Guan’s blog


Vox on AC/DC

25 Jan 2016

(Cross-posted from Ello with changes.)

Vox has a video explainer on why your laptop charger gets so hot. They make the interesting claim that it’s because of the conversion from AC to DC power.

In fact, the rectification itself is relatively efficient. But after you have rectified, you are left with a quite high (DC) voltages. Then there is circuitry that steps down the 170V (peak) DC voltage (in the US) to the roughly 18V that a Mac laptop expects, mainly for charging its batteries. It is this DC-to-DC conversion that accounts for most of the inefficiency in a laptop power supply.

The main reason we don’t get DC power into our houses is that a hundred years ago, it was hard or expensive to change the voltage of a DC power line, while AC voltages can be easily stepped up or down with transformers. A transformer sits near your (American) house to step down the 1.5kV to the 120/240V that an American house expects. Larger transformers sit in various places in the grid to convert 115kV to various lower voltages, and finally to the 1.5kV that runs down residential streets.

That is not to say that the traditional account of the Edison–Tesla rivalry is entirely incorrect, although Vox’s version is a bit simplified. However, even if DC had won some short-term victories, the technology of the day simply wasn’t adequate for widespread DC deployment, mainly due to boosting and bucking voltages, not AC–DC conversion.

Today high-voltage direct current is much more feasible with thyristors and integrated gate bipolar transistors, and is often used for undersea connections and long distance power lines from hydroelectric power plants. It is particularly useful for undersea connections, with AC–DC converters on each end, because it can be used to make connections between electrical grids that operate at different frequencies or phases. It is also useful for long distance because less conductor material can be used due to the skin effect, which limits how deep alternating current can penetrate copper wire. You also do not have to worry about capacitance as much, which is useful for both long-distance and undersea connections.

The real question that should be asked is: Why don’t we just get a lower voltage into our house? If the outlet had 18V for laptops, or perhaps 12V for a lot of other electronic devices or 5V for charging our phones, we wouldn’t need any voltage conversion. These lower voltages would cause problems for makers of higher power devices like hair dryers or ovens, but those could possibly be overcome, and who needs hair dryers in this new internet of things world. If there was a wall outlet with 18V, the laptop charger wouldn’t be necessary at all. (You can, in fact, get electrical outlets with 5V USB ports. They have an AC–DC(–DC) converter inside the outlet, and there is absolutely no way I have installed any because it would not be permissible for me to do so as I am not a licensed electrician in New York City, but hypothetically, if I had, I would think they are awesome and highly recommend them.)

Transmission voltages are not lower because lower voltage means higher current (for the same amount of power), which means more losses in the transmission lines. A 50 or 100 meter phone charging cord would be infeasible for this reason—there would be too much loss in the wire, or the cord would be infeasibly large. If we did receive DC from the electric company into our houses, it would probably be relatively high voltage DC, and we would still need inefficient DC-DC converters everywhere. Proposals for powering data centers with direct current usually call for 48VDC, with DC–DC converters at the point of load.

The voltage of overhead AC power lines is typically around 115kV and most large new HVDC lines are 800kV.

The video also makes the interesting claim that the United States has one of the best and most reliable power grids in the world? It is certainly physically and electrically large and has a huge amount of capital invested in it, which makes it a questionable proposition to replace it or adapt it to a new standard, but there are more blackouts and brownouts than residents of most wealthy countries would be satisfied with.

Also, the LED light bulb pictured in the video probably has a rectifier inside and runs on DC. Only very cheap LED lamps run directly off AC; I do own a set I bought at Home Depot that I installed as under cabinet lighting, but it’s relatively rare these days.

I know a MacBook charger works a little differently, has power factor correction, and you actually get 380VDC inside it. The point is that rectifiers are efficient. One amp times 1.4V, which is a worst case since most bridge rectifiers are based on Schottky diodes, means less than 2 watts dissipated as heat. It is also still the case that if your laptop did accept 170VDC, or 380VDC, the adapter would create much less heat. Despite what Vox claims, DC–DC conversion is the main source of efficiency gains, not AC–DC conversion.