4 ms·

18V, 5A would be a shock hazard.

XMPPwocky 8y ago
If you stick it in your mouth or something, maybe. Or if you're soaking wet and have a terrible heart. Otherwise, nobody's getting hurt off 18v.
gsich 8y ago
No. Skin resistance is at least around 100k Ohm at that voltage.
semi-extrinsic 8y ago
DC voltages below 50 V are considered "safe" in normal indoor conditions. It's often referred to as "extra low voltage", as opposed to "low voltage" (50 V - 1000 V) and "high voltage" ( > 1000 V). If you want to verify the safety of 18 V: ever touch both poles of a 9V battery with your fingers? You can't feel anything. Go ahead and try two in series, you still won't feel it.
- naikrovek 8y ago
  Really? I can feel 4 volts between my finger and thumb. Isn't painful or anything, but I can tell when a lipo battery is charged vs. discharged.
  - Maskawanian 8y ago
    Yes, here is a good video about this: https://www.youtube.com/watch?v=ZxBF7WC0TQk https://www.youtube.com/watch?v=ZxBF7WC0TQk
  - xxs 8y ago
    You can feel lower voltages if you fingers are injured, or if you... lick them. As kids (6-9y) we used to test the voltages of 3R12 (4.5v) by touching the poles with tongue, a sharp bites - the battery was good.
- dbuder 8y ago
  42V is the limit for reliable non lethal shocks.
userbinator 8y ago
That's roughly the voltage that laptop AC adapters output, and they do so continuously without needing any special negotiation/signaling. (Even those from manufacturers that try to lock this down using an EEPROM, e.g. Dell, still output their voltage like the others, but the laptop might not want to charge from an "unofficial" power source.)
basementcat 8y ago
The shock hazard is the 5A, not necessarily the 18V.
- XMPPwocky 8y ago
  Well, no. If 5A flows through you from hand-to-hand, sustained for longer than an instant, you're probably dead. But I've touched live power supply terminals for something rated for 20A without a second thought. And if there's a 10kV differential between your left and your right hand, sustained for longer than an instant... You're probably dead. But I've touched a 10kV power supply (not on purpose, this one), and...still here. So! What gives? Well. Place a 0.1 ohm resistor across that power supply. 18V across 0.1 ohms will produce a current of 180A. This will dissipate more than 3kW in the resistor, and it will very rapidly disassemble itself. But that doesn't happen! Okay, now put a 10 megaohm resistor across that power supply. It's 5A, right? So 5A across 1 megaohm will produce a voltage of 5000kV. This will dissipate 25 megawatts, and destroy the resistor even faster. This also doesn't happen! In fact, what will happen in the first case is that 5A, not 180A flows (assuming the PSU doesn't detect the apparent short and shut down entirely.). But Ohm's law says that if 18V is placed across a 0.1 ohm resistor, 180A must flow! Ah, but 18V isn't placed across the resistor. Instead, as current draw approaches the 5A limit, the power supply starts dropping the voltage. No more than 5A will flow, even if that means the voltage must be decreased. And 5A won't flow through the 1 megaohm resistor, either. Instead, no more than 18V will be placed across the load, even if that means the current must decrease. So, the power supply won't kill you UNLESS your skin resistance is low enough that 18V causes a lethal (very roughly roughly 1A) current to flow through you. Luckily, humans have a fairly high skin resistance, at least when we're dry, and 18V is fine. It's not the volts that kill you, it's the amps which the volts are directly related to. 10kV across your body, sustained for a bit, will kill you dead, modulo some sort of miracle. 10kV at 1mA across your body ... is impossible. If you touch a 10kV, 1mA supply- the voltage will drop so that only 1mA flows. It's not the voltage limit on the power supply that kills you. It's not the current limit on the power supply that kills you. Both need to be high enough to do you in.
  - fb03 8y ago
    Extremely interesting explanation. Thank you man. I would like to peruse your electronic skills for another quite unrelated question that has been hovering my head for quite a while. I know that normal USB (1,2) devices charge using the provided 5V rail off the USB connection. And I've read somewhere else that coming from a PC motherboard or other regulated (standardized) USB outlet, the port will provide about 500mA. What would happen if i took a ATX PSU and rigged usb ports directly (at the correct power pins) to the psu's 5V rails? Would I fry any devices plugged in? Or.. Would any plugged devices limit their intake and I'd be left with a 'super quick charger' which can charge any device at the maximum amount of power that the device can receive power? those 5V rails usually output at 30A+ and stuff. thank you again, and hope you can shed some light into this long doubt of mine (I always 'dreamed' of creating some atx-frankstein psu which i could use for electronic experiments but also fit some usb ports in it). o/
xxs 8y ago
Laptops tend to be 19V and you can hold them with both hands wet w/o any risk (it's DC, so even better). If you lick them it'd be quite unpleasant but that's that. Voltage below (roughly) 36 are safe.

If you stick it in your mouth or something, maybe. Or if you're soaking wet and have a terrible heart. Otherwise, nobody's getting hurt off 18v.

gsich 8y ago

No. Skin resistance is at least around 100k Ohm at that voltage.

semi-extrinsic 8y ago

DC voltages below 50 V are considered "safe" in normal indoor conditions. It's often referred to as "extra low voltage", as opposed to "low voltage" (50 V - 1000 V) and "high voltage" ( > 1000 V). If you want to verify the safety of 18 V: ever touch both poles of a 9V battery with your fingers? You can't feel anything. Go ahead and try two in series, you still won't feel it.

naikrovek 8y ago
Really? I can feel 4 volts between my finger and thumb. Isn't painful or anything, but I can tell when a lipo battery is charged vs. discharged.
- Maskawanian 8y ago
  Yes, here is a good video about this: https://www.youtube.com/watch?v=ZxBF7WC0TQk https://www.youtube.com/watch?v=ZxBF7WC0TQk
- xxs 8y ago
  You can feel lower voltages if you fingers are injured, or if you... lick them. As kids (6-9y) we used to test the voltages of 3R12 (4.5v) by touching the poles with tongue, a sharp bites - the battery was good.
dbuder 8y ago
42V is the limit for reliable non lethal shocks.

userbinator 8y ago

That's roughly the voltage that laptop AC adapters output, and they do so continuously without needing any special negotiation/signaling. (Even those from manufacturers that try to lock this down using an EEPROM, e.g. Dell, still output their voltage like the others, but the laptop might not want to charge from an "unofficial" power source.)

basementcat 8y ago

The shock hazard is the 5A, not necessarily the 18V.

XMPPwocky 8y ago
Well, no. If 5A flows through you from hand-to-hand, sustained for longer than an instant, you're probably dead. But I've touched live power supply terminals for something rated for 20A without a second thought. And if there's a 10kV differential between your left and your right hand, sustained for longer than an instant... You're probably dead. But I've touched a 10kV power supply (not on purpose, this one), and...still here. So! What gives? Well. Place a 0.1 ohm resistor across that power supply. 18V across 0.1 ohms will produce a current of 180A. This will dissipate more than 3kW in the resistor, and it will very rapidly disassemble itself. But that doesn't happen! Okay, now put a 10 megaohm resistor across that power supply. It's 5A, right? So 5A across 1 megaohm will produce a voltage of 5000kV. This will dissipate 25 megawatts, and destroy the resistor even faster. This also doesn't happen! In fact, what will happen in the first case is that 5A, not 180A flows (assuming the PSU doesn't detect the apparent short and shut down entirely.). But Ohm's law says that if 18V is placed across a 0.1 ohm resistor, 180A must flow! Ah, but 18V isn't placed across the resistor. Instead, as current draw approaches the 5A limit, the power supply starts dropping the voltage. No more than 5A will flow, even if that means the voltage must be decreased. And 5A won't flow through the 1 megaohm resistor, either. Instead, no more than 18V will be placed across the load, even if that means the current must decrease. So, the power supply won't kill you UNLESS your skin resistance is low enough that 18V causes a lethal (very roughly roughly 1A) current to flow through you. Luckily, humans have a fairly high skin resistance, at least when we're dry, and 18V is fine. It's not the volts that kill you, it's the amps which the volts are directly related to. 10kV across your body, sustained for a bit, will kill you dead, modulo some sort of miracle. 10kV at 1mA across your body ... is impossible. If you touch a 10kV, 1mA supply- the voltage will drop so that only 1mA flows. It's not the voltage limit on the power supply that kills you. It's not the current limit on the power supply that kills you. Both need to be high enough to do you in.
- fb03 8y ago
  Extremely interesting explanation. Thank you man. I would like to peruse your electronic skills for another quite unrelated question that has been hovering my head for quite a while. I know that normal USB (1,2) devices charge using the provided 5V rail off the USB connection. And I've read somewhere else that coming from a PC motherboard or other regulated (standardized) USB outlet, the port will provide about 500mA. What would happen if i took a ATX PSU and rigged usb ports directly (at the correct power pins) to the psu's 5V rails? Would I fry any devices plugged in? Or.. Would any plugged devices limit their intake and I'd be left with a 'super quick charger' which can charge any device at the maximum amount of power that the device can receive power? those 5V rails usually output at 30A+ and stuff. thank you again, and hope you can shed some light into this long doubt of mine (I always 'dreamed' of creating some atx-frankstein psu which i could use for electronic experiments but also fit some usb ports in it). o/

xxs 8y ago

Laptops tend to be 19V and you can hold them with both hands wet w/o any risk (it's DC, so even better). If you lick them it'd be quite unpleasant but that's that. Voltage below (roughly) 36 are safe.