# Usb output voltage and amperage relationship

### What is the power output of a USB port? - Super User

There are now four USB specifications — USB , , , and — in of actual current (milliamps or mA), there are three kinds of USB port. The USB 1.x and specifications provide a 5 V supply on a single wire to As power is equal to current times voltage, all you have to do is multiply . that its power capabilities are maxed out, it can refuse the connection. PC USB, 5 volts, amps, Watts the USB port is 1/4 the output of the iPad charger and 1/2 the output power of the iPhone charger.

If the city turns up the water pressure while you're doing this test, the flow past your thumb will increase.

The same thing happens electrically if you increase the voltage and this is what Ohm's law is about. You may be thinking about power supplies as if they were constant current supplies.

## Technical Details

There are such nthings but most are not. Constant current supplies are special purpose supplies that work by sensing the current they are supplying and actively adjusting their voltage as necessary to keep that current at the set value.

• Покупки по категориям
• Apple Footer
• USB Power Meter

A battery, your wall outlet, or a USB port power supply are more typical of constant voltage supplies. I'm waffling about the constant voltage part because simple power supplies don't actively maintain their output voltage; as they supply more current their voltage decreases. Usually only sightly as they supply low currents, but more, as they approach their maximum capability.

But a simple power supply such as a battery behaves more like a constant voltage power supply than it does like a constant current one, if the current draw is within its capability. And don't take "constant" too literally. A 5v USB supply might be a couple tenths above 5v with no load and drop a few tenths for typical loads, more for loads that draw more current, and in some cases, like my iMac's USB ports, may go to zero - disconnect - to protect themselves and the computer, if the load tries to draw too much current.

In terms of actual current milliamps or mAthere are three kinds of USB port dictated by the current specs: In the USB 1. The charging downstream and dedicated charging ports provide up to 1,mA 1. It also supports power draw of 1. USB-C is a different connector entirely.

### How USB Charging Works, or How to Avoid Blowing Up Your Smartphone - ExtremeTech

But there can also be older-style USB ports that support the 3. Can I blow up my USB device? There is a huge variance, then, between normal USB ports rated at mA, and dedicated charging ports, which range all the way up to 3,mA.

If you take a phone which came with a mA wall charger, and plug it into a 2,mA iPad charger, as an example, will it blow up? The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. Shortly thereafter, USB devices that implemented this spec started to arrive.

It might only work with old school, original mA USB 1. In some much older cases, USB devices can only be charged by computers with specific drivers installed, but this is now going back more than a decade.

The standard evolved quickly, and sits at version 4 at the time of this writing.