3 ms·

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 fo

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/
- Kirby64 8y ago
  EE/CE here: In order if your questions... 1. Assuming you hooked the 5V to the USB 5V and the GND to the USB GND pins, nothing bad would happen. 2. Nothing would get fried 3. Plugged in devices would respond according to how they're designed. Most cell phones will limit current and only draw 500mA if they don't have any other signalling to test otherwise. Some will slowly ramp current until their own internal limits if voltage drop isn't too bad. Some 'dumb' devices will pull as much current as they 'want' or need. You won't be able to push 30A into any device because there are almost no USB devices that 'want' that much current. Even really poor behaving devices (outside of outright broken ones) will probably only draw 3-5A max. Technically this is non-compliant, since there are defined USB specifications for how USB ports should be connected. If you wanted to make a 5V 'mega USB' charger, here's all you'd need to do: Take your 5V, many amp power supply and connect all USB-A ports you want to it, with a constraint that number of USB ports should be total amperage divided by 2.5. E.g. if you had a 10A supply, only use 4 USB ports. Short the D+ and D- pins together on the connector. 5V should be connected to your large 5V rail, GND should be connected to ground. That's it. You've just made a 'DCP' (Dedicated charging port) device. By USB standards any device connected can pull up to 1.5A; in practice they'll usually pull 2.1A or more if they can.
  - fb03 8y ago
    THANK YOU! that's what i needed to know. knowing shit about electronics, i was worried there would be 'too much current' and that i could fry something :-) have a good one, man o/

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/

Kirby64 8y ago
EE/CE here: In order if your questions... 1. Assuming you hooked the 5V to the USB 5V and the GND to the USB GND pins, nothing bad would happen. 2. Nothing would get fried 3. Plugged in devices would respond according to how they're designed. Most cell phones will limit current and only draw 500mA if they don't have any other signalling to test otherwise. Some will slowly ramp current until their own internal limits if voltage drop isn't too bad. Some 'dumb' devices will pull as much current as they 'want' or need. You won't be able to push 30A into any device because there are almost no USB devices that 'want' that much current. Even really poor behaving devices (outside of outright broken ones) will probably only draw 3-5A max. Technically this is non-compliant, since there are defined USB specifications for how USB ports should be connected. If you wanted to make a 5V 'mega USB' charger, here's all you'd need to do: Take your 5V, many amp power supply and connect all USB-A ports you want to it, with a constraint that number of USB ports should be total amperage divided by 2.5. E.g. if you had a 10A supply, only use 4 USB ports. Short the D+ and D- pins together on the connector. 5V should be connected to your large 5V rail, GND should be connected to ground. That's it. You've just made a 'DCP' (Dedicated charging port) device. By USB standards any device connected can pull up to 1.5A; in practice they'll usually pull 2.1A or more if they can.
- fb03 8y ago
  THANK YOU! that's what i needed to know. knowing shit about electronics, i was worried there would be 'too much current' and that i could fry something :-) have a good one, man o/