Simple question: Most computer power supplies are auto-sensing between 110V/120V and 220V/240V. This begs the question, can they use 240V in North America i.e. connected to both hots (red and black)?
Solution:
Nearly all computer power supply units (PSUs) are switched mode power supplies (SMPS). The mode of operation is:
- Rectify the mains voltage.
- Charge a high voltage capacitor to mains peak voltage.
- Feed the DC through a ferrite transformer and high-voltage switching transistor.
- Switch the transistor at high frequency. This generates an AC on the primary which gets stepped down in the transformer and appears on the secondary as low voltage AC.
- Regulate the secondary voltage by varying the pulse width on the primary.
The relevance to your question is that neither to positive DC (+V) or negative DC (-V) is connected directly to either L1 or L2 (as it appears in the schematic). In fact the DC rails are leaping around with the AC voltage.
Let’s take the simple case where L2 is Neutral and at zero volts.
- When L1 goes positive, current will flow in via D2 and out via D3. -V will, therefore, be 0.7 V above 0.
- When L1 goes negative, current will flow in through D4 and out through D1. +V will, therefore, be 0.7 V below 0.
The net effect is that both +V and -V, while keeping a respectable couple of hundred volts between them, are leaping up and down together with respect to earth / neutral.
Voltages during L1 positive and L1 negative, L2 connected to ground.
Since there is no neutral or ground reference on the high-voltage DC section the power supply should be fine provided:
- You operate within the min and max voltages.
- Neither L1 or L2 exceeds the insulation rating with respect to ground.
Thanks for the explanation, this provides insight but also raises more questions (I don’t quite have the background to fully understand it yet). Where does the 0.7 V come from, and is that brought back up to 12 V / 5 V / 3.3 V?
The first part of this question and the schematic deals with the high-voltage DC section of a typical SMPS. The intention of that part is to show that that section will work OK with either Hot – Neutral or Hot – Hot mains connection.
The reference to 0.7 V is the forward voltage drop across the diode when it is conducting. If it helps, you can think of the diode as a non-return valve in a water circuit: it blocks flow in the reverse direction but allows it in the forward direction with a 0.7 V ‘pressure drop’. It only affects the high-voltage side and has no effect on the low-voltage DC side.
What isn’t shown (other than a hint) is that the transformer isolates the mains from the secondary windings which are low voltage and rectified to produce the various DC voltages required. Typically there is a high current winding, diodes and smoothing capacitors for the 5 V supply (or 3.3 V supply on more modern devices) and separate windings, diodes and capacitors for each of the other voltages required.
Summary: the hot-hot connection does not affect the DC voltages as they are isolated from the supply by the switching transformer.