Why it is unsafe for common customers, to open up AC-connected appliances today.

Back when I was young, we also had appliances which connected to an AC power outlet in some way, but which operated at reduced, DC voltages. Those appliances used 60Hz transformers, to down-convert the AC voltage first, and then to rectify it.

This type of AC-to-DC power conversion is not commonly done anymore, because the transformers needed, were too bulky and expensive to manufacture. At the power-levels needed in many common appliances, one solution commonly used today is called a Buck Converter. This circuit assumes, that an arbitrary DC voltage is available, and that this voltage can be switched at frequencies of many Kilohertz. And, because Kilohertz-frequencies are used, the inductance of the main coil can also be made much lower – i.e., the coil now only needs to have far-fewer turns – of potentially heavy, enameled wire, than transformer coils needed to have, which operated at 60Hz.

But the very basis of a Buck Converter – an available DC input voltage – needs to be questioned in itself, not because the AC wall outlet offers AC power, but because again, the simplest way to rectify that AC power needs to be applied, which also uses the fewest and smallest physical components. And so one solution which people may imagine, is that we might just connect a rectifier to the wall outlet, and then some small capacitor to the output of the rectifier. But this is a complete non-solution, because if any load-current is drawn, this would also cause a strong, DC current-component to flow from the wall outlet. This would actually be counter to electrical codes and prohibited!

And so what the Electrical Engineer might think of next, if he did not know circuit theory, is just to connect the phase-wire of the AC to a decoupling capacitor, the capacitance of which is such, that it conducts at 60Hz, but not at much-lower frequencies. But the first approximation of a circuit which results, would also be wrong, and would look as follows:

rect_simp_3

The problem here would be, that as long as the load is drawing current, and D1 conducting, C2 would reverse-charge, and would stop conducting at some point, because the reversed voltage on D1 would get blocked – successfully – by D1. The above circuit is anon-functioning circuit, which I’ve provided for reference purposes only.

This circuit can be modified however, by just adding one more diode – D2 – so that it will draw AC from the source, and deliver DC to the load, as follows:

rect_simp_2

The problem with all this has to do with the actual voltages which result. When we say that the AC voltage of our outlets, in Canada and the USA, is 110VAC, this is the ‘RMS voltage’ – i.e., the ‘Root-Mean-Square voltage’ – and is meant to count as the DC-equivalent voltage. It actually corresponds to a ‘peak voltage’ of 155.5V . But, the circuit shown, which is also the simplest way to convert pure AC into DC, actually doubles this voltage, so that the no-load voltage that appears as output, will actually be the peak-to-peak voltage, that the input had! This is because, when D2 conducts and the AC input is at its most-negative value, the voltage across C2 is actually the peak voltage, forward. And, when the input voltage goes positive, the voltage which will appear across D2 will have the voltage of C2 added to it, and will be twice the peak voltage. When D1 finally conducts, it will also cause this voltage to be stored in C1.

There are many people who might not guess, that if they open up a 110VAC appliance, there would be voltages of 311VDC inside, and if we did get a shock from that, we’d additionally be getting a shock from a capacitor, which is a component which provides high currents to go with that, if there is a sudden voltage-change across the capacitor.

(Updated 06/10/2018, 17h30 … )

(As of 06/09/2018 : )

I suppose that there is another observation which I should add, about using a Buck Converter here. Back in the 1970s, the maximum collector-voltage of most bipolar transistors, which was also the maximum voltage they could switch off, was around 50VDC. But today, transistors are readily available, with maximum collector voltages of 350V, or even of 700V ! This is because when we look at an electronic component, we see the outside packaging, not what’s really inside. And so a component which has small, innocent external appearance, can actually be able to switch 350V on and off – at low, average current-levels.

This would not have been thinkable, back in the 1970s.

I suppose that another observation I should add is, that the type of voltage-doubling ladder I just described, tends to experience a considerable drop in the output-voltage, as soon as the load starts to draw current from it, and in fact, tends to provide poor filtering of the output-voltage, which therefore has strong, 60Hz ‘ripple’. The reason this does not really present a problem in some cases, is the fact that a Buck Converter can produce steady, well-filtered, reduced DC output-voltages, even if the input voltages are unstable. This is because in a Buck Converter, a feedback-loop simply causes its switching-component to stay on for a percentage-duration, which ‘rides out’ fluctuations in the input-voltage, almost until the input voltage starts to become as low as the intended output-voltage.

This also allows Electrical Engineers to use low capacitances in the rectifying circuit shown above, which means innocent-looking, physically small capacitors.


 

In the WiKi-Article I linked to above, the switching component was visualized as being complemented by a diode – a simple rectifier. In practice, diodes are seldom used here, for a combination of reasons:

  • The duration for which they stay conductive is one minus the duration for which the switching component stays conductive,
  • A real diode requires a forward voltage-drop of about 0.6 volts to become conductive,
  • Many low-voltage converters are only designed to deliver 5VDC, for which reason to lose 0.6V in such a diode, would also waste 10% of the total energy,
  • Some voltage converters may be designed to deliver less than 5VDC.

In modern electronics, special transistors have been designed which when switched ‘on’ (conductive), only ‘lose’ about 0.1V. They are not even bipolar transistors anymore. Those transistors make the situation feasible, by which a heavy load such as a laptop-computer, can switch itself off, and by which a much-smaller current, than what the laptop draws, can turn it on, without the transistor which is actually in series with the whole laptop, becoming hot.

When this type of transistor has its ‘gate’ controlled properly, equivalent small voltages between its gate and its source, will turn it on, in one direction. What results is a ‘rectifying transistor’.

I’m sure that the WiKi author knew this, but did not want to confuse the reader, by exposing the reader to the full complexity of a real voltage-converter.


 

I suppose that another question which some people might ask would be, why it’s not always possible just to use a bridge-rectifier. And the reason is the fact that most DC appliances require that their common supply – i.e. the rectified voltage that corresponds to zero – also not receive any high voltages – positive or negative – itself. Hence, it is not good enough for the design of DC appliances such as laptop-computers, only to have relative voltages between their positive terminal and their common, that are correct. They must additionally have ‘safe’ common voltages.

This means that a bridge-rectifier can usually not just be connected directly to the pins of an AC wall outlet, which has one neutral and one phase terminal. ( :1 ) If the DC-output-common voltage is the more-negative, when the phase terminal is positive, the neutral terminal will determine the DC-output-common voltage, but when the phase terminal is negative, it will determine the output-common voltage. This would result in the DC-output-common voltage swinging negatively by 155V !

Bridge-rectifiers are excellent, when connected to the secondary winding of a transformer, the common-mode voltage of which floats, just so that the DC-output-common voltage can be determined externally, let’s say by grounding. But in certain designs, the use of a transformer is eliminated.

Also, bridge-rectifiers would work in LED light-bulbs, because there, the LEDs themselves are protected from external contact, so that there, only their relative DC-supply-voltages matter.

Also, the diagram I showed above would have as problem, that the DC-common voltage would consistently be defined by the AC-neutral voltage, as shown. To allow the DC-output-common voltage to float, my circuit would require that another capacitor be inserted, between the AC-input-neutral terminal, and the DC-output-common voltage. Such a capacitor, like C2, would need to be conductive at 60 Hz, which means that an equivalent set of input-capacitors to a bridge-rectifier, would not decouple its DC-output-common voltage.

But then for the same reason, such an additional capacitor would not make my appliance ‘safe’, if its AC-input phase and -neutral terminals were reversed.

To the best of my knowledge, the way the power-supply of a (Canadian or US) desktop or tower computer works, the chassis-voltage is actually determined by ground, but the zero-volt, DC-output-common voltage, by the AC-input-neutral terminal. Additionally, the circuit-boards of desktop computers have metallic surfaces around their mounting-screws, that do not have continuity with this power-supply, zero-voltage, and which are electrically disconnected from anything else. And, even if some electrical contact was to take place between the output zero-voltage and the grounding of the chassis, the power-supplies refuse to apply power – such a computer fails to turn on. ( :2 )


 

1: )

Another approach which works, is that a bridge-rectifier is connected directly to the pins of the AC outlet, resulting in an intermediate DC voltage, the neutral side of which is known to be ‘hot’.

Within this relative voltage-set, the DC is chopped up at Kilohertz frequencies, and used to drive the primary winding of an actual transformer. This transformer can be made smaller than a 60Hz transformer, because instead, it operates at kHz frequencies.

The secondary winding of this transformer can be fed to a simple rectifier-circuit, and the DC-output-common voltage is decoupled from the AC-input voltages, simply because the secondary winding of the transformer is decoupled from the primary.

USB-wall-chargers are typically designed this way.

The only problem with this design is, that it’s not related in any way to designs, that use Buck Converters. ( :3 )


 

(Update 06/10/2018, 17h30 : )

2: )

When the desktop or tower -computer power-supplies do use a Buck Converter, this starts to create an issue, with what types of USB devices can be plugged in, simply because the USB ports pass through the DC zero voltage, as well as the +5V supply-voltage.

Ideally, any sort of USB device could be plugged in, and such devices as tablets and phones will not pose a problem, as long as they are ‘free at their end’, so that their zero-voltage can adapt to that of the PC.

But this situation would become problematic, if the connected USB device had its own power-supply because then, both devices’ power supplies may try to determine the zero-voltage, which is then connected by way of the USB cable.

Because the desktop or tower-computer is considered to be the main device, and the other device is named ‘The Peripheral’, designs would make sense in which the peripherals do not attempt to steer either DC voltage. In other words, the 0V and +5V leads of the cable could act as a power-supply to certain peripherals, but when a peripheral is connected that has its own, active power supply, those leads simply act to facilitate the passage of high-frequency signals – by allowing induction to take place – and may be met with capacitors at the other end. Only this time, the capacitors would be chosen to conduct at Megahertz frequencies, but no longer at 60Hz.

What I do notice is,

  • An external USB-hub which I have connected to one of my tower-computers, has a power-adapter itself, which is noticeably powered by an old-fashioned, 60Hz transformer / adapter, and therefore does not attempt to determine the DC, zero-voltage.
  • We do not tend to connect monitors to our computers by way of USB, but monitors definitely have their own power-supplies.

 

3: )

I think that in general,

If the AC plug is a non-polarized plug, such that phase and neutral could be switched arbitrarily, then the power adapter will possess an isolation transformer. But in modern times, there is no real need for that transformer to operate at 50 or 60Hz, since it can operate at Kilohertz frequencies in that case.

Dirk

 

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