live circuit, and multimeter, noob questions

[andybgr]

I know that what I am about to ask is a noob question, because i have no experience what so ever in electronics, I am trying to learn myself through youtube videos and tutorials etc.

I've seen many people in youtube videos, they take off completely a laptop motherboard and use the multimeter to check the voltages so they can identify the problem.

anyway, when they plug the motherboard to the powercord, and it turns on. they touch the motherboard with their bare hands so easily with no worries. so I have 2 questions about this,

1) can touching the motherboard on a live circuit damage the motherboard components because of static electricity on hands?

2) is it safe for you to touch a live motherboard and not get electrocuted? I know it transforms into DC and DC is safe, but is it in this case?

and... 3) since I am just learning about this stuff, I have no experience with a multimeter, in order to count voltage with a multimeter on a motherboard, the motherboard has to be powered on right?

I know that these answers can be obvious, and maybe I know the answers already, or I think I do, and since I am learning myself about this, yes I am a noob on this, so I just need to get these questions out of my head as easy as they are, just need to get my head straight.

thanks

EDIT: I have just seen soldering a live circuit motherboard, can it be done? or I did not see clearly?

 

[zyh1987]

1.generally it's safe, these guys usually grounds themself before touching the electronics. i won't worry too much about static discharge.
2. it's not recommended, but i don't see the harms in that. wear gloves to prevent debris and oil get onto the motherboard.
3. it depends on what you are doing with the multimeter, if you are measuring voltage, then yes, it has to be turned.
if you are measuring resistance/capacitance. then no, you don't need to.

i still recommended soldering on a circuit with power off. you can easily short something and fry the board if you are not careful.

 

[grimfox]

Touching a live motherboard is not a good idea. Low DC voltage is safe because it can't overcome the resistance of your skin in most circumstances. Generally 12VDC or lower is safe. But there are exceptions, wet/moist skin, damaged skin (cuts scrapes). Voltages higher than 12V have the ability to shock you. A motherboard in a laptop does have AC voltage on it which can really hurt you. The battery charge circuit may have high voltage DC as well depending on the battery design.

The static in your hands is dangerous to the components on the motherboard. When handling a PCB touch the edges first away from components to discharge any static and maintain contact to prevent the charge from building up again. Static discharges can happen below your ability to feel them and still damage components.

The motherboard must be powered on to measure most voltages. However, certain parts of the motherboard may maintain a voltage after shutting off and being disconnected. Probing a laptop motherboard is very much like diving into the ocean to learn to swim. I would visit sparkfun.com and check out some of their basic circuitry primers to understand what you are doing. There is a "learn" tab at the top of their website.

Edit: Soldering a live circuit can be done. There is no reason to do that. Other than a useless sense of instant gratification. It can also be dangerous. If you short something while soldering, easy to do without much experience, you can damage the board. Shut it down, disconnect the power, do your work, check your work thoroughly, reconnect the board and then power it back on.

 

[nobspls]

You need to know what you are doing is going be "KEY" to this matter.

Static discharge can certainly destroy your computer, but the odds of static discharge happening is very low unless you deliberately try to charge yourself up, i.e. stacking the deck. But that is not say, in cold dry winter climate, wearing wool and fleece hoodies and such, will naturally stack that deck, and just dragging your feet over the carpet will probably build up plenty of static over time. Short of buying the static ESB protection straps, for your hand or feet, on the things you can do discharge static is to ground yourself periodically. The steel case of PC (assuming it is properly grounded with 3 prong AC power cable connected to the wall) will actually drain your static charge. It won't be pleasant feeling, the static will discharge, so touch the bare metal of the case before you touch anything else is usually a good practice. I've been doing this when I am lazy to get the ESD strap since the days of the 8086 when I was first taught as kid how to mess with a PC by old time hacks at the repair shop.

As for getting electrocuted, the odds of that happening is even lower than ESD static discharge damage. Most PC motherboards should not have any high voltage components, but this can not be guaranteed without knowing more about a specific board. If you were messing with an old CRT board you better bet you can get yourself a nasty shock and perhaps even an hospital visit. The important thing is you need to know how distinguish one board from another. But I've even accidentally got jolted by 120V AC from the wall and I am alive to tell you it is extremely not pleasant, but you'll probably still live, assuming you don't have a pacemaker or something that gets blown up by it. The odds are if there is high enough voltage on the board to jolt you, then the short you created might very well destroy something on the board. There is place that I worked for, and they made these little home router boards, but you better be careful because there are caps on there that can get charged up to 90V. That jolt stings.

The biggest challenge of using the multimeter is getting the probes to actually contact the right pins, leads, etc. Many probes are often too big and the pins and leads are too small or too tightly spaced, and you would actually be shorting pins and potentially wrecking stuff. Extreme care must be taken to make sure you don't force the probe into the wrong places, or there may be many tears. I know because I've been there done that. Also double check the polarity, basically the measure twice cut once rule.

 

[andybgr]

thank you guys, for your answers. they are really helpful and understandable to me. So far I thought I knew a lot about computers, and suddenly I felt like a noob because this is a new area for me which I am really interested to keep on going and learning more stuff about this

grimfox, that sparkfun.com is very good, I am already into it, thanks