Author Topic: Comparator problem  (Read 1256 times)

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Offline NASKTopic starter

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Comparator problem
« on: April 03, 2019, 06:49:01 am »
plz help me to figure it this issue.  + input is 12.5 mV  & - input is  1.7V  then LED is ON. when  + input is 12.5 mV & - input is 2.7V   LED is off.  how this is possible ? is it issue in ic ?
 

Offline not1xor1

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Re: Comparator problem
« Reply #1 on: April 03, 2019, 07:16:19 am »
plz help me to figure it this issue.  + input is 12.5 mV  & - input is  1.7V  then LED is ON. when  + input is 12.5 mV & - input is 2.7V   LED is off.  how this is possible ? is it issue in ic ?

LM741 cannot work in that circuit because:
-1) supply voltage is below the minimum value specified by the datasheet (20V)
-2) input voltage range is beyond the limits specified in the datasheet (2-3 volts above/below negative/positive supply)

you need either a different operational amplifier (rail-to-rail or so called single supply operational amplifier) or different supply voltage and input voltage values

BTW while there is no problem in using an operational amplifier as comparator to switch on a LED, there are many cases where one can only use real comparators
 
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Online Zero999

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Re: Comparator problem
« Reply #2 on: April 03, 2019, 08:11:30 am »
It's a typical comparator application, so you ideally need a comparator, rather than an op-amp.

If you replaced the 741 with a comparator, which can work at 5V, such as the the MCP6541 or TLC3702 (note that this has a different pin-out) it would work.

If you don't have either of the above, then you could try the LM358, an op-amp designed for use at lower voltages and its inputs work down to the negative rail.

The best cheap and commonly available part for your application is the LM393, but it has an open collector output, which means its output can only be pulled down to 0V, so the LED will need to be wired from the output to +V and the inverting and non-inverting inputs exchanged.

Shouldn't R2 be 47k?
« Last Edit: April 03, 2019, 08:13:57 am by Zero999 »
 
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Offline NASKTopic starter

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Re: Comparator problem
« Reply #3 on: April 03, 2019, 09:37:56 am »
thank you both of you , any one can tell me where did you find this information in datasheet because I cant find it.
 
Quote
input voltage range is beyond the limits specified in the datasheet (2-3 volts above/below negative/positive supply)

 Zero999
it a given circuit to me , 47R
 

Online Zero999

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Re: Comparator problem
« Reply #4 on: April 03, 2019, 10:05:54 am »
thank you both of you , any one can tell me where did you find this information in datasheet because I cant find it.
 
Quote
input voltage range is beyond the limits specified in the datasheet (2-3 volts above/below negative/positive supply)
It's the common mode or input range specification, page 5. ±12 to ±13, which is  specified with a 15V positive and negative supply, a difference of 2V to 3V.

Quote
Zero999
it a given circuit to me , 47R
It doesn't look right. with R2 = 47R, the LED will only turn on, when the potentiometer's wiper is right at the end, which may or may not be the desired effect.
 
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