Hi all
I have a bit of a query about a circuit I'm designning I'm using a LM311 as a comparator to compare a voltage reference I've designed with an input voltage. At the moment the LM311 is pully high to 5V when the input voltage is above the reference voltage, however I want this the other way around i.e. when the input voltage is below the reference voltage the comparator goes high can anyone please help or point me in the right direction.

Thanks
Art

 

I had a look again all I did was change the pins, would this work i.e. change the reference voltage to the inverting pin and the input voltage to the non-inverting pin. Can anyone confirm this please?

Thanks Art

 

Hello,

Have a look at the following page for the use of comparators:
http://www.electronics-tutorials.ws/opamp/op-amp-comparator.html

Bertus

 

Hello,

Have a look at the following page for the use of comparators:
http://www.electronics-tutorials.ws/opamp/op-amp-comparator.html

Bertus

Hi I extracted the following:
"
VIN is less than the DC voltage level at VREF, ( VIN < VREF ). As the non-inverting (positive) input of the comparator is less than the inverting (negative) input, the output will be LOW and at the negative supply voltage, -Vcc resulting in a negative saturation of the output.

If we now increase the input voltage, VIN so that its value is greater than the reference voltage VREF on the inverting input, the output voltage rapidly switches HIGH towards the positive supply voltage, +Vcc resulting in a positive saturation of the output."

Can I assume then if I swap over pins for Vref and Vin, that if Vin is <Vref the output would then becoming high? I just need confirmation sanity check if you like

 

Just in case, could you show the complete schematic you are using?

 

I had a look again all I did was change the pins, would this work i.e. change the reference voltage to the inverting pin and the input voltage to the non-inverting pin. Can anyone confirm this please?

Yes, that's how you do it.

 

Hi I extracted the following:
"
VIN is less than the DC voltage level at VREF, ( VIN < VREF ). As the non-inverting (positive) input of the comparator is less than the inverting (negative) input, the output will be LOW and at the negative supply voltage, -Vcc resulting in a negative saturation of the output.

If we now increase the input voltage, VIN so that its value is greater than the reference voltage VREF on the inverting input, the output voltage rapidly switches HIGH towards the positive supply voltage, +Vcc resulting in a positive saturation of the output."

Can I assume then if I swap over pins for Vref and Vin, that if Vin is <Vref the output would then becoming high? I just need confirmation sanity check if you like

In general, yes, swapping the inputs swaps the output effect. There are exceptions. Post your full circuit to clarify or, use a light load, a fuse and give it a try.

 

Yes, that's how you do it.

great thanks

 

Since you won't show your schematic, you basically are asking a blind man for advice about colors. How confident would you be in using that advice?

ak