(2007?Xianning) as shown in the figure is a thermostatic soldering iron circuit diagram, when not using the soldering iron welding press, disconnect the switch S, so that it is in the insulation state

(2007?Xianning) as shown in the figure is a thermostatic soldering iron circuit diagram, when not using the soldering iron welding press, disconnect the switch S, so that it is in the insulation state; need to When S is closed, R′ is short-circuited, the power of the soldering iron is rated power;

By the power formula P=

U2

R

can be found

R's resistance value R=

< tr>

U2

P

=

(220V)2

40W

=1210Ω;

When S is disconnected, the two resistors are connected in series with a total resistance of R total=R+R′=2×1210Ω = 2420Ω;

then the total power consumed by the circuit P total =

U2

General R

=

(220V)2<

2420Ω

=20W;

So choose A.