1% of the total power supply current right?

if the Faulty stage is an Open current, the Total power supply current won't go UP or Down Right?

Only A short circuit will cause the total power supply current to go UP or Down right?

---------- Post added at 17:25 ---------- Previous post was at 17:22 ----------

The Voltage across the short would be if it's 99%? would the voltage be 99% of Vcc?
No! A short has no resistance so there is never any voltage across it.

The Voltage across the short would be if it's 1%? would the voltage be 1% of Vcc?
No. Same reason.

I mean the component surround the SHORT, Will the Voltages Be Either HIGHER or LOWER around the SHORT? That how you can Hunt for the SHORT?

---------- Post added at 17:30 ---------- Previous post was at 17:25 ----------

1.) 20mA - 20uA = the 99% shorted current?
No. The total current is 20uA. All of that current goes through the short, so the current through the short is 20uA

.

Oh ok so the 1% will go to the other stages/networks of the circuit
And there voltage drops will be in the millivolts because of the SHORT

So the Total Power supply voltage is not 20mA anymore it's 20uA because of the SHORT?

THe other stages/networks/branches are going to have nanoAmps of current because the SHORT has 20mA of current, its hogging all the current right?

 

[godfreyl]

Mostly the Faulty stage or network will either be 99% of the total power supply current or 1% of the total power supply current right?

What if the faulty stage normally only takes 10% of the total supply current?

• If the fault causes that stage to take no current at all, the total supply current will only go down by 10%.
• If the fault causes that stage to double it's current, the total supply current will only go up by 10%, and the current through the faulty stage will still be less than 20% of the total current.

if the Faulty stage is an Open current, the Total power supply current won't go UP or Down Right?

An open circuit may cause the total power supply current to go up or down.

Only A short circuit will cause the total power supply current to go UP or Down right?

No, there can be other reasons.

So the Total Power supply voltage is not 20mA anymore it's 20uA because of the SHORT?

The total power supply current is not 20mA anymore. It's 20uA because of the short.

THe other stages/networks/branches are going to have nanoAmps of current because the SHORT has 20mA of current, its hogging all the current right?

The other stages/networks/branches are going to have nanoAmps of current because the short has 20uA of current. Its hogging all the current.

 

[walters]

Thanks for correcting me, That is what i ment to say i just typed it wrong

An open circuit may cause the total power supply current to go up or down.

Why would an open circuit cause the total power supply current to go up or down?

Does an open circuit cause the Total power supply Voltage to go up or down?

Does a short circuit cause the total power supply voltage to go up or down or stay the same?

Because from my experience , A short circuit only changes the total power supplys current either up or down and the voltage stays the same

When there is a short circuit somewhere on the board, all the voltage drops are in millivolts and the currents are in microamps or nanoamps

When the circuits are in normal operations, the voltage drops are in volts not millivolts and the current is in either milli-amps or microamps

For Open Circuits, There is no current flowing through them. But their is DC Voltage on both sides of the component with the same potential?

For Open Circuits, The current

kirchhoff's never made it clear as to make his current and voltage laws applied to open circuits and short circuits

Because Kirchhoffs KCL states that the total power supply currents will equal whatever the whole circuit is Drawing from it.

But Kirchhoff never explains what happens to his KCL and KVL laws when there is a Open circuit or short circuit

Yes, when you have a open circuit or short circuit somewhere in a circuit or network the KCL and KVL will have to equal the total power supply voltage and current

The KCL and KVL doesn't explain what a open circuit or short circuit does to the current or voltage LOOP because the open or short takes a percentage of the total power supply current.
0.) All the Currents and voltages should Equal the Total power supply current and voltage
1.) Each SHORT is different and will take from a range of 1% to 99% of the Total power supply current.
2.) A Short will change the circuits Drawing current value of normal operation.
3.) An Open Circuit will change the circuits current or voltage drawing value of normal operation right?
4.) An Open Circuit should draw LESS current from the power supply, So the Total powers supply current will be Lower it, will go down?
5.) A SHORT has no voltage drop across it, but has very high current
6.) The Component around the SHORT circuit will have Big or lower voltage drops compared to normal operation?
7.) A Open Circuit changes the voltage drops only in that faulty stage but doesn't affect the other stages or networks?
8.) An Open Circuit Has the same DC potential voltage on both side of the component?

The total power supply current is not 20mA anymore. It's 20uA because of the short.

Yes I get it now, The SHORT circuit changed the circuits "DRAWING current"

 

[godfreyl]

Walters, you are trying to invent too many rules and most of them are wrong.

There are only two things we can know for sure about a short circuit or an open circuit:

1. The voltage across a short circuit is very low or zero, definitely zero for a perfect short circuit.
2. The current through an open circuit is very low or zero, definitely zero for a perfect open circuit.

That is all.

It is important to understand these points:

1. The current (if any) through a short circuit will depend entirely on the rest of the circuit.
2. The voltage (if any) across an open circuit will depend entirely on the rest of the circuit.
3. The total current drawn from the supply when there is a short circuit will depend entirely on the rest of the circuit.
4. The total current drawn from the supply when there is an open circuit will depend entirely on the rest of the circuit.

There are many different kinds of circuit. Each one behaves differently.

 

[ahsan_i_h]

@godfreyl
Hello godfreyl,
from post number 19,

When there is a short, the tech at my work will always reference the 100mA at the total current of the circuit. The tech will measure the current from a Resistor tied to Vcc to know what that stage is being Feed. Mostly It will be microamps because the SHORT is hogging all the current right?

Do you think that the tech was using a current source( I mean short circuit protected power supply and set the short circuit current to 100mA)?

May be a current source is making wrong feelings about SHORT to walters.

 

[ahsan_i_h]

Hello walters,
Two types of SHORTS may happen in a circuit.

First type:This type of SHORTS(from component failure) makes the power supply rail(Vcc and groud) CLOSE TO SHORT(low ohms) or EXACT SHORT(just zero ohms).

Second type: This type of SHORTS(from component failure) affect the power supply rail(Vcc and ground) but do not make them near SHORT. Finding this type of SHORTS, are the job of advanced troubleshooters.

Lets discuss about First type of SHORT(power supply rail is CLOSE TO SHORT or EXACT SHORT):
In this situation a high current will flow. Why a high current will flow? because your circuits Vcc and ground resistance is low(for CLOSE TO SHORT) or zero(FOR EXACT SHORT). You know that, like....

A short from Vcc to Gnd is easy, just check continuity between Vcc and Gnd if they are common they are shorted
Ya, but which stage of the circuit is the Short from Vcc to ground?

As your power supply voltage is 12v(for example), So the current from your power supply will be 12/low or zero= very high. For this reason you can not connect your circuit to your power supply. There are special type power supply which can understand that, threre is a SHORT in its terminal and they automatically reduce their voltage so that a high current can not flow. Then what current will flow? There is a knob in the power supply to set that.
Now you have to take a such power supply and set its short circuit current limit (power supply will reduce its own voltage and not let this current limit to cross) to say 100mA or 200mA or 300mA like that. Now give power to your circuit. Now cosider two cases,

Case1(for EXACT SHORT): If your circuits power supply rail resistance is EXACT zero, then everywhere you will get zero voltage. Why? because zero*anything=zero. So you can not measure voltage anywhere. So you have to go to Method 1(troubleshooting without power).

Case2(for CLOSE TO SHORT): This case was handled by the Tech and he was doing the right thing. Like...

The Tech at my work find shorts this way:
1.) Measures the Currents Total from the power supply
2.) Then measures the current from each resistor that is tied to a Vcc supply voltage in each stage

If the Current total is 100mA out of the power supply, So Kirchhoffs Law states that current needs to Equal it's loop or Current Total

If you measure the current from each branch or resistor that is tied to Vcc then you will know how much current is being Feed to that stage of the circuit

When there is a short, the tech at my work will always reference the 100mA at the total current of the circuit. The tech will measure the current from a Resistor tied to Vcc to know what that stage is being Feed. Mostly It will be microamps because the SHORT is hogging all the current right?

So if the total current is 100mA
You measure The Resistor tied to Vcc of each stage, you will mostly get 10uA or 50uA
he will always say where is there rest of the of the current going?
100mA - 10uA =
100mA - 50uA =

I just have a hard time finding where the Rest of the current is going to lead you to the SHORT or whats SHORTED , How do you find where the Rest of the current is going?

Now the REST OF THE CURRENT was flowing through a component which was connected to the power supply rail and which has no series resistor connected with it, like a power supply filter capasitor. A power supply filter capasitor is a capasitor which is connected directly to power supply rail(Vcc and ground). If you see any circuit, then you will see that capasitor.When any one connect a power supply filter capasitor to a circuit, he never put any series resistor with it. As there is no series resistor with it then the Tech could not measure current flowing through it.

Now you asked...

The stages that don't have the SHORT, will have very low current in microamps

The Stage that has the SHORT will have 97% of the Total Current referenced from the power supply Total Current?

The other 3% will be in microamps for the other stages that don't have the short?

Yes to all. the power supply filter capasitor(for example) was the SHORT and that was taking the 97% of that 100mA.

As the Tech could not find the SHORT so he should go to method 1(troubleshooting without power). As he knows that 10uA or 50uA networks are not SHORT. So he need not to check them. Now he will find that there are many power supply bypass capasitors and all are parallely connected, So he will treat them as a BLOCK OF PARALLELS. Then he will open each capasitor and will check them by multimeter.

 

[walters]

If Vcc to Ground Is Shorted, How do I find which stage is causing the SHORT to Vcc To Ground?

 

[ahsan_i_h]

Some stages have a series resistor. Some stages do not have a series resistor.

The stages which has a series resistor, all those stages are already checked by the Tech by measuring current(10uA,50uA etc) through the series resistors.

Now we know that the SHORT is in the stages which do not have a series resistor. There are many stages like that and are parallely connected. Examples of such stages are power supply bypass capasitors, ICs, A coil with a transistor etc. As they are all parallely connected so it is tough to find the SHORT.

Do not turn off the power

Increase the power supply current to a high value like 1A(for example)

Now touch(by finger,carefully so that you do not get electric shocked or burned) those stages like capasitors,ICs, transistors and keep it mind that which is cold, which is hot, which is hotter and which is hottest.

Turn off the power.

Now start from the hottest component. Generally, de-solder and take out the component from the board and check for SHORT in the power supply rail (Vcc and Ground). If the SHORT is cleared then that component was the SHORT. You do not always need to take out the component from the board. For example, de-soldering one leg of a diode is enough to check for SHORT. How to take out ICs? Now if the IC is in a base, pull it out and, again check for the SHORT in the power supply rail. If the SHORT is cleared, then that IC was the short. What if the IC is soldered, Then you have to de-solder and take out the IC or you may cut the TRACK which is giving power to the IC. Some times de-soldering and taking the IC out, may damage the ICs connection tracks due to the lack of experience. In that case If you do not want to cut the track and if the IC is cheap, use a small cutter, then cut all of its legs then take out the IC. Now de-solder and take out each leg from the board. When you need the IC, buy the IC and fix it with base. Now like this, serially check from hottest componet to coldest component.

As you started from hottest componet so there is a high probability that you will find the SHORT fast.

Note: If you do not feel any heat at all, you may increase current, but carefully.

 

[walters]

Is there another way to Find a Vcc to ground Short besides increasing the power supply current to 1A?

Some stages have a series resistor. Some stages do not have a series resistor.

Yes why is that some stages have a series resistor and other stages don't have a series resistor , why is that?

 

[FvM]

Is there another way to Find a Vcc to ground Short besides increasing the power supply current to 1A?

Yes, by measuring voltage drops of supply traces with a mV meter. If the board doesn't use power planes, you can usually localize the component drawing excessive current.

In case of shorts in power plane supplied components, voltage drops along the vias and supply pins may be still detectable.

A TTI i-prober is a nice tool for similar problems, by the way https://www.current-probe.info/

 

[ahsan_i_h]

Yes why is that some stages have a series resistor and other stages don't have a series resistor , why is that?

When a stage need to know the current of another stage, then a series resistor is provided.

For example if you see the "offline flyback regulator" circuit from the UC3844 datasheet (texas instruments), you will see that there is a 0.55 ohms/1W resistor is connected in series with the main switching mosfet. By measuring the voltage drop across that 0.55 ohms/1W resistor, the controller IC UC3844 is getting the current information of the main coil. This current information is used to form a current feedback loop.

Another example is, stage1 is controlling stage2, stage2 is driving a motor. The motor can rotate forward/reverse. We know that if the motor gets jammed suddenly, then it will get burned due to over current. So we need to feedback motor's current information to stage1. Stage1 will control the stage2 to give low voltage to the motor when the motor will try to take more current. Now, as stage2 is giving current to the motor, so stage2's current has some mathematical relation with motor's current. Thats why stage1 will need the stage2's current information to protect the motor.

 

[walters]

To Find a short from Vcc to ground, you have to turn the power OFF?
And use a DVM ohm meter to measure if there is zero ohms at any stage of the circuit?

If the Vcc to Ground Are Rails or Bussed, If i put a ohm meter across the Vcc to ground anywhere in the circuit or stage it will measure zero ohms if there is a short

How do I know which stage of the circuit is the faulty stage that is causing Vcc to ground short?

Do I have to Disconnect each stage of the circuit one by one from Vcc?

 

[ahsan_i_h]

Do I have to Disconnect each stage of the circuit one by one from Vcc?

Yes do it, or take this circuit board to a repair person and ask him to find SHORT, then watch what does he do and tell us about that. We learned many things from the Tech you were talking about.

 

[FvM]

How do I know which stage of the circuit is the faulty stage that is causing Vcc to ground short?

Do I have to Disconnect each stage of the circuit one by one from Vcc?

Not necessarily. Did you read the answers to your previous question?

If "disconnecting" is easily achievable, it's the preferred method. Mostly it's not feasible.