Need help for LM311 Comparator

[ginebra]

Comparators

Good day. I am using an LM311 to obtain a square wave from the output of a receiver which is a sinusoid. Problem is when I check the output of the LM311 I see a sawtooth like wave but when I simulate it in simetrix I see a square wave. Am I doing something wrong? Hope someone could help me. Thanks in advance.

 

[keith1200rs]

Comparators

I think you need to show your circuit. What frequency? What amplitude? Are you sure what you built is what you simulated?

Keith

 

[betwixt]

The LM311 has an open-collector output, do you have a pull up resistor or load on it's output pin?

Brian.

 

[abbeyromy]

@ginebra

yes LM311 has an open collector output which means if that you need to add a pull up resistor in order to take the output. Also when internal transistor is being driven high then you would get a low output as transistor gets Forward biased.

I think you are trying to design something like zero crossing detector. Please post schematics in jpg format.

 

[ginebra]

Sorry I forgot to post the schematic. The input of this circuit is either about 1V sinusoid when there is something detected by the transducer or a 10 mV peak-to-peak sine when the sound-to-voltage transducer does not pick up anything (when there is nothing transmitted). Problem for me is that when I checked the output of this circuit when there is something transmitted instead of a perfect square wave i see a sawtooth like wave.

 

[IanP]

The output [pin 7] can be connected (via a resistor) to almost anything you like, but pin 8 [+V] HAS to be connected to MIN +5V ..

Rgds,
IanP

 

[ginebra]

Ah I see that is the reason I see a sawtooth, right sir? Its just weird in simetrix I produce a square wave with max of 3.3 and min of 0V.

The sinusoid is about 40 KHz. Problem is I just need a square wave with a maximum of at least 2.5V. So in simetrix I just replaced the 3.3V of the LM311 with a 14.4V supply and I fed the output of the 311 to a voltage divider to get my square wave. Would it work as expected?

Attached is the edited schem. Thanks for the replies by the way.

 

[IanP]

On picture looks OK but why would you force the output stage to sink 14.4V/330Ω => 47mA of current whereas all what is going through voltage divider is roughly 14.4V/[20kΩ+4.7kΩ+.33kΩ] => 0.6mA
More efficient option would be to connect 330Ω to +5V and divide it by ≈2 to get 2.5Vpp output ..

Rgds,
IanP

 

[karesz]

Hi,
Whats your supply pls?
If you have 3V3 on pin4: connect the ref attenuator (R1)too to these potential, and if you can need max 2.5V as output; connect between the pull-up & +3V3 a small Zener (a diode) for the difference
So you will not need R8/R9... Btw: I think you must not drive a 330 Ohm with the output Transistor_check it with some higher resistor; maybe 2K2 (depend of loading)?
K.

 

[ginebra]

More efficient option would be to connect 330Ω to +5V and divide it by ≈2 to get 2.5Vpp output ..

Um sir I would just implement it via a simple R-R divider? Sorry sir I was confused on how it was inefficient.

PostPosted: 21 Mar 2010 14:10 Post subject: Need help for LM311 Comparator
Hi,
Whats your supply pls?
If you have 3V3 on pin4: connect the ref attenuator (R1)too to these potential, and if you can need max 2.5V as output; connect between the pull-up & +3V3 a small Zener (a diode) for the difference Smile
So you will not need R8/R9... Btw: I think you must not drive a 330 Ohm with the output Transistor_check it with some higher resistor; maybe 2K2 (depend of loading)?
K.

Sir I changed the supply to 14.4 V on the advice of sir IanP.

 

[keith1200rs]

As a general point, simulations are only as good as the models. Some of the older device models from manufacturers are a bit crude and inaccurate.

Keith