Hello;

I've found this schematic which implements a trigger Schmitt using a LM393N. I'd like to know the explanation of the circuit, why are the resistors chosen and how to calculate their values. Thank you in advance.

Find the files attached.

 

Welcome to AAC!

Find the files attached.

Could you post a picture of the schematic? Don't feel like opening a zip file from a new member.

 

Welcome to AAC!

Could you post a picture of the schematic? Don't feel like opening a zip file from a new member.

No problem. Here you have it.

Thank you in advance.

 

Here you have it.

Where did you find that circuit? The way it's drawn obscures function, resistor values are odd, and Q2 is being abused.

It's the 3 resistor Schmitt comparator variant.

From the 1994 National Semiconductor Linear Applications Handbook:

Resistor value calculations are in the attached PDF.

 

Where did you find that circuit? The way it's drawn obscures function, resistor values are odd, and Q2 is being abused.

It's the 3 resistor Schmitt comparator variant.

From the 1994 National Semiconductor Linear Applications Handbook:
View attachment 257221
Resistor value calculations are in the attached PDF.

Thank you! A friend of mine sent it to me, and asked me to check it out.

 

A friend of mine sent it to me, and asked me to check it out.

Your friend doesn't appear to know what he's doing. He can't even draw the schematic properly.

 

The output of the Schmitt Trigger determines two different thresholds. When the output is at Vcc, the threshold is set at the lower trip point, and when the the output is at GROUND the threshold is set at the upper trip point. The rest is just the relentless application of KCL.

 

That is a bizarre set of resistor values, and none of the ones that matter (R1, R2, R3) are standard values. The LM393 input offset and bias errors far outweigh the precision of a 4-digit resistor value.

Also - Because R4 is so close to the value or R3, it affects *almost all* of the hysteresis calculations. In the PDF above, this is bypassed with an assumption stated in equation 5. Your values for all of R1 - R4 are so close together that most of the math in the PDF does not apply directly to your circuit; almost every equation needs an additional Rpullup term. In your circuit, R4 appears in series with R3 when the U3B output is high, but not when it is low. This is a messy complication, but not complex.

ak

 

Could you please show me the calculations to obtain the R values in this circuit and explain me how to do it? Thank you.

 

The calculations are on pages 2 and 3 of the PDF. Page 3 has a design example based on figure 7, which is the one you want for your circuit. To be more accurate, equations 1, 3, 6, and 8 need to have the Rpull-up term added in series with R3. this is PDF R3, not your schematic R3.

ak

 

Also note that the pull-up resistor needs to be small compared to the resistance of the load.

 

Also note that the pull-up resistor needs to be small compared to the resistance of the load.

That is equation 4 in the PDF.

ak