Touching a PCB while its on?

[GagHalfrunt]

it might kill you, or you might kill it, or nothing would happen. only one way to find out.

It's like Schrödinger's cat. The wave function won't collapse until you actually do it, so at this point it kills you, you kill it and nothing happens all at once.

 

[SKORPI0]

In the context of computers, only a moron would think that this would hurt you or the part.

Everything outside of the PSU is 12v or less. Dry skin will not conduct given that kind of voltage. You can touch bare traces and nothing will happen.

So you would have no problem trying to "fix" a computer while it's turned on?
I'm not worried that the person would get shocked. More on making more damage on that computer than it already has if it wasn't turned off.

 

[BUTCH1]

This is ATOT. There is nothing so idiotic that someone here won't do it.

Yea, what was I thinking...

 

[Red Squirrel]

Depends on the voltage of the traces and the nature of the PCB.

Take a PSU for example, you don't want to be touching anything before the transformer. Possibly even a bit after.

Also if you are standing/sitting on something fairly insulated chances are if you touch only one trace you wont get a shock. But when you are testing for voltage using your fingers (not the smartest thing in first place) you should really put one finger on a known ground. You are better off having voltage go finger to finger than through your whole arm and down a leg if you do happen to be on something grounded.

 

[phucheneh]

So you would have no problem trying to "fix" a computer while it's turned on?
I'm not worried that the person would get shocked. More on making more damage on that computer than it already has if it wasn't turned off.

I would have no reason to attempt such. Just stating the facts of the matter- human flesh has a decent bit of resistance...particularly, the dry outer layer of skin. And Ohm's law is, well...law.

Assuming 12v DC, which is obviously a worst case scenario for the voltage level seen outside of a PSU (inside, of course, having some parts getting the obviously-dangerous 110v AC), a human fingertip is not any more likely to provide an alternate path for current flow than, say, the air that is surrounding all the parts. If two closely located traces/contacts/ect are not at risk of arcing through the air and shorting, they're also not at risk of arcing through your fingertips. Not with 12v.

Yes, it WILL take less voltage to arc through said finger than it will the air, but it doesn't change that 12v is simply not enough for either.

Sparks CAN be made with 12v, but it will be as two conductors (i.e. metal) are brought into/away from immediate contact. E.g. plug in a molex connector for a fan while the circuit is live, and you'll see some little arcs. But it won't hurt it. Said arcs are more likely to hurt an actual PCB component with IC's and whatnot on it, but even then, I don't see it. Worst case, you get a burnt contact that needs to be cleaned up.

The real danger (as far as damaged parts) is in something with lots of little connections, like the rows of contacts on an expansion card, where parts that are not supposed to make contact may do such for a brief moment during insertion/removal of the part.

Not saying 'sure, take your PC apart while running'...why would you? That's silly. Again, just stating the facts that answer the OP, since we're a bunch of semantical nerds.

 

[Red Squirrel]

You can get 24 volts inside a computer (there is a -12v rail) but even then that's not going to do much. If your fingers are wet you MIGHT feel it but even the 48v (actually 54v due to the rectifiers being set to the battery's float voltage) we work with here you don't feel with bare hands. It's funny how people are very scared of current but don't realize you still need voltage to drive that current.

This guy, while hilarious, actually does explain it well:

https://www.youtube.com/watch?v=XDf2nhfxVzg

 

[disappoint]

You can get 24 volts inside a computer (there is a -12v rail) but even then that's not going to do much. If your fingers are wet you MIGHT feel it but even the 48v (actually 54v due to the rectifiers being set to the battery's float voltage) we work with here you don't feel with bare hands. It's funny how people are very scared of current but don't realize you still need voltage to drive that current.

This guy, while hilarious, actually does explain it well:

https://www.youtube.com/watch?v=XDf2nhfxVzg

What about a 100,000 volt stun gun? How is that safe?

Heck they even have stun guns in the millions of volts now. Still non lethal though.

It's the current that kills you not the voltage. Yes you need high voltage to drive it though.

 

[Red Squirrel]

What about a 100,000 volt stun gun? How is that safe?

Heck they even have stun guns in the millions of volts now. Still non lethal though.

It's the current that kills you not the voltage. Yes you need high voltage to drive it though.

Well you need both. In the case of very high voltage stun guns and tasers they limit the current to a very low, safe level, but it still hurts as hell because the little current it takes for you to feel something is being driven through you. Meanwhile if you put your finger in a 15 amp socket you're not actually getting 15 amps through you, you're getting a very tiny amount of amps but if you double the voltage you'll get double those amps due to ohms law.

So really you are safer around very high current low voltage systems than vise versa. I feel more comfortable working near huge batteries capable of dumping 10,000 amps than in a live electrical panel. Though, the batteries have a bigger danger than getting zapped: arc flash. You drop a wrench on the terminals and you are history. I cringe a little when I see somebody put an aluminum ladder near or against the battery rack here. Both situations require respect.

 

[phucheneh]

I guess it's really just not even worth mentioning Ohm's law. The people who don't understand it...will continue to not understand it.

A taser is not an appropriate analogy because the current is artificially limited. Were the the current AVAILABLE, yes, you would die if the electricity went through the right place. As in, your heart, or possibly others. But if both electrodes just touch, say, an arm...pretty much no way to kill you with electricity short of being horrifically burned to death. This is how people can survive lightning strikes...current takes a path that is not across their vitals.

OTOH, the kind of example that SHOULD be looked at when dispelling the thoughtless 'it's the amps that gets ya!' statement...look at a car battery. Capable of supplying HUNDREDS of amperes. And far less than 1a can kill you.

Lick your fingers and grab both posts of said car battery, providing a direct path for current across your heart. Do you die? Nope. At most, you feel a slight tingle, because your body, even with wet hands, is presenting a resistance of hundreds of thousands of ohms.

 

[disappoint]

Well you need both. In the case of very high voltage stun guns and tasers they limit the current to a very low, safe level, but it still hurts as hell because the little current it takes for you to feel something is being driven through you. Meanwhile if you put your finger in a 15 amp socket you're not actually getting 15 amps through you, you're getting a very tiny amount of amps but if you double the voltage you'll get double those amps due to ohms law.

So really you are safer around very high current low voltage systems than vise versa. I feel more comfortable working near huge batteries capable of dumping 10,000 amps than in a live electrical panel. Though, the batteries have a bigger danger than getting zapped: arc flash. You drop a wrench on the terminals and you are history. I cringe a little when I see somebody put an aluminum ladder near or against the battery rack here.

Yes. Stun guns have current limiting resistors in them. It's both voltage and current that can be lethal. I wonder who started the "it's the current that kills and not the voltage" rumor? Like most soundbites too much information is left out. DANGER! RISK OF MISINFORMATION! DO NOT TOUCH! Leave the soundbites to the politicians and lamestream media.

 

[disappoint]

I guess it's really just not even worth mentioning Ohm's law. The people who don't understand it...will continue to not understand it.

People can be taught to understand things they don't currently understand.

A taser is not an appropriate analogy because the current is artificially limited.

I wasn't making an analogy. I was making a point. The point was that high voltage alone isn't necessarily lethal. Just as you pointed out that high current alone isn't either with your car battery capable of hundreds of amps example.

But artificially limited? Are your resistors all natural or something? Do you grow your resistors in your garden and pick them off a plant or a tree? A bush maybe, or a vine?

 

[Red Squirrel]

Tasers use only 100% organic resistors, as opposed to the GMO ones.

Think what he meant is the taser actually has circuitry that hard limits the current, as opposed to relying on the battery's capacity.

 

[Matt1970]

Define "electrocuted"

The traces on the board, or the metal bits/traces at the back.



Receiving an electric shock of some kind.

Electrocution is death. Anything less is just a shock.

 

[NoTine42]

I once worked on some Printronix line printers that ran 120v AC in some PCB's....it wasnt an ideal, reliable setup.

 

[TurtleCrusher]

I've been shocked by 2500v and a very low amount of current. A-ok. Wiring a house and accidentally touched a hot 20 amp circuit and might have had an insurance policy payout for my wife if I had latched on. Current kills, not voltage.

And electrocuted means death by electricity. Noobs here need to know that he means shocked.

 

[Maximilian]

Okay so dry skin dosent offer much of a conductive path. But what if someone licked the traces on the motherboard?

Also! Are these traces actually on the surface of the board? Or are the conductive parts buried under a few layers of plastic (or whatever motherboards are made of) and the traces we see is just a result of the manufacturing process? Or is that all wrong...

 

[PottedMeat]

Okay so dry skin dosent offer much of a conductive path. But what if someone licked the traces on the motherboard?

Also! Are these traces actually on the surface of the board? Or are the conductive parts buried under a few layers of plastic (or whatever motherboards are made of) and the traces we see is just a result of the manufacturing process? Or is that all wrong...

alright where the fuck is this going?


the traces are usually protected with a conformal coating against moisture/damage. but there may be exposed test points ( gold circles ) on the traces

 

[BUTCH1]

Okay so dry skin dosent offer much of a conductive path. But what if someone licked the traces on the motherboard?

Also! Are these traces actually on the surface of the board? Or are the conductive parts buried under a few layers of plastic (or whatever motherboards are made of) and the traces we see is just a result of the manufacturing process? Or is that all wrong...

Really, "lick your motherboard", are you in love with your rig and want to pleasure it?, I think it would prefer a good session with a can of canned air to blow out dust bunnies..

 

[Matt1970]

I've been shocked by 2500v and a very low amount of current. A-ok. Wiring a house and accidentally touched a hot 20 amp circuit and might have had an insurance policy payout for my wife if I had latched on. Current kills, not voltage.

And electrocuted means death by electricity. Noobs here need to know that he means shocked.

I used to get hit by the 30K in TV and monitors. Damn that would hurt, but it wouldn't kill you.

 

[Matt1970]

Okay so dry skin dosent offer much of a conductive path. But what if someone licked the traces on the motherboard?

Also! Are these traces actually on the surface of the board? Or are the conductive parts buried under a few layers of plastic (or whatever motherboards are made of) and the traces we see is just a result of the manufacturing process? Or is that all wrong...

I guess it depends of where you are touching. Touch the power coming in to the power supply and you have the full potential of the house circuit. Kill you? Probably not, but it's a hell of a time to find out you have a heart condition. Our rule was if you are messing with the higher voltages that you stick your other hand in your back pocket so as to not create a path that leaves your other hand and consequentially goes across your heart.

 

[mmntech]

Caps can pack quite a nasty punch when they're charged up. Probably won't kill you in smaller applications, but it will hurt like a mofo. Electricity is one of those things that deserves a healthy respect.

 

[Eric1987]

Only a moron would touch a PCB while it's on. Is there a reason why device can't be turned off? :hmm:

You're a moron for thinking that.

 

[Red Squirrel]

Okay so dry skin dosent offer much of a conductive path. But what if someone licked the traces on the motherboard?

Also! Are these traces actually on the surface of the board? Or are the conductive parts buried under a few layers of plastic (or whatever motherboards are made of) and the traces we see is just a result of the manufacturing process? Or is that all wrong...

https://www.youtube.com/watch?v=XDf2nhfxVzg :biggrin:

Similar subject: https://www.youtube.com/watch?v=hp97GjuULX8

It wont kill you, but I don't recommend it. Also some people have a different threshold for feeling voltage. For example the 9 volt battery trick does not work for me, if I lick it, I don't feel a thing whether it's charged or not.

Was experimenting with lot of 9 volt batteries once to see how many it takes before I feel a shock. I had like 12 of em hooked up together and still was not feeling anything. Then I licked my finger... that changed everything. OUCHY!

I can also confirm that 12 volt fans do not like 100 volts. :awe:

 

[Red Squirrel]

Caps can pack quite a nasty punch when they're charged up. Probably won't kill you in smaller applications, but it will hurt like a mofo. Electricity is one of those things that deserves a healthy respect.

They make a nice light show too. :biggrin:

 

[gorcorps]

The traces on the board, or the metal bits/traces at the back.



Receiving an electric shock of some kind.

That's not what electrocuted means... to be electrocuted means to DIE of electric shock.