back to back zener diodes does what?

[Davewalker5]

What does these back to back zener diodes do?

Why are the reverse polarity back to back?

 

[davenn]

they will symmetrically clip a signal appearing across them to the approx. voltage of the diodes (6.8V)
so if there is an AC signal coming out of that Op-Amp, through R133. the resulting output past the diodes towards R132 will be a rough looking square wave

 

[Davewalker5]

they will symmetrically clip a signal appearing across them to the approx. voltage of the diodes (6.8V)
so if there is an AC signal coming out of that Op-Amp, through R133. the resulting output past the diodes towards R132 will be a rough looking square wave

I was thinking that also but, isn't U27 a comparator?

So U27 is not outputting an AC waveform i'm thinking

U24 is an intergator

 

[davenn]

regardless of the function of U27, the zeners are still acting as a symmetrical clipper

U 24 is also a low pass filter and you wouldn't need a LPF if the signal into U24 was plain DC

 

[Harald Kapp]

I was thinking that also but, isn't U27 a comparator?
So U27 is not outputting an AC waveform i'm thinking

Even a comparator can output an AC signal. AC is not limited to sinusoidal signal. AC means Alternating Current and encompasses any waveform that is not DC. A square wave from a comparator is AC, too. The zener diodes limit the amplitude of the signal to approx. +-7.5V whereas the output from U27 can be much higher, depending on the power supply +V and -V.

 

[davenn]

yup, agree totally there Harald ... got tied up at work ... doing my job haha
didn't get a chance to respond with the info you did ... thanks

Dave

 

[Colin Mitchell]

The maximum voltage appearing across them will be 6.8v + 0.7v = 7.5v in either direction

 

[Harald Kapp]

The maximum voltage appearing across them will be 6.8v + 0.7v = 7.5v in either direction

I think that's what I wrote

 

[Davewalker5]

How can you tell from a Low Pass filter to an Intergator? they both look the same to me

Is it the Capacitor and Resistor values that make the difference?

I can't tell on schematics if it's a low pass filter or an intergator circuit

Any hints to know without knowing the input waveform is AC or DC

 

[davenn]

they are the same

 

[Davewalker5]

they are the same

So they can have the same RC values? so how can you tell which is which? if you don't know what the input waveform is either AC or DC

How would a tech know by looking at a schematic

 

[Davewalker5]

I found another back to back zener diode on the op amps input pin

These diodes must clamp or clip the waveform symmetrically

 

[davenn]

So they can have the same RC values? so how can you tell which is which? if you don't know what the input waveform is either AC or DC

How would a tech know by looking at a schematic

an integrator is just another name for a low pass filter

 

[davenn]

I found another back to back zener diode on the op amps input pin

These diodes must clamp or clip the waveform symmetrically

no, not symmetrically

why do you think it would be symmetrical ?

 

[Davewalker5]

So what does these back to back diodes do?

 

[davenn]

they still do clipping

 

[Davewalker5]

symmetrical clipping but only on the feedback loop?

The other circuit symmetrically clipped the output of the op amp

What is the difference between symmetrically clipping the feedback loop compared to symmetrically clipping the output?

 

[davenn]

Original question answered

thread closed