TABLE OF CONTENTS Tim Williams
Typically you will be designing for 230V AC in the UK and continental Europe and 115V in the US. Other countries have frustratingly minor differences. The usual supply voltage variability is 10%, or sometimes +10%/ ?15%. In the UK the supply authorities are obliged to maintain their voltage at the point of connection to the customer's premises within 6%, to which is added an allowance for local loading effects. If the voltage tolerance is applied to the UK/Europe nominal then the input voltage range becomes 207-253V or 195-253V. This range must be handled transparently by the power supply circuitry.
To cope simultaneously with both the American supply voltage, which may drop below 100V, and the European voltages is difficult for a linear supply although it is possible to design "universal" switch-mode circuits which can accept such a wide range (see the comment at the end of section 36.5). Historically, this problem was handled by using a mains transformer with a split primary (Figure 36.1) which can be connected in series or parallel by means of a discreetly mounted voltage selector switch. This has the disadvantage that the switch may be so discreet that the user doesn't know about it, or else it may not be discreet enough and the user may be tempted to fiddle with it. This is not a real problem in the US, but applying 230V to a unit which is set for 115V will at least annoy the user by blowing...
