Schmitt Trigger Design using LM358

[manishanand14]

Hello Everyone!

I want to design a schmitt trigger using LM358 with UTP=12V, LTO=10V


This is the diagram given in LM358 datasheet.How will I set the Vref.

Can someone give me the verified design with resistor values.

 

[goldsmith]

Hello Everyone!

I want to design a schmitt trigger using LM358 with UTP=12V, LTO=10V
lm338-schmitt-trigger.png

This is the diagram given in LM358 datasheet.How will I set the Vref.

Can someone give me the verified design with resistor values.

Hi manishanand14
Vref can be obtained by two voltage divider resistors through the supply of your op amp .
For values of resistors , you have all of parameters except value of resistors , so you can use a desired range instead of one of them for example a 10 k ohm . then the equation will give you the other one .

Best Wishes
Goldsmith

 

[manishanand14]

Thanks Goldsmith!

I want to use Zener Diodes for Vref.
Please tell me how to select the resistor value to be used for zener diode current limiting.

 

[D.A.(Tony)Stewart]

Zeners like LEDs have a threshold voltage and ESR drop that is current dependant.

Compare two white LEDs at 3.0 using only 1 or 2 mA instead of 3.2~3.4@ 20mA
Check the VI curves to measure the slope or the ESR. For LEDs are rated current above typ= <<20 Ohms each.

You choose the bias resistor to raise the voltage and lower ESR.

As long as ESR << than your comparator resistors, you can neglect the ESR in the hysteresis calculations.

The LED production tolerance as it happens is on the ESR and not the Vthreshold, so Vf at rated current has a tolerance variation, but at lower currents it is tighter.

 

[goldsmith]

I want to use Zener Diodes for Vref.
Please tell me how to select the resistor value to be used for zener diode current limiting.

Hi again

because the input impedance of op- amp is too high , then the zener diode won't need too much current handling . so you can consider 2 or 5 ma for IZ , hence R=(Vcc-VZ)/IZ [ohm] .
Best Wishes
Goldsmith