### Author Topic: LM393 hysteresis calculation vs real circuit  (Read 5704 times)

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#### getchar

• Beginner
• Posts: 2
##### LM393 hysteresis calculation vs real circuit
« on: May 16, 2013, 09:02:48 AM »
Hi all,
I am newbie here and I would like to ask you for help.
I am working on thermal protection module but comparator which I used doesnt work properly.
Calculations are from this literature:
http://saaubi.people.wm.edu/TeachingWebPages/Physics252_Spring2009/Week13/LabManual_Chpt13.pdf

My circuit:

Calculations:
Vref = 6V
Vp+ = 8V
Vp- = 4V
-> hysteresis 4V

I measured points when it was toggling:
Vp+ = 4,46V
Vp- = 3,86V
-> hysteresis 0,6V

Where did i make mistake? Thank you very much for any hint...
« Last Edit: May 16, 2013, 11:22:25 AM by getchar »

#### jwatte

• Supreme Robot
• Posts: 1,345
##### Re: LM393 hysteresis calculation vs real circuit
« Reply #1 on: May 16, 2013, 01:06:43 PM »
When looking at your circuit, consider the equilibrium states when output is high, and output is low.

High: Effective 50 kOhm pull-up; 100 kOhm pull-down, so 8V input to +. - would have to go above 8V to turn off.
Low: Effective 100 kOhm pull-up; 50 kOhm pull-down, so 4V input to +. - would have to go below 4V to turn on.

Why would this particular circuit not behave correctly? I don't know exaclty, but maybe the current drive provided by the 100 kOhm resistors is not sufficient. Try 10 kOhm resistors instead. That may still not be enough for properly driving BJT transistors -- 5 mA seems to be a magic place, so perhaps you should actually try 2 kOhm resistors instead of 100 kOhm resistors.

From what I've found, a comparator is just like an opamp, except with worse performance. I'd use a rail-to-rail single-ended opamp rather than a comparator.

#### getchar

• Beginner
• Posts: 2
##### Re: LM393 hysteresis calculation vs real circuit
« Reply #2 on: May 17, 2013, 02:48:38 PM »

I changed values of resistors to each 2kohm, but it still doesnt work correctly. Hysteresis is still about 600mV. I havent got any rail-to-rail opamps now, so i have used TL072 (it is pin-compatible) - hysteresis is still about 600mV.
I am confused...

#### jwatte

• Supreme Robot
• Posts: 1,345