the best way to find short circuit?

[shortchange]

Re: the best way to find short circuit?

I would say using your VOM is fine.
All you need to see is a needle movement.
With digits I have to see number change.
That is alot of needle movement.

[senz_90]

Re: the best way to find short circuit?

4 ohms is what your ohm meter may read read for a typical short cicuirt.
It may read something else.
The point is you will see a large change with your ohm meter monitoring the "shorted" power supply voltage "line" (WHILE THE POWER IS OFF and no external power supply) by chilling the bad part.
The good parts will not change the ohm meter.

Your DMM does not have an ohm setting?
Mine is a Fluke model 16.


I have been using this method for many years now, while other techs lug their power supply around.

Accurate reading on voltage drop means you might notice a dip in voltage.
After probing around 10 accurate voltage reading you notice a dip.
So you remove a 100 pin SMD ic to find that was not shorted
but nearby is a cap that is bad.

You would see a large change in the ohm meter chilling the ic or cap!

my VOM has ohm meter but it's ashame to tell you that i don't have DMM, just an analog VOM. almost all people around my small town repairer still using that VOM, i am just realize that VOM become an absolete one on abroad. hahaha.
what is normal value indicate short on ohm meter, 0 ohm? or is that have not exact value? or i just have to notice a big dip on measurement voltage drop? for example value please between normal and shorty one??

[budm]

Re: the best way to find short circuit?

You can buy electronic freeze spray for troubleshooting:
http://www.allelectronics.com/make-a...-OZ-CAN/1.html

or use canned air duster upside down to get really cool air.

[dj_ricoh]

Re: the best way to find short circuit?

what chilling/freez spray do you use?
or how do you cool it ?

[shortchange]

Re: the best way to find short circuit?

4 ohms is what your ohm meter may read read for a typical short cicuirt.
It may read something else.
The point is you will see a large change with your ohm meter monitoring the "shorted" power supply voltage "line" (WHILE THE POWER IS OFF and no external power supply) by chilling the bad part.
The good parts will not change the ohm meter.

Your DMM does not have an ohm setting?
Mine is a Fluke model 16.


I have been using this method for many years now, while other techs lug their power supply around.

Accurate reading on voltage drop means you might notice a dip in voltage.
After probing around 10 accurate voltage reading you notice a dip.
So you remove a 100 pin SMD ic to find that was not shorted
but nearby is a cap that is bad.

You would see a large change in the ohm meter chilling the ic or cap!

[senz_90]

Re: the best way to find short circuit?

You do NOT need to lug a power supply around.
The ohm will show a large change when you heat or cool the shorted parts.
With much better resolution the your finger.
You do not need to know or have a schematic to find the power connections in hopes to see a difference in voltage drops.
To find a short or multiple shorts.
Say the 3.3v supply has 4 ohms.
attach your ohm meter to the 3.3 volt line.
And using chilling spray or a heat gun.
The component that causes a change in the ohm is or are most likely at fault.

how do you get 4 ohms calculation? everyone's around the world using a good DMM that can measure voltage drop easily. i just think have to purchase one DMM later instead always use my VOM that is couldn't makes an accurate reading on voltage drop..

[senz_90]

Re: the best way to find short circuit?

Some tricks I use for locating shorts:
Inject 1V or so at max. 1 amp (~1watt) and see what is getting warmed up. Higher current can melt thin PCB traces. Usually you can feel the hot spot if you wait a while, or use an IR thermal cam if you have one.
Use a high-resolution multimeter (HP34401 6-1/2 digit) and you can measure μV voltage-drops along PCB traces and wires, to locate the short.
I also lift pins/leads of parts to isolate them. Some boards get hit with overvoltage and many things short, so you can get lost by thinking it's just one bad part.

that's a good trick with IR cam. i ever heard that voltage drop measurement. but i don't have a good multimeter, just analog one, is that possible using that? i have an ESR meter adapter for my VOM also that any site discuss how to find short with ESR meter. i just think that is a mili ohm meter, because need a high resolution on the digit likes u said. did u lift pins all of parts or just the all good one's tested by your DMM? and both leads or just one lead of the component?

[shortchange]

Re: the best way to find short circuit?

You do NOT need to lug a power supply around.
The ohm will show a large change when you heat or cool the shorted parts.
With much better resolution the your finger.
You do not need to know or have a schematic to find the power connections in hopes to see a difference in voltage drops.
To find a short or multiple shorts.
Say the 3.3v supply has 4 ohms.
attach your ohm meter to the 3.3 volt line.
And using chilling spray or a heat gun.
The component that causes a change in the ohm is or are most likely at fault.

[redwire]

Re: the best way to find short circuit?

Some tricks I use for locating shorts:
Inject 1V or so at max. 1 amp (~1watt) and see what is getting warmed up. Higher current can melt thin PCB traces. Usually you can feel the hot spot if you wait a while, or use an IR thermal cam if you have one.
Use a high-resolution multimeter (HP34401 6-1/2 digit) and you can measure μV voltage-drops along PCB traces and wires, to locate the short.
I also lift pins/leads of parts to isolate them. Some boards get hit with overvoltage and many things short, so you can get lost by thinking it's just one bad part.

[shortchange]

Re: the best way to find short circuit?

To find a short or multiple shorts.
Say the 3.3v supply has 4 ohms.
attach your ohm meter to the 3.3 volt line.
And using chilling spray or a heat gun.
The component that causes a change in the ohm is or are most likely at fault.

[senz_90]

Re: the best way to find short circuit?

Yes. For example, I'd set it to around 1V.

ok. really appreciated for the answer.. thank you so much. my knowledge for repairing increase now so i don't waste my time to found any short circuit hardly.

the other's have a another advice or technic to find the short circuit without wasting the time?? it's so fun to learn about troubleshooting technic..

[tom66]

Re: the best way to find short circuit?

Yes. For example, I'd set it to around 1V.

[senz_90]

Re: the best way to find short circuit?

Be certain to set the power supply voltage to something lower that the rails's normal operating voltage. This is to avoid causing lots more damage if the short clears itself.

oh. yeah.. i see. thank you advicing. so for example if the rail that maybe shorted give 5V output e.g atx psu rail. so i have to give that rail lower than 5V. is that right?

[rievax_60]

Re: the best way to find short circuit?

aha..!! i am understand clearly now..so the other must be connect to whichever power input predicted short on the board..thank you for advice
maybe i have to get that freeze spray on supply market (store) to get a better result instead using my hand.

Be certain to set the power supply voltage to something lower that the rails's normal operating voltage. This is to avoid causing lots more damage if the short clears itself.

[senz_90]

Re: the best way to find short circuit?

One on ground, the other to whichever power input or voltage input is shorted on the board.

aha..!! i am understand clearly now..so the other must be connect to whichever power input predicted short on the board..thank you for advice
maybe i have to get that freeze spray on supply market (store) to get a better result instead using my hand.

[tom66]

Re: the best way to find short circuit?

One on ground, the other to whichever power input or voltage input is shorted on the board.

[senz_90]

Re: the best way to find short circuit?

I use 3 amps but if the components are small, 500mA can work. 3 amps makes just about anything get very hot. I only apply the current for half a second, I'm not looking to burn the failed component. If the current is sufficient, the component will get warm, melting the freeze spray, accurately pinpointing the fault. I find that using a finger alone isn't precise enough... you don't actually have great thermal "resolution" on your finger... just enough to tell you if it's hot or cold... freeze spray makes it obvious exactly what is getting hot. Poor man's thermal imaging camera.

It's important that the supply you have or have made limits current non-destructively, i.e. it can safely drive a short. For example, when I short out my power supply set to 2 amps, it becomes a 2 amp constant current source (output voltage nearly zero) and can do so continuously without damage, though it does get rather warm due to the extra heat produced in the power transistors.

ok. i got it. but where is the place for me to attach PSU probe?? i just don't understand clearly " line" word means about ,because i am not a fluent english user on science concepts.. one on ground surely. another one? and ground with red(+) probe or black(-) probe?

[tom66]

Re: the best way to find short circuit?

I use 3 amps but if the components are small, 500mA can work. 3 amps makes just about anything get very hot. I only apply the current for half a second, I'm not looking to burn the failed component. If the current is sufficient, the component will get warm, melting the freeze spray, accurately pinpointing the fault. I find that using a finger alone isn't precise enough... you don't actually have great thermal "resolution" on your finger... just enough to tell you if it's hot or cold... freeze spray makes it obvious exactly what is getting hot. Poor man's thermal imaging camera.

It's important that the supply you have or have made limits current non-destructively, i.e. it can safely drive a short. For example, when I short out my power supply set to 2 amps, it becomes a 2 amp constant current source (output voltage nearly zero) and can do so continuously without damage, though it does get rather warm due to the extra heat produced in the power transistors.

[senz_90]

Re: the best way to find short circuit?

High current from constant current power supply plus freeze spray.

i build an adjustable power supply a couple weeks ago. but it just have low current with 500mA transformer. maybe i can't use it. what is the minimum current for the power supply? i have read around googling and i have read it, but i just didn' t understand how to practicing.
from my understanding, we connect a red probe high current PSU to the "line" ( means live or neutral mains?) and the black probe to ground for a while. release the probes, any warm or hot by touched is the faulty one.. is that right?

[tom66]

Re: the best way to find short circuit?

High current from constant current power supply plus freeze spray.