| 摘要 |
In a toaster, the bread carriage is latched in the down position by a solenoid coil 312. When the carriage is in the down position, current flows to the toaster's heating elements. Switch 310 is also closed, allowing current to flow to a timing circuit 318. This incorporates a capacitor 328 that slowly charges via diode 302 and resistors 320, 322 and variable resistor 324. When the voltage across capacitor 328 reaches a predetermined level, SCR 334 is triggered via emitter follower transistor 330. This connects the two terminals of the coil 312 to ground, and causes the bread carriage to be released. The time required to charge capacitor 328 to the predetermined voltage is determined by variable resistance 324, which can be set by the user, and by the voltage that appears at terminal 314 of coil 312. This voltage is dependent on the resistance of coil 312, the coil and resistance 306 forming a voltage divider. The resistance of coil 312, which may be made of copper, increases with temperature. The temperature will increase due to both the current passing through it, and residual heat from the toaster. As the resistance of the coil increases, the voltage present at terminal 314 increases, reducing the time required to charge capacitor 328. This means that for a given setting of variable resistance 324, the toasting time will reduce with repeated use of the toaster. This compensates for the reduced toasting due to residual heat within the toaster.
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