[BC] hysterusis

[Harold Hallikainen]

Hysteresis is the difference between the upper and lower trip points in a
switching circuit. The example I use for my students is your household
thermostat. If it turns the heater on at 70F and turns it off at 72F, it
has a hysteresis of 2F. Going electronic, an analog comparator has no
hysteresis, so the output goes high if voltage on the non-inverting input
is above that on the inverting input. A Schmitt trigger, on the other
hand, has its output go high when the voltage is higher than the upper
trip point, but the output does not go low until the voltage is below the
lower trip point. Again, the difference is the hysteresis. This assumes a
non-inverting Schmitt. A chip like the 74HC14 is an inverting Schmitt, so
the output goes low when the input goes high.

The Schmitt trigger is often used to "square up" noisey signals,
especially those with slow edges. If the noise voltage riding on the
signal is less than the hysteresis, the output will not "chatter" during
an input transition. A comparator output would chatter due to the noise,
again, especially on slow moving edges.

One of my favorite circuits is the Schmitt trigger oscillator. Use an
inverting Schmitt (like the HC14). Put a capacitor between the input and
ground. Put a resistor between the input and output. You have an
oscillator! The capacitor charges and discharges such that the input to
the Schmitt oscillates between the two trip points. The output of the
Schmitt is a square wave.

Harold


>
> I can't spell it to save my life right now, but one way to describe it is
> the natural amount of reactive response lag to an event.
>
> Like when a motor is started, there is a delay between when the EMF is
> applied to the coil and when the rotor starts turning at it's rated RPM
> and load.
>
> There are others which better describe this I'm sure.
>
> MM
>
>
>> Wha is this histerusis stuff;
>>
>> loks like a little like histherwatsus to me..
>>
>>
>>
>> hick......
>>
>>
>>
>> Stanley Adams
>>
>> Memphis
>>
>>
>>
>>
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>>
>>
>>
>>
>>
>>
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