SHORTS, voltage across resistor converts to current

[walters]

How to find SHORTS or Shunts in a circuit

1.) You measure the voltage drop across the resistor and convert it to current.
2.) You compare the current value with the supply voltage?
3.) If it's a SHORT or shunt would the current value be zero or very HIGH?

3volt supply feeding a 10mega resistor , the voltage drop is 2.62volts across the resistor, the current would be 2.62picoAmps its PICOamps right?

 

[Ratch]

walters,

What on earth are you talking about?

Ratch

 

[enjunear]

You typically find/remove short circuits to ground by three methods.
1. Visual inspection of the PCB for shorts between traces or between pins on devices

2. Isolating parts of the circuit (remove components to "disconnect" parts of the circuit from the supply). Often you need to physically cut DC supply lines to isolate circuits (repair them with a wire soldered over the cut, or a piece of copper ribbon, if you have it)

3. (for those who like to live dangerously) if you suspect the short-circuit is in the artwork of the PCB, you can try to "blow it open". Use a power supply with a high current-capability, set the voltage to your normal operating voltage (or less), and turn the current limit to the maximum, then flip on the power supply. The high current through a thin piece of shorting metal will make it get very hot and melt, which removes the short-circuit. However, if the short is pretty thick, the metal can get hot and cause burning on the PCB. If the short-circuit is inside of a failed component, you'll make it smoke REALLY GOOD, and likely stink up the entire lab when the part melts.

V = I*R
2.62 V / 10e6 ohms = 0.262 uA (microamps, x10^-6), which is 262 nA (nanoamps, x10^-9), which is 262,000 pA (picoamps, x10^-12).

If you are feeding 3V into a 10M resistor, where are the other 380 mV being dropped? Is that voltage lost in the wires from the power supply to the PCB? That seems like a lot of voltage drop if there is just a 10M ohm resistor on the PCB. If your power supply is at 3V and the traces on the PCB are at 2.62V, then you don't have a short-circuit... unless its a short that is going through a resistor. If it was a short from Vcc to ground, then voltage on the PCB should be very close to 0.0 volts.

If you have a short-circuit, then the current draw is VERY high... think about putting a screwdriver across the terminals of your car battery. That's a short circuit and makes a heck of a lot of sparks because of the high current through a very low resistance (12V across a few ohms).

 

[walters]

If you are feeding 3V into a 10M resistor, where are the other 380 mV being dropped? Is that voltage lost in the wires from the power supply to the PCB? That seems like a lot of voltage drop if there is just a 10M ohm resistor on the PCB

Where or how do you measure with your DVM meter to know where the 380mV drop is going?

If it was a short from Vcc to ground, then voltage on the PCB should be very close to 0.0 volts.

If the Voltage is 0.0 , What is the Current?

If I put my DVM meter across a resistor and the Voltage drop is Zero, Than I Have a Short?

If I put my DVM meter across a resistor and the voltage drop is the same voltage as the power supply, It's a Open resistor

Often you need to physically cut DC supply lines to isolate circuits

If I cut the DC supply lines then the circuits or networks aren't going to work

You would have to have external power supplys to feed the other "Networks" that your CUT right?

---------- Post added at 06:47 ---------- Previous post was at 06:41 ----------

If it was a short from Vcc to ground, then voltage on the PCB should be very close to 0.0 volts

What if the Short doesn't go to Ground or VCC

What are some signs to find it?

What would the voltage drop across a resistor be Higher? and the current would be higher?

 

[tpetar]

Ratch I can click only one time on helped me, they dont allow multiple clicking on that button from the same user, but your post deserves at least few clicking. :-D This is first post I read this morning, and I can say this make start of this day with smile. :-D

 

[walters]

low resistance where you expect to find high resistance

If i put my DVM meter across a resistor lets say 100K or 1meg, it's going to measure 100K or 1meg , why would it read a lower resistance if there is a short?


The way the electronic tech. at my work finds shorts is putting the DVM meter across a Resistor and measuring the voltage across the resistor, then you convert the voltage drop to current. Then you measure another resistors voltage drop that's in the signal path and you keep measuring the voltage drops and convert it to current until you find a high current?

I just don't know, how a tech would know which resistors voltage drop and converting it to current would tell you there is a short? because it would depend on the resistors value, so a 1meg would give you a big voltage drop so the current would be high but that doesn't mean there is a short there cause that is normal

I'm just confused on how you would know which voltage drop in relationship with the resistors value a tech would know that the current is a short?

 

[FvM]

A way to clarify what you are exactly talking about would be to sketch a circuit of your imagined resistor configuration. Hopefully it will help to better understand the problem yourself.

 

[ahsan_i_h]

If i put my DVM meter across a resistor lets say 100K or 1meg, it's going to measure 100K or 1meg , why would it read a lower resistance if there is a short?

Because troubleshooting's first law is SHORT PARALLEL TO ANYTHING IS SHORT.

 

[walters]

If you measure a resistor in circuit you will generally read a lower value than the resistor because not all the current from the probes goes through the resistor. Some part (maybe almost all of it if there's a short) will take another path,

If the resistor is 100K , I put my DVM to measure it , it's should read 100K

What you are saying is that if there is short it's not going to read 100k it will read 1ohm?

What I don't get is that If the 100K is not the short, but there is a short in the circuit somewhere else, will the 100K read 100K or measure as a SHORT?

To find shorts in a circuit, do you have to have the power turn on or Off ? can't you find shorts with the power off?

The Tech at my work showed me kinda of something to find shorts, If there is a short in a circuit somewhere, You use the ohm meter can what ever component has a resistance that measures a SHORT on the meter then you have located your shorted network??

Cause he was using the Ohm meter measuring the resistance component to component either if its a capacitor or transistor or resistor still using the ohm meter to measure the "resistance of the component" from stage to stage and then he found a stage that made he meter SHORT, so he isolated the network

 

[Ratch]

walters,

Did you follow the instructions in the attachment I sent you in the previous thread you started? Why did you start a new thread asking questions not relevant to what the material in the attachment was about?

Ratch

 

[enjunear]

If i put my DVM meter across a resistor lets say 100K or 1meg, it's going to measure 100K or 1meg , why would it read a lower resistance if there is a short?


The way the electronic tech. at my work finds shorts is putting the DVM meter across a Resistor and measuring the voltage across the resistor, then you convert the voltage drop to current. Then you measure another resistors voltage drop that's in the signal path and you keep measuring the voltage drops and convert it to current until you find a high current?

I just don't know, how a tech would know which resistors voltage drop and converting it to current would tell you there is a short? because it would depend on the resistors value, so a 1meg would give you a big voltage drop so the current would be high but that doesn't mean there is a short there cause that is normal

I'm just confused on how you would know which voltage drop in relationship with the resistors value a tech would know that the current is a short?

It sounds like the technician is looking for abnormal amounts of current going through the resistors. The easiest way to figure out what he's doing is to draw your circuit schematic and determine the normal current in each path, and normal voltage at each node. By measuring the voltage across a single resistor and knowing it's resistance, you can calculate the current flowing through it (Ohm's Law, V=I*R). By using the voltmeter, you can determine the voltage at each node. If you compare the nominal values from your schematic to the values you determined by probing in the lab, you will start to see trends... some voltages are too low, current is zero through some resistors. By seeing those discrepancies, you can identify what part of the circuit is having a problem by drawing more current than it's supposed to. Then you start looking for bad solder connections, traces shorted together by thin fingers of metal, etc.

To answer your first question about the 100k resistor and a short. If the resistor is in shunt to ground (pin 1 has a voltage supplied to it in normal operation and pin 2 is connected to ground), and you have a short-circuit on that same node (the metal trace that connects to pin 1 of the resistor is accidentally shorted to ground), then you will short out the resistor when you probe it. It's effectively like putting a wire across the pins of the resistor... you will measure a very low resistance because the wire (or short-circuit on the PCB) is a very low resistance. From the math, if you have multiple resistor in parallel, you sum their admittances using the following formula (admittance = 1/impedance).

Using 100K (resistor) and 0.1 ohms (shor-circuit)...

Using 200 ohms and 0.1 ohms, Rt = 0.0995 ohms... still looks like a short, not anywhere near 200 ohms.

 

[walters]

some voltages are too low, current is zero through some resistors.

What does that mean, when voltages are too low and current is zero through some resistors? its a short to ground?

---------- Post added at 04:43 ---------- Previous post was at 04:31 ----------

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?

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?

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?

 

[godfreyl]

Walters,

You need to learn something about electronics, or at least electricity. Then these things will be easy for you to understand. I suggest you start by learning about Ohm's law and how current flows in a circuit.

It pains me to see that you have been asking elementary questions here for almost 7 years, but don't appear to have learned anything. Perhaps there is a college nearby where you could study? If not, you could try to read a book. All these things will help.

Regards - Godfrey

 

[tpetar]

https://www.facstaff.bucknell.edu/mastascu/elessonshtml/Basic/Basic4Ki.html

https://www.facstaff.bucknell.edu/mastascu/elessonshtml/Basic/Basic5Kv.html

https://www.facstaff.bucknell.edu/mastascu/elessonshtml/Basic/Basic2i.html

https://www.facstaff.bucknell.edu/mastascu/elessonshtml/Basic/Basic3v.html

 

[walters]

kirchhoff's current and voltage law Applied to SHORTS

When applying Kirchhoff's current and voltage Laws to SHORTS

If you have a SHORT on a circuit and you're trying to narrow down where the SHORT Is coming from or where the alternative path is

You measure the voltage drop and convert it to current

Since there is a SHORT on the board the current is going to be HALF of what it is suppost to be

So Kirchhoffs Law states that current needs to Equal it's loop

So where is the other Half of the Current going? and how do you find it?

The SHORT should have the other half of the current, but how do u find it?

 

[Ratch]

Re: kirchhoff's current and voltage law Applied to SHORTS

walters,

Did you receive and follow the instructions of the attachment I sent you a few posts ago?

Ratch

 

[ahsan_i_h]

Hello walters,
You can find a short in a circuit by two methods. First method is troubleshooting without power and second method is troubleshooting with power.

Method 1(troubleshooting without power):
A circuit is a combination of many components and many tracks. The tracks are copper metal, so they must be a SHORT. So we can say that, a circuit is a combination of many components and many SHORTS.( Consider parallely connected components as a single component and lets give a name as BLOCK OF PARALLELS.)

Now the second law of troubleshooting is A SHORT AND A COMPONENT CAN NOT STAY PARALLELY. If it is true then if we measure each component's resistance by a DVM we will not get zero ohms. But if we get zero ohms then we will suspect that the component may be SHORT inside. Why? because we know that there must not exist any SHORT parallel to any component. So the component itself is the SHORT. Thats why your tech was doing the right thing like...

The Tech at my work showed me kinda of something to find shorts, If there is a short in a circuit somewhere, You use the ohm meter can what ever component has a resistance that measures a SHORT on the meter then you have located your shorted network??

Cause he was using the Ohm meter measuring the resistance component to component either if its a capacitor or transistor or resistor still using the ohm meter to measure the "resistance of the component" from stage to stage and then he found a stage that made he meter SHORT, so he isolated the network

Now what will happen in case of BLOCK OF PARALLELS, if we find it as a SHORT? Then we have to open each componet of it and have to check it.

There are some components which are very close to SHORT. These are coils of transformers, Fuses, Low value shunt resistors etc. So you should not be happy when you find a SHORT using DVM. You only SUSPECT there is a SHORT. Then you will visualy trace the circuit connection and make confirm that those CLOSE TO SHORT COMPONENTS are not parallely connected with the component which you suspect to be SHORT. What if those CLOSE TO SHORT COMPONENTS are parallel to your component? Then you have to open your component and have to check it. What if the CLOSE TO SHORT component like a coil itself is SHORT? Then you will need an inductance meter.

 

[ahsan_i_h]

hello walters,
Actually there is no laws of troubleshooting, I just made for you for easier understanding.

 

[walters]

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?

---------- Post added at 03:54 ---------- Previous post was at 03:20 ----------

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?

 

[enjunear]

Generally, the amount of current flowing through the short will be high, but the percentage of total power supply current going through the short depends on the particular circuit design. As I mentioned before, you start by looking for places where the current draw through parts is not in line with the nominal/designed current draw. When you find the region of the circuit that is not behaving correctly, you begin to isolate parts of the circuit by lifting leads, cutting traces, and uninstalling parts until you find a bad part (may be shorted internally) or the metal trace that is being shorted out.

Most of it's trial and error, but you can be assured that KVL and KCL must stand true... the difficult part is that you don't know where the short circuit is, so you have to hunt for it by looking at it's impact to adjacent or downstream circuits/components.