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Voltage comparator LM741

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nmbg011

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Hi,

I need voltage comparator to sense 5,7V or 5,8V and turn on some transistor at that voltage level.

Input voltage is 12V, I use LM317 to limit current of NiMh charger for 4x cells at 100mA, and whant to monitor voltage of charging, and make cut off at 5,7 5,8V.

LM741 ?
 

Hello Dear nmbg011
It is simple . 741 op amp has height pin . pin number 2 is inverting input and pin number 3 is non inverting input . ok , if you use it , open loop ( without feed back ) it will work as a comparator . ( if pin 3 become higher than pin 2 , thus the out put will be +vcc , and if the pin 2 become higher than pin 3 , thus the out put will be about -vcc ) .
Good luck and Best Wishes
Goldsmith
 
LM741 is suitable to work as an amplifier.
A comparator, like LM339 could be a better choice.


+++
 
Dear andre
Hi
We can use all of the om amps as a comparator , but not at high frequencies. as i understood , he want do comparing between 2 constant DC voltage , and i think there is no problem with that .
But if he want work at high frequencies , surely , he will nee comparators , such as LM311 or LM339 orLM319 or ....
Best Regards
Goldsmith
 
To make a very simple battery status monitor need to make comparison between a voltage and a stable reference. And no matter what operational amplifier we intend to use it, still need to use an external reference voltage. So, why not to use comparators in which a reference voltage is integrated on to the chip? One example is TL431 precision voltage comparator with an on-chip voltage reference.
We had some discussion on same topic here:
https://www.edaboard.com/threads/239384/
@nmbg011 did you find this circuit on the local shop?
 
did you find this circuit on the local shop?
Click to expand...

No I find that circuit on internet, and works fine, this is original circuit, but its just constant current, I try to make charger to stop charging at some voltage level. To add some led indicating charging, and when charging finished at 5,8V completing charging, user must press some taster, if whant new cycle of charging.

currentsource.jpg
 
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Use an LM10. It has an opamp that can be used as a comparator plus an adjustable voltage reference all in an 8-pins case. It works fine from a low supply voltage and the output of the opamp swings very close to the positive supply and close to ground.
 
On the other hand, I have reason to doubt he will find that circuit at the local store, if he had not found a very common circuit used in most of the TV power supplies...
But, as long as we have a very large range for the threshold voltage (5.7 –5.8V), then we can use simple circuits that may give acceptable results. See some example using bipolar transistors only.
 

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    chargers _v 3.GIF
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There is no LM10 in market.

Ok. MisterRF I will try thise circuits.
 

Hello Dear nmbg011
Do you want some op amps ?
Try LM358 or LF356 or TL072 or TL074 or ......
Good luck
Goldsmith
 
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