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semiconductor
Joined: 04 Apr 2003
Posts: 294
Helped: 3
Location: France
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 09 Oct 2004 18:53 Strange phenomenon with Op-Amp |
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I’m implementing this circuit: temperature controller
Orginal site: http://www.progsoc.uts.edu.au/~timj/temprat/
First, This is the differential circuit using Op-Amps.
I have soldered as it shows, all values of resister are the same as in this pictures. But I can not achieve the result as stated in this website. The differential amplifier always work whether in saturation or in cutoff state.
The output voltage can not swing between 1.07 and 4.3 V, it sends out 0V or 3-4V only. I don’t know why[/img]
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 09 Oct 2004 19:01 Strange phenomenon with Op-Amp |
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I never used it but... What if instead of LM335 use a voltage source as if is output of LM335 and try to simulate it, do you get what you want?
Try that!
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semiconductor
Joined: 04 Apr 2003
Posts: 294
Helped: 3
Location: France
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| 09 Oct 2004 19:08 Re: Strange phenomenon with Op-Amp |
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| djalli wrote: |
I never used it but... What if instead of LM335 use a voltage source as if is output of LM335 and try to simulate it, do you get what you want?
Try that! |
Yes, I Have used a voltage source instead of LM335. I use a resistor VAR so I can adjust the voltage sent out from pin 8 of LM324 U8C. But the phenomenon is still the same!!!
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 09 Oct 2004 19:41 Re: Strange phenomenon with Op-Amp |
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| First check R4, it looks you have comparator rather then amplifier. Next thing is that calculations on web page about voltages are done with R12=R13=47k not 33k.
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DarkJedi
Joined: 25 May 2004
Posts: 145
Helped: 9
Location: Lima, Perú
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| 09 Oct 2004 19:55 Re: Strange phenomenon with Op-Amp |
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| mmm try powering the opam not with ground and 12v, try using -12 and 12 it mustn't do much change but i have had some surprises myself wich i can't explain yet.... Crazy opams.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 09 Oct 2004 20:12 Re: Strange phenomenon with Op-Amp |
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| This can be throught. OP AMP's sometimes latchup when output is driven "over" negative supply rail.
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DarkJedi
Joined: 25 May 2004
Posts: 145
Helped: 9
Location: Lima, Perú
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| 10 Oct 2004 2:08 Re: Strange phenomenon with Op-Amp |
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How can it be a comparator if you have negative feedback???
What could be is that the gain is too much for the signal you are trying, if i were you i'll first change the suply to +/- 12, if it doesn't work, changfe R4 for a potentiomenter an vary the gain till you get the signal.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 10 Oct 2004 6:59 Re: Strange phenomenon with Op-Amp |
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| In case of broken R4 you have no negative feedback and amplifier becomes comparator.
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nicleo
Joined: 06 Sep 2004
Posts: 717
Helped: 60
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| 10 Oct 2004 13:37 Re: Strange phenomenon with Op-Amp |
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The circuit, after combined with other parts from the link, is given here for convenience. In my opinion, the circuit looks OK. If 33k is used, the gain will be 220/33 = 6.67. So, you should expect to see 1.53V ~ 6.20V instead of 1.07V to 4.35V. And, LM324 is a single supply opamp (earliest single supply opamp, claimed by Intersil). So, the circuit should work for +5V positive supply and 0V negative supply. I guess, most likely, it's the problem of connection when transferred to PCB, or defective componets.
Last edited by nicleo on 17 Oct 2004 7:38; edited 1 time in total |
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semiconductor
Joined: 04 Apr 2003
Posts: 294
Helped: 3
Location: France
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| 14 Oct 2004 23:11 Re: Strange phenomenon with Op-Amp |
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Dear members,
I have made the PCB for the following circuits and I can't not explain why it does not work.
When I measure the voltage at PIN 1 of LM324, it displays 3.000 V, when I measure the voltage at PIN 5 of LM324, it displays 2.500V
When I measure the voltage at PIN 8 of LM324, it displays 2.730V (I use this voltage as my reference voltage, instead of 2.500V as used in the TempRAT)
But When I measure the voltage at PIN 6 of LM324, it display only 2.720V. I can't not understand why? It leads to Up of LM324 is different from Un (about 0.2V different).
I hope at output PIN 7, it would be 1.3V (Difference voltage is (3.000 - 2.730) * 5 = 0.27 * 5 = 1.35V)
But exactly, it sends out 0V at output PIN 7. I think due the the difference between Un and Up.
I can't understand why.
My due date is coming soon and I really need your helps, urgely!
Thank you in advance!
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ozy_
Joined: 27 Sep 2004
Posts: 6
Location: ROMANIA
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| 14 Oct 2004 23:40 Re: Strange phenomenon with Op-Amp |
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In your last schematic from "TEMPSENS.JPG" the R7 resistor is wrong conected!
It must be conect to PIN 6 of CI, (at the negative input) . Not to Vref.
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Moof
Joined: 21 Nov 2003
Posts: 154
Helped: 6
Location: Argentina
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| 15 Oct 2004 3:42 Re: Strange phenomenon with Op-Amp |
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I work a little with op amps, and my experience is that you dont have to use lm324 for DC, cos this op amp has a very big offset.
If you change this op amp for another one, you will have what you spectate.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 15 Oct 2004 7:05 Re: Strange phenomenon with Op-Amp |
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| Your schematics tempsens.jpg if it is correct drawn is wrong. OP Amp U4B is connected as comparator not as an amplifier! Compare original schematics with tempsens and you will find the difference.
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vtt
Joined: 15 Oct 2004
Posts: 2
Location: SR, Vietnam
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| 15 Oct 2004 9:14 Re: Strange phenomenon with Op-Amp |
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| Borber wrote: |
| Your schematics tempsens.jpg if it is correct drawn is wrong. OP Amp U4B is connected as comparator not as an amplifier! Compare original schematics with tempsens and you will find the difference. |
I don't think so.
The result is the difference between tempsens and volref, and then the difference is multiplied with (R2/R1) to produce the result.
As stated by Semi, the multiplication factor is 5 and the result would be
5*(VSens-Vref)
But in fact, he does not receive this result. But I don't know why!
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vtt
Joined: 15 Oct 2004
Posts: 2
Location: SR, Vietnam
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| 15 Oct 2004 9:18 Re: Strange phenomenon with Op-Amp |
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| ozy_ wrote: |
In your last schematic from "TEMPSENS.JPG" the R7 resistor is wrong conected!
It must be conect to PIN 6 of CI, (at the negative input) . Not to Vref. |
Refer to Nicleo attachment, you will find that your comment is wrong@
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tutx
Joined: 04 Mar 2004
Posts: 149
Helped: 16
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| 15 Oct 2004 9:35 Re: Strange phenomenon with Op-Amp |
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| semiconductor wrote: |
Dear members,
I have made the PCB for the following circuits and I can't not explain why it does not work.
When I measure the voltage at PIN 1 of LM324, it displays 3.000 V, when I measure the voltage at PIN 5 of LM324, it displays 2.500V
When I measure the voltage at PIN 8 of LM324, it displays 2.730V (I use this voltage as my reference voltage, instead of 2.500V as used in the TempRAT)
But When I measure the voltage at PIN 6 of LM324, it display only 2.720V. I can't not understand why? It leads to Up of LM324 is different from Un (about 0.2V different).
I hope at output PIN 7, it would be 1.3V (Difference voltage is (3.000 - 2.730) * 5 = 0.27 * 5 = 1.35V)
But exactly, it sends out 0V at output PIN 7. I think due the the difference between Un and Up.
I can't understand why.
My due date is coming soon and I really need your helps, urgely!
Thank you in advance! |
To Semeiconducter:
I had the same your problem three year ago. Pls wait, I will give you the answer as quickly as possible. I don't remember exactly now because it happened a long time ago. But I think maybe we can use LM741 with the voltages V+ and V-.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 15 Oct 2004 10:21 Re: Strange phenomenon with Op-Amp |
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| Please compare tempsens circuit and circuit in first semiconductor post. The difference is obvious. In first circuit Vref is connected through 33k resistor to feedback resistor and inverting input of op amp. In tempsens Vref is connected to inverting input and thus feedback resistor plays no role, so op amp is connected as comparator and circuit must operate as semiconductor's troubles has been described.
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nguyenquoccuong
Joined: 21 Sep 2004
Posts: 4
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| 15 Oct 2004 11:15 Re: Strange phenomenon with Op-Amp |
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HI,
I think that R7 is wrong.
In theory, input current of op-amp equal zero. So that Voltage at pin 6 will be ~ Vref. As the same as the Voltage at pin 5 is 2.5V (divided by resistor netword R4 and R5). If the difference beetwin pin5 and pin6 is -0.2V. then Vout will be 0V.
You need connect R7 with pin6.
Do you think like me ?
Cuong
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 15 Oct 2004 11:40 Re: Strange phenomenon with Op-Amp |
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You are right. In connection like in tempsens where R7 directly connects output of U4B and Vref it plays no role in feedback loop and is redundant. Because Vref is practically connected to pin6 (very low input current of U4B) the R4, R5 and U4B (it has very large gain) elements forms a comparator where a millivolt input voltage to op amp causes it to saturate.
Circuit in firs post is correct and works correctly. Circuit in tempsens is absolutely wrong.
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Tornado
Joined: 26 Apr 2002
Posts: 340
Helped: 1
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| 17 Oct 2004 6:33 Re: Strange phenomenon with Op-Amp |
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What is the voltage at VCC? Is it 12V?
Tornado
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semiconductor
Joined: 04 Apr 2003
Posts: 294
Helped: 3
Location: France
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| 17 Oct 2004 6:56 Strange phenomenon with Op-Amp |
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to Borber and nguyen quoc cuong,
Both are right and I have solved my problem.
I'll make a PCB today and I hope that it would work properly!
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