Author Topic: Problem with 4-20mA loop  (Read 1317 times)

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Offline MarkTopic starter

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Problem with 4-20mA loop
« on: November 03, 2023, 03:57:48 pm »
This circuit (attached) takes a 0-1mA input and converts it to 4-20mA loop powered at 24V.

Some BOM items:
IC1 = LM358N
CR2 = 1N5300 (1.30mA Current Regulator Diodes)
Q1 = HARRIS D44C2 or SOLID STATE D44C8
CR3 = LM329BZ (6.9V Precision Voltage Reference)

As you can tell from the scan, this is old.  It has worked for decades, but now it draws around 90mA when powered up from a fast-rising supply. 
A slow ramp up to 24V does not show the issue. 

I don't want to muddy the waters at the moment, but does anyone have any pointers? 
I had thought it might be latch-up or output phase reversal but I'm not sure now. 

thank you. 

Edited to clarify: this circuit thas been in production for decades without issue.  Only recently the problem has been observed on newly built circuits.  Looking at the parts list, a few things have gone obsolete over the years and replaced by equivalents (mostly same part number, different manufacturer)

Alternatives are listed for IC1:
NSC LM358N
ST MICROELECTRONICS LM358N
FAIRCHILD SEMICONDUCTOR LM358N
« Last Edit: November 04, 2023, 06:39:33 am by Mark »
 

Offline nfmax

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Re: Problem with 4-20mA loop
« Reply #1 on: November 03, 2023, 04:09:45 pm »
Check the value of C2 - it looks as though it's there to slow the rate of rise of IC1's supply. Most OPAMPS go through funny states as they power up.
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #2 on: November 03, 2023, 04:14:03 pm »
Check the value of C2 - it looks as though it's there to slow the rate of rise of IC1's supply. Most OPAMPS go through funny states as they power up.
I tried adding another 1.0uF across the op amp but no joy. 
edit: even another 10uF doesn't fix the problem. 

Even if it worked, wouldn't extra capacitance slow the response of the circuit (to the 0-1mA input)? 
« Last Edit: November 03, 2023, 04:20:23 pm by Mark »
 

Offline pickle9000

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Re: Problem with 4-20mA loop
« Reply #3 on: November 03, 2023, 06:14:00 pm »
Any overshoot on the power supply?
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #4 on: November 03, 2023, 06:34:05 pm »
Any overshoot on the power supply?
No overshoot.  The 90mA issue is observable down to about 17V with a fast rising edge on the power supply. 
With <17V it starts up with the expected 4mA. 
 

Offline IanB

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Re: Problem with 4-20mA loop
« Reply #5 on: November 03, 2023, 06:46:16 pm »
As you can tell from the scan, this is old.  It has worked for decades, but now it draws around 90mA when powered up from a fast-rising supply.
A slow ramp up to 24V does not show the issue.

I don't want to muddy the waters at the moment, but does anyone have any pointers?
I had thought it might be latch-up or output phase reversal but I'm not sure now.

If the physical device worked for decades, but now it doesn't, then you are looking for a mechanical failure like a bad solder joint, or a component failure like a capacitor drying out.

Your approach should be to carefully examine all solder joints with a good magnifier and reflow any suspect ones. Then, start replacing components one by one based on the probability of failure. You may need to remove them from the board and test them, or just replace them regardless. Start with the cheaper components that are more likely to have age related failures. Semiconductors probably work forever, so they would be last on your list.
 

Offline pickle9000

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Re: Problem with 4-20mA loop
« Reply #6 on: November 03, 2023, 07:38:32 pm »
Before doing anything. Take some good images of the board both sides. You can repeat the problem so that is really good. If you have a thermal camera see whats heating up, otherwise use your finger. Give the board a good cleaning but don't submerge is spray iso and blow if off. Don't test until dry.

Test between every change.

 

Offline Miti

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Re: Problem with 4-20mA loop
« Reply #7 on: November 03, 2023, 08:17:46 pm »
Did it work for decades with the same fast rising power supply or this is new?
Fear does not stop death, it stops life.
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #8 on: November 04, 2023, 06:18:45 am »
Thanks for the reponses, I edited the first post to clarify that the circuit has been in production without issue for decades until now. 
I'm wondering if we have a bad batch of LM358N. 

 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #9 on: November 04, 2023, 06:35:17 am »
If I reduce the supply voltage I get to around 18V when this happens:

The op-amp supply voltage spends time on a plateau at 7.6V before reaching maximum voltage of 17.4V. 
I'm using a signal generator pulse to drive a transistor to power the circuit on and off (from a bench power supply). 
When I see the phenomenon in the image below, the behaviour is very temperature sensitive. 
Slightly cooling the LM358 op-amp makes the op-amp voltage rise correctly and the circuit takes 4mA as it should. 

Slightly warming the LM358 makes the plateau extend.  When warmer, the op-amp supply voltage stays stuck at 7.6V and the circuit takes ~90mA. 
« Last Edit: November 04, 2023, 01:42:41 pm by Mark »
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #10 on: November 07, 2023, 12:51:53 pm »
Thanks for the replies and ideas so far. 

Does anyone have any suggestions of what to measure or test next? 

Testing parts? Trying different op-amps? 
 

Online Doctorandus_P

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Re: Problem with 4-20mA loop
« Reply #11 on: November 07, 2023, 12:59:05 pm »
You only have a few possible current paths. If you hack in a few shunt resistors (about 1 Ohm) then you can measure where the current is going though, and you can either confirm or exclude things like latchup.

Do the inputs of the opamp ever get higher then it's supply voltage? That can be a reason latchup occurs. And for a generic LM358, try some other brands?

Note: I did not look very closely at your schematic because it's picture quality is so bad.
« Last Edit: November 07, 2023, 01:02:45 pm by Doctorandus_P »
 

Offline Vovk_Z

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Re: Problem with 4-20mA loop
« Reply #12 on: November 07, 2023, 07:44:52 pm »
What is a resistance value between +24Vin and +Vred?
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #13 on: November 07, 2023, 07:57:39 pm »
What is a resistance value between +24Vin and +Vred?
0-600 ohms.  I am testing it with 9 ohms. 
 

Offline Andy Watson

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Re: Problem with 4-20mA loop
« Reply #14 on: November 07, 2023, 09:01:19 pm »
From the TI  LM358-N datasheet:
Quote
The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3 V (at 25°C).
It does not say what happens when the input(s) exceed -0.3V, however, your table of voltages suggests that Vout of IC1B is not what it should be., i.e IC1B is not functioning as intended. CR5 - 1N4933 - does not prevent the input voltage from exceeding the -0.3V level. Perhaps a schottky germanium diode would be better?

« Last Edit: November 10, 2023, 09:46:35 pm by Andy Watson »
 

Offline PCB.Wiz

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Re: Problem with 4-20mA loop
« Reply #15 on: November 07, 2023, 09:30:03 pm »
Thanks for the replies and ideas so far. 

Does anyone have any suggestions of what to measure or test next? 

Testing parts? Trying different op-amps?

Check the input voltages of IC1B.

Here is a nice scope shot of the inversion effect in vanilla opamps

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/447545/opa-output-inversion-during-input-overdrive

 

Offline Vovk_Z

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Re: Problem with 4-20mA loop
« Reply #16 on: November 07, 2023, 10:18:55 pm »
What is a resistance value between +24Vin and +Vred?
0-600 ohms.  I am testing it with 9 ohms.
Then what is "-Vout" pin?  :-//
 

Offline MarkTopic starter

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Re: Problem with 4-20mA loop
« Reply #17 on: November 08, 2023, 09:47:38 am »
What is a resistance value between +24Vin and +Vred?
0-600 ohms.  I am testing it with 9 ohms.
Then what is "-Vout" pin?  :-//

+V RED and OUT-(Black) are the two pins available for customer connection. 
A load resistor of between 0 and 600 ohms is connected between +V RED and a +24V supply and is used to convert the current back to a voltage. 
 

Offline Vovk_Z

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Re: Problem with 4-20mA loop
« Reply #18 on: November 08, 2023, 10:14:12 am »
Ok. So, you may use 1000 uF capacitor across the rails. It will give us Trise= 9 R * 0.001 F = 0.009 s = 9 ms.
Or use e.g. 47 R so Trise= 47 R * 0.001 F = 0.047 s = 47 ms.
 


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