External Power supply current limit set to MAX amps to find SHORTS on a PCB

[danny davis]

The Techs that sit next to me at work , use a technique of finding shorts

When there is a SHORT from VCC to a component or a SHORT to ground to a component or a SHORT from VCC to ground

The use an external power supply and set the current limit to MAX 3 to 5 amps , but the voltage is at 5 volts or less

They turn OFF the circuit , And Pulse or trigger the Power supply rails with an external power supply and set the current limit to MAX 3 to 5 amps , but the voltage is at 5 volts or less

The Shorted component or what is causing the short, will cause a component to get HOT or explode the component, plus it leaves damage to the blown traces on the PCB board

How can you do this technique without cause blown traces? they have the current limit set to high? and would be a good current limited setting?

Also , Is this technique done for only a SHORT that is for VCC to ground? without damaging other components?

Cause I would think if you had A SHORT that is a SHORT from VCC to a component or a SHORT to ground to a component , and use an external power supply and set the current limit to MAX 3 to 5 amps , but the voltage is at 5 volts or less , that it will damage the other components or STRESS them out

I think this technique is only used when you have a SHORT from VCC to ground right? without cause STRESS damage to other components?

 

[Mithun_K_Das]

its not a proper way to find a short circuit at all. Just use your Digital Multi-Meter and test selecting connectivity test.

Passing a 3-5A of current may damage to the PCB layers. Also may damage other parts too.

 

[betwixt]

I've seen this done too but it is dangerous and frequently causes other damage. Take for example a short across 0V and 5V supply, you apply 5V from a high current supply, something goes pop and the short disappears. While it may look as though the fault is cured, what might happen is a power supply track or ground track on the board has just burned out. The original fault is still there and a new one is masking it. Further to that, when powered up normally with open supply or ground tracks the voltages around the isolated part of the circuit are now wrong and more damage is caused. It becomes far more difficult to effect a repair, especially if the new fault in on a middle layer of a multi-layer PCB.

To find power rail shorts the method should be:
1. visual - check for reversed capacitors, solder bridges, connectors fitted backwards and obvious signs of stressed areas.
2. resistance checks, if the short has such low resistance that a DVM can't locate it, use a special low resistance measuring unit "Tone Ohm" or similar with 4-wire measurement capability. They can usually localize a short to within a cm or so, even on boards with gridded power rails or power/ground planes inside the board.

Obviously NEVER apply power across a short anywhere, or a voltage in excess of normal, except power rails. Trying elsewhere is almost guaranteed to cause damage.

Brian.

 

[danny davis]

when powered up normally with open supply or ground tracks the voltages around the isolated part of the circuit are now wrong and more damage is caused.

Is the power rails are OPEN or ground tracks are OPEN, how can this cause damage? the power is OPEN i don't get it

It becomes far more difficult to effect a repair, especially if the new fault in on a middle layer of a multi-layer PCB.

Yes, they are using this technician to find the SHORTS in multi layer PCB boards , because the SHORT is inbetween , 3 layers up to 17 layers on the new PCB boards

 

[chuckey]

You should use a Hewlett Packard current tracing probe and its pulse probe - can detects currents as low as 10mA
Frank

 

[danny davis]

give an example how to use them? u pulse the power supply rails and use the current probe to measure it?

 

[chuckey]

This has been dealt with before in the "shorts on digital circuits" topic.
Frank

 

[Audioguru]

A "Tone Ohm" lets you hear where a short circuit is without using a high current and without destroying anything. So use it.

 

[danny davis]

u act like they make tone ohms anywhere, they don't make them anymore , u have to build them

U use an external power supply and set the voltage low around 2 or 1 volt and put the current limit to 1 or 2 amps, you feel with your finger which component is getting HOT and there is your short?

 

[Audioguru]

U use an external power supply and set the voltage low around 2 or 1 volt and put the current limit to 1 or 2 amps, you feel with your finger which component is getting HOT and there is your short?

That is a good way (a stupid way) to destroy a pcb.
A complete short DOES NOT GET HOT. A partial short, the pcb traces feeding your power to a short, a transistor, a diode or a low value resistor feeding a short will burn out because they have some resistance.

My Fluke meter on its 40 ohms range sends a very low voltage at a very low current to measure down to 0.01 ohms without destroying anything. It even calibrates away the tiny resistance of its leads and probes.

 

[danny davis]

the pcb traces feeding your power to a short, a transistor, a diode or a low value resistor feeding a short will burn out because they have some resistance.

So how does a transistor, a diode or a low value resistor feeding a short will burn out because they have some resistance?.

The SHORT should have all the voltage and HIGH CURRENT

Why does the transistor , diode , low value resistor get burned out?

 

[Audioguru]

Power equals voltage times current. A short has a very low resistance then it has no voltage across it. Since there is no voltage then there is no power and power creates heat. There is no heat.
But a pcb trace, a transistor, a diode or a low value resistor has some resistance and has voltage across it AND the high current from your damaging power supply in it which creates a lot of power and heat. They become a FUSE which is a low value resistor that burns out when it feeds power to a short.

A guy built a very high power audio amplifier. He tested it using railroad tracks as its load. A short occurred and the electrical power wiring in the neighbourhood burned because he bypassed the circuit breakers.

This is some (more) simple basics of electricity THAT YOU NEVER LEARNED.
Why did you not learn this simple stuff??

 

[danny davis]

A short has a very low resistance then it has no voltage across it. Since there is no voltage then there is no power and power creates heat. There is no heat.

Why is it , when a IC chip is shorted it is very HOT , creating HEAT?

I put my finger on the IC chip and it's very hot

a transistor, a diode or a low value resistor has some resistance

A transistor and a diode are Low resistance?

 

[betwixt]

Another experiment is called for:

Danny or whatever, take a known good new transistor, diode or IC, connect your 2V at 2A power supply across its pins and see if it gets hot. Try swapping the connections around to see if it makes any difference. If we are right, it will get hot and be permanently damaged, if you are right, a good component wil stay cold. Tell us the results.

Regarding the venerable "Tone Ohm", other similar devices are still made and to build one yourself is quite easy. It's only a voltage controlled audio oscillator and an couple of op-amps, hardly high-tech. It's the way the measurement is taken that makes it able to read such small resistances.

Brian.

 

[chuckey]

Danny you must learn a bit more about power, volts and amps. As mentioned before, PCB tracks generally have a very low fusing current, a couple of amps would damage them. So a one inch long thin track would have a resistance of .05 ohms, so just before the track burns out, it will be dissipating, 2 X 2 X .05 = .2W which would not be apparent unless you had a thermal camera to detect the overheating.
Most low ohm meters need to have VERY superior probes, otherwise the change of contact resistance when you are probing about will totally overwhelm any change in tone due to you being closer or further from a short circuit.
Years ago in electronic inspection we used a transformer coupled oscillator buzzer arrangement. The advantage was that the test leads were connected to a one turn winding on the transformer, so it was extremely sensitive, the tone went all over the place depending on the resistance. I now have a commercially made op-amp version, that seems to be more sensitive to contact resistance then to the short circuit. The transformer version also ignored mains transformer windings.
With the HP kit, you can actually trace the current and find out where its going, as said before the peak current was high but last for a very brief time so semiconductots were not damaged.
Frank

 

[danny davis]

I don't get it, What is causing the HEAT than when there is a SHORT?

When I put my finger on components and if they are really hot my manager says the IC is shorted internally, so I'm confused now since you guys say it's not a shorted IC , what is it than?

I don't get if there is a SHORT on the power rails that transistors and diodes look like low resistance, how so?

 

[Audioguru]

I don't get it, What is causing the HEAT than when there is a SHORT?

When I put my finger on components and if they are really hot my manager says the IC is shorted internally, so I'm confused now since you guys say it's not a shorted IC , what is it than?

A low resistance dissipates power and gets hot. A partial short is a low resistance. A dead short has NO resistance so it does not get hot. If your circuit board has a dead short then your damaging external power supply will burn pcb traces, transistors, diodes and other items with low resistance.

I don't get if there is a SHORT on the power rails that transistors and diodes look like low resistance, how so?

When your damaging external power supply causes a transistor or diode to conduct and become a low resistance.

How low a resistance can be measured with your multimeter? Will its very low voltage and current damage anything? Will its very low voltage cause a transistor or diode to conduct? Can you push its button to calibrate-away the resistance of its leads and probes? Why does your meter have this expensive very useful feature?

 

[j33pn]

Danny, you are describing thermal runaway, not quite the same as a short.

https://en.wikipedia.org/wiki/Thermal_runaway#Electrical_engineering

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And in your defense, Danny, I have heard of PCB manufacturers using high voltages to clear whisker shorts on PCB's that have failed electrical testing. I don't know if that's still a normal practice. Those boards are not populated. It would be pretty dicey trying something like that on a populated board.

 

[Audioguru]

Danny does not understand about simple electrical heating.

My daughter bought a new home but the electrical circuit breakers were not marked. How do I find out which breaker powers which outlet?
A lamp, a walkie-talkie (or running up and down stairs), a second person and flipping the breakers all day?

Simple. I made a power cord with a dead short on its end. I plugged it in and POP goes a breaker for me to mark.
Why didn't the shorted power cord catch on fire when it had 15A in it? Why did the breaker disconnect the short?

 

[danny davis]

thermal runaway, not quite the same as a short.

What do u mean it's not the same as a short?

Why do u think it's thermal runaway?

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Why didn't the shorted power cord catch on fire when it had 15A in it? Why did the breaker disconnect the short?

If no break or if the break box was broken , your daughters shack would have went up in flames

My daughter bought a new home but the electrical circuit breakers were not marked.

Sounds like a seedy part of town , be a real man and dad and buy your daughter a nice house not a shack in the seedy part of town