Using oscilloscopes 87
Some modern mains-operated scopes use a direct off-line
switching power supply. In addition to cost and weight reduction,
this arrangement has other advantages. The supply can act as a
preregulator for the stabilizers used by the various voltage rails,
and can in addition cope with any mains input voltage from 90 V
to 260V without any mains voltage adjustment selector. How-
ever, many mains-operated scopes use a conventional mains
transformer, so that one of the items required in Figure 5.13 is
already available - it just needs another low voltage, low current,
secondary winding. Thus for very little extra cost, a component
test facility can be incorporated, although usually a single fixed
value of resistor is used, resulting in a rather restricted range of
values for which good discrimination can be obtained. Never-
theless, the arrangement is very popular and useful and is
consequently found in a number of low-cost instruments.
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