+ Post New Thread
Results 1 to 18 of 18
-
5th November 2018, 00:41 #1
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
Voltage Comparator Design
Hi all,
Has anybody worked with LM741 and used it for voltage comparison? I am trying to using LM741 to compare two voltages, and essentially then use this as a thermostat. The issue is that once V+ > V-. the output voltage is 1.3 -1.6 v, but not 0 volts. Do I need to add offset? I am only using the 5 pins atm.
Any help on this, or a precise design for comparing the voltages will be highly appreciated.
Thanks.
-
Advertisement
-
5th November 2018, 00:47 #2
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
My recommendation: If you want a comparator, then use a comparator.
There's a good reason why they designed comparators as well as amplifiers.
There are already threads discussing this in detail.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
-
5th November 2018, 01:09 #3
- Join Date
- Jan 2008
- Location
- Toronto area of Canada
- Posts
- 8,757
- Helped
- 2057 / 2057
- Points
- 53,786
- Level
- 56
Re: Voltage Comparator Design
The 741 opamp is 50 or 51 years old. There are many much better opamps today.
The 741 opamp was designed to use only a +15V/-15V supply or 30V, some do not work if the total supply is less than 10V.
Its inputs and output do not work within a few volts from the positive or negative supply.
It is too noisy (hiss) for audio and its high frequency response is poor. Its distortion might be too high.
-
5th November 2018, 04:39 #4
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
-
5th November 2018, 07:45 #5
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
Yes, it is a comparator and will do it's job as a comparator.
Are you aware that you don't give any requirement / specification?
Thus it is impossible to say if it fits your needs.
Which ICs you reckon will be appropriate?
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
-
5th November 2018, 22:58 #6
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
Re: Voltage Comparator Design
Hi,
Yes, my apologies for not being very clear. I am essentially looking to design a thermostat, which can only perform on/off operation based on the basis of temperature recorded by the sensor, which most likely will be a RTD. The temperature range which has to be measured is between -10 to about 100 C. ( the sensor will be mounted on roof, which is about 10m high from the ground). Ideally looking for a comparator which has a very stable output..
Thanks.
-
6th November 2018, 01:12 #7
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
A comparator compares two voltages.
It doesn't care about:
* thermostat
* sensor
* RTD
* room temperature range
* roof mount
* height from ground
Thus your post is useless.
--> show your circuit, give electrical specifications for the comparator
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
-
Advertisement
-
6th November 2018, 01:58 #8
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
Re: Voltage Comparator Design
I am simply looking for a comparator that does not play around like LM741. I have ordered LT7041 from TI, but it will take some time to be shipped from US, as I am in Australia. Have you any experience with some of the older/easily available ICs which can work fine as a voltage comparator? Ideally with 24vdc Power Supply, but can manage to provide 12v as well as the supply. There are no more limitations I can think of.
Thanks
-
Advertisement
-
6th November 2018, 07:48 #9
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
does not play around like LM741
I have ordered LT7041
Maybe you mean the previously mentioned "TLV7041"?
But as you now (eventually) mention a specification:
Ideally with 24vdc Power Supply, but can manage to provide 12v as well as the supply.
It clearly mentions on the first page "Features":
Wide Supply Voltage Range of 1.6 V to 6.5 VHave you any experience with some of the older...
... available...
But you write as if a comparator is something special. It is almost as often used as an Opamp. It surely is no exotic electronic part, there are many many different types (thousands), and more than 50% of them are able to be supplied with >12V.
In post#5 i recommended to use freely available selection guides. Are you too lazy to use it and look for the single specification you - until now - gave "power supply range"?
There are no more limitations I can think of.
I recommend to show a little effort of your own.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
-
6th November 2018, 20:45 #10
Re: Voltage Comparator Design
Hi,
"...older/easily available ICs which can work fine as a voltage comparator" - LM339
I'm getting a strange sense of déjŕ vu about comparator use and people not bothering to read anything about anything so am preempting the next possibly predictable thread you might be tempted to create: Please read about comparators, they need a pullup resistor otherwise they won't work.
I agree with Klaus, minimal effort required on your part.
-
6th November 2018, 22:49 #11
- Join Date
- Jan 2008
- Location
- Bochum, Germany
- Posts
- 45,810
- Helped
- 13929 / 13929
- Points
- 262,101
- Level
- 100
Re: Voltage Comparator Design
A usual feature of comparator ICs is that their output directly fits a logic interface standard. Because there are many different logic types, comparator output voltage ranges are different as well. By adding a suitable level converter, you can use any OP to drive your logic, also 741. In other words, using a specific comparator IC is more a matter of convenience than requirement.
There are other parameters like input voltage range, speed, supply voltage, but you didn't tell complete specifications.
-
6th November 2018, 23:23 #12
- Join Date
- Jan 2008
- Location
- Toronto area of Canada
- Posts
- 8,757
- Helped
- 2057 / 2057
- Points
- 53,786
- Level
- 56
Re: Voltage Comparator Design
Go to www.farnell.com and click on the flag of your country. They have warehouses all over the world and are also called Element14. They probably have many comparators close to you.
1 members found this post helpful.
-
7th November 2018, 00:04 #13
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
Re: Voltage Comparator Design
I am unable to add any images, but I can explain the issue I am facing with using LM741. The output when non-inverting input is less than the inverting input should be ideally 0v (as this is what I am providing to pin 4), but practically it is 1.6 v. I have tried pull up resistors of different values, but in vain. Could this possibly a problem of my design? Or this is normal? If I get a Comparator IC instead of Op-Amp, will that help? Thanks.
-
7th November 2018, 07:01 #14
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
Did you look into the datasheets for the "output voltage" specifications?
What do they say?
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
1 members found this post helpful.
-
9th November 2018, 03:18 #15
- Join Date
- Jun 2017
- Posts
- 14
- Helped
- 0 / 0
- Points
- 332
- Level
- 3
Re: Voltage Comparator Design
The output voltage swing can be upto 16v with Supply voltage of 20v.. I am afraid I understand what this means?
Does this mean that there can be a difference of 4v between the ideal and actual output?
Thanks
-
9th November 2018, 06:09 #16
- Join Date
- Apr 2011
- Location
- Minneapolis, Minnesota, USA
- Posts
- 13,023
- Helped
- 2589 / 2589
- Points
- 53,276
- Level
- 56
Re: Voltage Comparator Design
741 output does not get within 2V or so of the bottom supply rail. So it became common to hear that op amps need a bipolar supply (meaning positive and negative). That is how you can achieve output down to 0V.
There are newer types of op amps which go down to 0V even when the power supply is positive only.
-
9th November 2018, 08:19 #17
Awards:
- Join Date
- Apr 2014
- Posts
- 16,084
- Helped
- 3644 / 3644
- Points
- 79,246
- Level
- 68
Re: Voltage Comparator Design
Hi,
* An Opamp output - during normal operation - never touches the supply rails. It is designed to control/regulate the output voltage well within the supply rails.
* in opposite to this - a comparator output is designed to switch to the supply rail(s). It does not control/regulate the output voltage to stay somewhere in the middle of the supply rails.
Thus a comparator output specification tells you how close - in millivolts - it can switch to the supply rail(s).
In opposite to this with an Opamp you can see the specification how large a signal (maybe think about a sinewave) at the output can be.....not touching the supply rails.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
-
Advertisement
-
9th November 2018, 22:15 #18
Re: Voltage Comparator Design
Hi,
Also, it's related to the internal design of the op amp. Think of bipolar transistors in series and go adding up ~0.7V dropped across each BJT, if from V+ there are 3 BJTs (not to mention resistors perhaps, I think that the 741 has current sensing resistors inline with the output stage used for IC protection), that's 2.1V you'll never see at the output because it's being used to do something to make the op amp function. It's that and/or Vce voltage drops. The same applies to the negative rail swing/magnitude but the other way round, you have to sum an unwanted voltage rather than subtract a wanted voltage if you get me.
As well, true comparator outputs have a pull-up resistor to V+, the load has a resistance value too, and the comparator internally has a finite resistance to ground, so there's always a voltage divider there, whether the comparator is high or low, besides the current used which is lost as a voltage across the resistor. Good to have as small a pull-up as possible and as large a load as possible to get as close the the V+ rail as can be hoped for based on design/circuit limitations.
Hope I haven't said anything too dreadfully incorrect there, experts...
- - - Updated - - -
I am not an expert on this kind of analysis, so presumably wrong about some things, some-one more learned will correct my nonsense but the general idea will help to understand why the more paleolithic than merely monolithic 741 can't reach V+ or V- (Vcc and Vee). "rail-to-rail" op amps have a very low voltage drop of a few mV which is the VDS (RDSon) or Vce, due to unavoidable internal resistance of the MOSFETs or BJTs used.
This is a typical comparator output:
Pathetic analogy: They're both gggreat species (and apex predators in their respective niche environments), but even so, tigers aren't lions and vice versa...
Sorry for any confusing mistakes but hope the general idea helps to understand the devices you may or may not use...
+ Post New Thread
Please login