Demystifying Electricity

The relationship between volts, amps, and watts–and how it relates to USB charging capabilities.
Demystifying Electricity

Understanding the Terms

In the realm of electricity, three units of measurement are essential to understanding the flow and consumption of energy: Volts, Amps, and Watts.

Understanding the terms

Understanding the Formula

This simple formula describes the relationship of electricity measurement units:

VOLTS x AMPS = WATTS

Let’s say that we have a 15A outlet operating in the US where the standard wall voltage is 120V. Using the formula, we’d see that the outlet can deliver up to 1800W [120V x 15A = 1,800W]. But, it’s not that simple. If we double the Amps in the equation from 15 to 30, one would assume we’d end up with 3,600W [120V x 30A = 3,600W], right? Wrong! While this works in theory, it won’t work in practice because: 

Caveat 1 = There are limits to how much voltage and current a wire can tolerate. 

Let’s take a USB phone charging cable as an example: the conductor (the metal wire inside the cable) is very small in diameter (about 0.5mm) because charging cables need to be light, compact, and inexpensive. Something this small can only tolerate a small amount of current, – you can’t force all the water in the Mississippi through a small little creek! If you try to force a high current through a small conductor, the conductor gets very hot and will eventually melt the plastic insulation and even the copper itself (see exactly what happens in this video). 

 So now we know that forcing too much current through a small conductor is a bad idea. What about doubling the voltage from 120V to 240V. This would increase watts to 3,600 [240V x 15A = 3,600W], right? Wrong again! This too, works only in theory and not in practice because:

Caveat 2 = Voltage has practical limits because as it increases, more insulation is required to protect people and equipment from damage. 

High voltage powerline


As voltage gets really high, electricity can actually jump through insulation and even air (as you can see in this video), so it’s kept intentionally low for safety reasons. 

Understanding the Relation to USB-C

The specifications for USB are controlled by an association called the USB Implementers Forum (USB-IF). Back in 1996, the association developed the first specifications for USB-A, which were:

  • 5V Maximum
  • 2.1A Maximum Current
  • Which equals an 11W Maximum [5V x 2.1A = 11W]

These specifications ensured that the cables designed by all the companies using the USB standard would be light, small, and inexpensive because of the relatively low voltage and amperage. The association eventually increased the maximum allowable current to 3 amps, resulting in a maximum power of 15 watts [5V x 3A = 15W], which was sufficient for the devices and applications of the time.

Fast-forward to 2014, after the engineers at the USB-IF association realized that USB was being used as the main method of charging devices and released specifications for USB-C with Power Delivery (PD) to meet the growing power demands of newer devices. With the requirement for a small, lightweight, and inexpensive cable still in place, the only way to increase power was to increase the voltage. 

USB-C (PD) version 2.0 and later allows for (5) different voltages: 5V, 9V, 12V, 15V, and 20V. (There are even higher voltages allowed starting with version 3.1 but we don’t yet make any products that go that high, so I won’t cover those). Except for large laptops using a special, higher current cable, the max current (amperage) remains at 3A:

Current USB-IF Specifications for USB-C (PD)

Based on this foundation, we now have the ability to understand the basic specifications for Docking Drawer’s USB-C (PD) outlets. Let’s see how this plays out in a testing scenario:

Docking Drawer Product Testing

Product Testing

By applying the equation we covered earlier [Volts X Amps = Watts], you can see above that the only way to get 65w is to operate at 20V. Anything lower than 20V cannot reach the 65w capacity because of the 3A limitation. (Wondering why the max current at 20V in the picture above is 3.25A? The answer is: I don’t know, but I guess they are cheating a little bit or there was a change in the USB-C standard to allow for slightly higher current.)

What these examples prove is that a device can’t take advantage of the max capacity unless it can operate at a higher voltage. Not all devices are capable of operating at 15V or 20V, but such specifications are difficult to find and typically aren’t published by the manufacturer. Look at the specifications for example A in the chart above (30 Watt Docking Drawer Blade Outlet), as we list them on our website (astute readers will notice that, our specs aren’t quite complete, as they’re missing the 12V,15V, and 20V specs, all of which do appear in the test analysis displayed above):

Specifications

One characteristic of all USB charging outlets is that they share the same DC power supply inside that provides the charging power:

Charging Power Testing

This means that if you connect a device capable of only 5V charging and a device capable of 5V, 9V, 15V, or 20V charging to the same USB outlet, the outlet will default to the unit with the lowest voltage. Along the same lines, power is shared between the two ports so if the outlet is rated at 30W, that means a total of 30W which is shared between the two ports. This illustrates what is probably the biggest advantage of the 65W outlet. Provided that both devices are capable of charging at a minimum of 9V, then:

The 65W outlet is capable of delivering over 30W to multiple devices for the fastest possible charging speeds.

Device Testing Examples

Device Testing Examples



 

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