23

Because of a broken power button on the front of my computer I've had to take the cables connected to the button and create a sort of "hotwire" setup to start my computer. This is done by simply pressing the ground and power switch cables against each other, and having those cables loosly taped to the outside of my case.

Could these cables contain any dangerous amount of electricity, if a person or cat would accidentally touch them?

  • 5
    Dangerous and electricity depends on a whole host of things. The voltages on those wires should be at most 12V which is classed as a "safe" low voltage. If you simply touch a 12V supply you should be fine. It would be difficult to kill yourself without really trying, but having them exposed and loose could cause them to short against the case, could cause other problems and potentially charge the case up with enough current that if you were earthed somewhere else it could give you a sharp electric shock. It might only "buzz" you, but could harm smaller creatures. Just replace the switch. – Mokubai Mar 6 '18 at 22:11
  • 8
    If a simple "touch" starts the computer, those are not the main power cables, but rather the "on/off" button wires and run on 5V. A chance contact may initiate a computer shutdown, obviously. I'd go for a new switch. – LSerni Mar 6 '18 at 22:12
  • Thanks alot for the answers! (yes it's the "on/off" button wires I'm talking about). Simply getting a new switch button then. – JazzMaster Mar 6 '18 at 22:22
  • 40
    There is no answer to any question beginning with "How much electricity is there in ...". ;-) – Mike Waters Mar 6 '18 at 23:36
  • 1
    You should also be considering the risks to the machine from you - it's not obvious that the switch connectors have any protection against a discharge of static. – Toby Speight Mar 7 '18 at 13:57
51

The power switch provides an active low input to a logic gate within the power supply circuit.

enter image description here

This provides a low current logic low (ground) gate pulse to a BJT transistor biased to an isolated 5V source with the maximum current likely much less than 10mA. This portion of the circuit is also electrically isolated from the rest of the power supply circuit as can be seen in the diagram.

So to answer your question, No there are no dangerous voltages present.

Source: https://en.wikipedia.org/wiki/Power_supply_unit_(computer)

  • 38
    On the really unlikely chance the person has a really old power AT supply, the switch will be the same voltage as what you have on the wall. Don't mess with those. – Zoredache Mar 7 '18 at 1:15
  • 7
    The power button is not connected directly to the PSU's PS_ON input. It goes through more logic on the motherboard (otherwise, the power switch would have to be held down to keep the power on). Having the wires dangling can be problematic, because it's possible that one of the wires is a voltage rail, and shorting it to the chassis could damage something on the motherboard. – Fake Name Mar 7 '18 at 2:13
  • 5
    As @FakeName noted, the above answer is for switches on power supply units, not the switch on the front of the computer. +1 for interesting related information, though this answer doesn't seem to address the question as-asked. – Nat Mar 7 '18 at 13:46
  • 1
    @FakeName I'd expect one pin to be 0v / Chassis / Ground, and the other to be pulled high to 3.3v / 5v, possibly by as small as ~1k - shorting to the chassis will likely achieve the same result as shorting the two wires. It's quite unlikely that the logic will be the other way up (pulled down, and fixed high). – Attie Mar 7 '18 at 16:27
  • 1
    @ChrisH XP for Point of Sale systems (XP-POS for short, what's in a name) still has support until some time in 2019. So you may still encounter XP "in the wild". – Tonny Mar 7 '18 at 16:32
15

The wires should only have low voltages on them (<= 12V), so they're safe to touch.

However, you should be careful, because if one of the wires is connected to the standby voltage rail, shorting it to the chassis could potentially damage the motherboard.

2

The wires are safe to touch as they are powered from a 3.3V chip on the motherboard, and current is limited by a resistor.

However, if you carry an electrostatic charge, and you touch the wire, you will cause an electrostatic discharge (ESD). This is harmless for you, but it can zap the integrated circuits at the other end of the wire if they are not protected.

Normally, the manufacturer would have taken appropriate measures to ensure all input/outputs are protected against ESD, which begs the question: did they consider it worthy to protect this particular IO since it is normally not an externally accessible IO, but rather an internal wiring?

For example, looking at this motherboard schematic (pages 23-24), the power button signal is filtered by a 100nF cap which should be fine, however the RESET button signal goes straight to a IC pin without any protection.

While these chips include ESD protection diodes, such diodes are sized to protect against ESD events during fabrication, which are quite limited in charge as people manipulating chips are expected to take proper precautions. More robust protection diodes on every pin would use more silicon area and make the ICs more expensive, while providing no benefit unless the pin is actually accessible on the finished product and can be zapped by the user.

Therefore, your arrangement carries a risk of zapping the motherboard. It will probbaly be safe if the floor is tile and you are wearing cotton. However, if the floor is insulating, and you're wearing wool and sneakers, and the weather is dry, when touching metallic objects your fingertip can turn into a pretty impressive lightning gun!

  • That schematic is for the signaling between mobo and power supply, not for the front-panel switch. I'd assume it's ESD-protected on a PC because the mobo just has headers for the user to attach headers from the case wiring. So it's expected that semi-skilled human end-users will have their un-grounded hands near those pins. And BTW, a multimeter on my Asus Z170 board shows 3.36V across the front-panel power switch when the computer's on. I didn't check for source resistance or anything because I don't want to power down or standby my desktop. – Peter Cordes Mar 8 '18 at 6:12
  • 1
    @PeterCordes Thanks for the clarification, I've looked at a mobo schematic and updated the post. ESD protection is actually a lot less than I thought. – peufeu Mar 8 '18 at 9:58
2

Other answers have explained that the voltage difference between these two wires is almost certainly very low, and has next to no chance of harming you when you touch both of them - the more likely consequence is that you might harm your computer through static discharge.

There is another consideration for the electrical safety of low-voltage circuits though, and that is the voltage difference between these wires and ground (earth).

Your PC's power supply should either have its ground (0V) rail connected to mains ground (earth), or it should have adequate isolation between the mains side and the low voltage side making it a Class II device, and in either case it should be safe to touch the low voltage side even if your body is also touching something grounded. However if there's anything wrong, for example:

  • the power supply is faulty
  • the power supply is cheaply made and substandard
  • the power supply is not correctly connected to mains (e.g. if it needs to be grounded but is connected via a 2-core cable or 2-pin plug with no ground)
  • there is a fault with your outlet or your house wiring

then the result could be a dangerous voltage, relative to ground, on the low-voltage side of the PC power supply. Just in case of one of these, it's safer not to touch the wires.

1

The logic voltage on your motherboard is probably ~12V or less, as said in @FakeName's answer; I'd guess ~5V in most cases. But, why guess?

If you're going to be messing with electronics, seems like you should have a multimeter that could directly test it. It'd seem safer to actually check the voltage than just assume.

As for the danger of electricity, in general:

  • A power source can assert a voltage differential (e.g., 5V).

  • How much electricity comes from that voltage differential depends on the effective resistance between the terminals.

  • The lower the resistance, the more current will flow due to the same voltage differential.

  • Most power sources are limited in how much current they deliver.

So, how much voltage/current is dangerous?

To provide an arguably alarmist warning from @RussellMcMahon's answer (currently at +24 votes), while noting that this observation is extremely unlikely to apply to the current scenario:

FACT:

  • 12 VDC CAN kill and has killed people.

  • While 12V is almost always safe, worst case situations can and have lead to death.

  • Mechanism may be ventricular fibrillation BUT paralysis of the respiratory muscles occurs at about 20% of the current needed to introduce fibrillation.

  • See discussion and references at the end of this answer.

@Neelix's answer above speculated that the equivalent power switch on a PSU may be current-limited to <10mA. 10mA of direct current would presumably be perceivable-but-not-painful, according to the table from @OliGlaser's answer here:

Current safety table

So, it's probably not an immediate hazard upon human contact in most reasonably foreseeable cases, but it's still probably something to measure and be careful about anyway.

  • 6
    definitely alarmist and misleading in this context, omitting details on the conditions when such low voltage might be dangerous. if the mains powered computer and its user are submerged in salt water, the real danger does not come from the silly 12 or 5 volts... – szulat Mar 7 '18 at 15:15
  • 2
    The odds of 12V killing you are absurdly low. I'm sure that falling coconuts kill more people every year than 12V does. – J... Mar 7 '18 at 15:35
  • 1
    Guess I have to admit that I'm a tad phobic of telling people that something can't be dangerous. I mean, I have to agree with the majority opinion on this question that it's absurdly unlikely that this proposed power-switch hot-wiring presents a real danger, but I find myself unable to say that there's no danger. – Nat Mar 7 '18 at 15:43
  • 1
    "while noting that this observation is extremely unlikely to apply to the current scenario:" without explanation of why, I'm not sure why you wrote it anyway. – user366447 Mar 7 '18 at 17:05
  • 2
    "Can kill", yes. However you need two electrodes inserted fully through the skin on either side of the heart to do so, which is not something that happens by accident. For the OP's question about anyone accidentally touching them, they are unequivocally safe and categorically cannot be injured. Note that licking an exposed 12V will definitely hurt, but it is not harmful to life. And as has previously been noted, there's a resistor limiting current anyway. – Graham Mar 8 '18 at 12:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.