# Find Hysteresis

Discussion in 'General Electronics Chat' started by dileepchacko, Aug 28, 2009.

1. ### dileepchacko Thread Starter Active Member

May 13, 2008
102
1
Can anybody find out the Hysteresis of the attached circuits. Consider the vltage source will ingreese from 0 to 270V.

• ###### COMPARATOR_01.pdf
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Apr 5, 2008
18,610
3,606
Hello,

Here is the circuit you posted in the PDF:

I assume the output of the comparator will go to 0 Volts when low and to + 5 Volts (=V2) when high.
The upper switch limit will be when the output is high:
The resistor R5 will be parallel to R2.
The voltage at the input will be 5 * R1 / (R2||R5+R1)
The lower switch limit will be when the output is low:
The resistor R5 will be parallel to R1.
The voltage at the input will be 5 * (R1||R5) / (R2 + (R1||R5))

The hysteresys will be the difference between these values.

Greetings,
Bertus

• ###### Comparator_hysteresys.png
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Last edited: Aug 28, 2009

Apr 5, 2008
18,610
3,606
Hello,

The calculations I gave will not work.
The comparator has open drain output, so a resistor to the powersupply is needid.
See datasheet.
This resistor will add to the R5 when the high switch voltage is calculated.

Greetings,
Bertus

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4. ### dileepchacko Thread Starter Active Member

May 13, 2008
102
1
Thanks Bertus...

5. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
$V_{\small{(-H)}}=\Large {{\frac{\frac{5}{20}}{{\frac{1}{20}}+{\frac{1}{1}}+{\frac{1}{68}}}}}\, \normal {=\,0.234807\,Volts}$

$V_{\small{(+H)}}=\Large \frac{{\frac{5}{20}}+{\frac{5}{(68+R_{pu})}}} {{\frac{1}{20}}+{\frac{1}{1}}+{\frac{1}{(68+R_{pu})}}}$

Here are the two Millman's Theorem equations.

Once you choose a value for Rpu, you can plug it into the second equation and solve for the positive hysteresis setpoint.

hgmjr

Last edited: Aug 28, 2009
6. ### Wendy Moderator

Mar 24, 2008
21,376
2,923
To restate what has been said a different way, think in terms of voltage dividers. When the two inputs are at the same voltage, that is when it will switch.

Each leg with 2 resistors will divide the voltages to fixed values, which sets the hysteresis.