Need help for LM311 Comparator

Status
Not open for further replies.
G

ginebra

Junior Member level 3
Joined
Mar 7, 2010
Messages
28
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,558
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.
 

Comparators

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

Keith
 

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

Brian.
 

@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.
 

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.
 

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
 

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.
 

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
 

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.
 

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

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


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

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
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…