Hi all
I am basically trying to create a circuit, where I can set a reference voltage say 2.8V as one input, then I want another voltage V1 to be compared to this but I only want an output from this circuit of 5V ONLY WHEN V1>2.8V or whatever reference I set. Can anyone please provide any tips of what kind of topology I should use or where I could start.

Thanks
Art

 

Look at op amp Comparator Circuits, like Lm358, or better still Lm3914. For better stability introduce Hysteresis.

https://www.allaboutcircuits.com/textbook/experiments/chpt-6/voltage-comparator/

 

In addition to the link Dogydave was kind enough to provide you may also want to give this link a read, Voltage Comparator Information And Circuits. While operational amplifiers do work in a comparator roll I would try and stay with a chip labeled as a comparator. Also per Dogydave read up on hysteresis in the links as it can be an important feature in comparator circuits.

Ron

 

In addition to the link Dogydave was kind enough to provide you may also want to give this link a read, Voltage Comparator Information And Circuits. While operational amplifiers do work in a comparator roll I would try and stay with a chip labeled as a comparator. Also per Dogydave read up on hysteresis in the links as it can be an important feature in comparator circuits.

Ron

ok great thank you, with comparators is there a setup where the output is either 0 or 5V I know with differential op-amp setups there isn't this almost binary option but rather you get a range of values

 

Yes the output of the comparator will hit the supply rail, or as near to it as it will go, so if you want only a 5V output, supply the comparator with 5V.


http://www.ti.com/product/LM311

 

Yes the output of the comparator will hit the supply rail, or as near to it as it will go, so if you want only a 5V output, supply the comparator with 5V.


http://www.ti.com/product/LM311

brilliant thank you do you have any suggestions about what I should use to get a consistent voltage reference? Would potential dividing to get a reference voltage of say 2.8V be sufficient or would I need something more substantial?

 

If you want stability, use a Tl431 zener, better to post the circuit you want to use, then we can assist you further to modify it.

 

If you want stability, use a Tl431 zener, better to post the circuit you want to use, then we can assist you further to modify it.

brilliant thank you I will get something to you over the next two days thanks J

 

If your voltage is greater than 7 volts I would think about using a regulator like an everyday 7805 voltage regulator. I mention 7 volts as regulators like this have a "dropout voltage" exceeding the voltage the regulate at. There are also "low dropout regulators". Either regulator produced by a dozen manufactures so the links are just examples of using a regulator to get your 5 volt reference.

Ron

 

If you want stability, use a Tl431 zener, better to post the circuit you want to use, then we can assist you further to modify it.

Hi so the following is a description of the circuit I am after: It's to ensure the "current" of a system I'm designning doesn't go over a treshold. At the moment, I need a comparator that will compare a voltage that will be proportional to the current that has a range of 0 to 3.3V, the reference voltage that I need this input to be compared to is 2.87, or if that isn't possible 2.8 but 2.87 is preferred (if you can provide suggestions that would be really useful). The problem I am having at the moment is choosin components. If you could provide any suggestions that would be extrememly useful.

Kind Regards
Art

 

Ok so you need the resistors to give a Ratio of 0.148 for a voltage of 2.87V,

So you can use a 1.48K and a 10K, or any combination to give that Ratio.

If you look at any datasheet for the TL431

R1 is 1.48K, R2 is 10K.

The formula is :

V= 2.5 x (1+R1/R2)

 

Ok so you need the resistors to give a Ratio of 0.148 for a voltage of 2.87V,

So you can use a 1.48K and a 10K, or any combination to give that Ratio.

If you look at any datasheet for the TL431

R1 is 1.48K, R2 is 10K.

The formula is :

V= 2.5 x (1+R1/R2)

ok brilliant thank you so much, do you have any suggestions of what comparator I should use a bit confused with that point I was looking through a lot of datasheets but there are so many to choose from, I need it to give out near 5V or near Vcc when input voltage is greater than 2.87, if you have any suggestions for that would be greatly appreciated.

Thanks
Art

 

Yes start by looking at the Lm311, see if it meets your specific requirements.

http://www.ti.com/product/LM311

 

Yes start by looking at the Lm311, see if it meets your specific requirements.

http://www.ti.com/product/LM311

ok great thank you really appreciate the help

 

Yes start by looking at the Lm311, see if it meets your specific requirements.

http://www.ti.com/product/LM311

does the strobes feature have any effect can I bypass this?

 

Just looked at datasheet, it wont do rail to rail. So try
http://m.linear.com/product/LT1711

 

Just looked at datasheet, it wont do rail to rail. So try
http://m.linear.com/product/LT1711

ok great thanks, when I design a circuit can I double check with you then as well

 

Ok, search for Rail to Rail Comparator Ic,

EDIT, THE LM311 is an Open Collector, comparator so you can use it, you just put a resistor to pull the out up to 5v.


http://www.learningaboutelectronics.com/Articles/LM311-voltage-comparator-circuit.php

 

Ok, search for Rail to Rail Comparator Ic,

EDIT, THE LM311 is an Open Collector, comparator so you can use it, you just put a resistor to pull the out up to 5v.


http://www.learningaboutelectronics.com/Articles/LM311-voltage-comparator-circuit.php

quick question about pull up and pull down, just to confirm with the pull up: unless there is an ouput from the comparator (i.e. comparator gives out 0), there will be 0 totally, but once there is an output it gets pulled up to 5V?

 

Correct, you need to provide a Positive via a Resistor.