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Unformatted text preview: EECS 314 Winter 2009 Homework set 3 Students name ___________________________ Discussion section # _______ (Last, First, write legibly, use ink) (use ink) HW problem for extra credit Schmitt trigger a better comparator 2009 Alexander Ganago Page 1 of 5 The Big Picture The greatest disadvantage of comparators is so-called chattering (isnt it a highly scientific term ;), which means that even the slightest variations of the difference between the sensor voltage V 1 and the set voltage V 2 would lead to abrupt changes of the output voltage. For example, variations of temperature by a small fraction of a degree would make (V 1 V 2 ) vary only by a millivolt or so but it would be enough to turn on and off a powerful air conditioner every few seconds, without due reason (and with possible damage to its motor, etc.), the big switch would close and open very often, making a chattering sound. A better circuit solution would involve a comfort zone in which nothing should be done. For example, in temperature control, you turn on the heater if the room air gets cooler than 68 F, you turn on the air conditioner if the temperature rises above 72 F, but the range of temperatures between 68 and 72 F is your comfort zone: you are comfortable and do need to control the temperature until it goes beyond this range. An improved comparator, which has a comfort zone, is called the Schmitt trigger , based on an Op Amp whose non-inverting (+) input terminal is connected to the output terminal (this is called positive feedback). As in any comparator, the Schmitt triggers output voltage saturates either at a certain high level V OUT, HIGH which is smaller than the positive supply voltage (shown on this diagram as +12 V) or saturates at V OUT, LOW which is higher than the negative supply voltage (shown on this diagram as 12 V, but it can equals zero = ground in other circuits). For simplicity, we may assume that saturation occurs at the supply voltages such as -12 and +12 V....
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