The circuits shown are based on the LM339 Quad Voltage Comparator chip or the LM393 Dual Voltage Comparator chip. These devices are functionally identical. The LM311 Voltage Comparator can be used for these applications as well and also has a number of unique features.
The diagram shows a hysteresis ‘loop’ that describes how a comparator functions. The horizontal ‘X’ axis is the input, and represents the difference of the two input voltages. The vertical “Y” axis represents the comparator’s output state.
If the comparator is initially ‘OFF’, the MINUS input voltage has to become slightly above the PLUS input voltage before the comparator output turns ‘ON’. This is represented by moving right along the bottom part of the loop.
Once the comparator is ‘ON’, the MINUS input voltage needs to drop slightly below the PLUS input voltage before it turns ‘OFF’ again (moving left along the top of the loop).
The width of the loop outlined by an off-on-off cycle is the input hysteresis voltage.
The hysteresis voltage for most comparators is in the millivolt range and usually only affects circuits where the input voltage rises or falls very slowly or has voltage spikes knoown as “noise”.
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