Problem with 4-20mA loop - Page 1
Forum section: electronics / projects

author: Mark
author_rank: Frequent Contributor
timestamp: November 03, 2023, 03:57:48 pm
content: This circuit (attached) takes a 0-1mA input and converts it to 4-20mA loop powered at 24V.Some BOM items:IC1 = LM358NCR2 = 1N5300 (1.30mA Current Regulator Diodes)Q1 = HARRIS D44C2 or SOLID STATE D44C8CR3 = 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 beenin productionfor 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 LM358NST MICROELECTRONICS LM358NFAIRCHILD SEMICONDUCTOR LM358N

author: nfmax
author_rank: Super Contributor
timestamp: November 03, 2023, 04:09:45 pm
content: 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.

author: Mark
author_rank: Frequent Contributor
timestamp: November 03, 2023, 04:14:03 pm
content: Quote from: nfmax on November 03, 2023, 04:09:45 pmI 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)?

author: pickle9000
author_rank: Super Contributor
timestamp: November 03, 2023, 06:14:00 pm
content: Any overshoot on the power supply?

author: Mark
author_rank: Frequent Contributor
timestamp: November 03, 2023, 06:34:05 pm
content: Quote from: pickle9000 on November 03, 2023, 06:14:00 pmNo 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.

author: IanB
author_rank: Super Contributor
timestamp: November 03, 2023, 06:46:16 pm
content: Quote from: Mark on November 03, 2023, 03:57:48 pmIf 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.

author: pickle9000
author_rank: Super Contributor
timestamp: November 03, 2023, 07:38:32 pm
content: 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.

author: Miti
author_rank: Super Contributor
timestamp: November 03, 2023, 08:17:46 pm
content: Did it work for decades with the same fast rising power supply or this is new?

author: Mark
author_rank: Frequent Contributor
timestamp: November 04, 2023, 06:18:45 am
content: 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.

author: Mark
author_rank: Frequent Contributor
timestamp: November 04, 2023, 06:35:17 am
content: 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.

author: Mark
author_rank: Frequent Contributor
timestamp: November 07, 2023, 12:51:53 pm
content: 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?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 07, 2023, 12:59:05 pm
content: 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.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 07, 2023, 07:44:52 pm
content: What is a resistance value between +24Vin and +Vred?

author: Mark
author_rank: Frequent Contributor
timestamp: November 07, 2023, 07:57:39 pm
content: Quote from: Vovk_Z on November 07, 2023, 07:44:52 pm0-600 ohms. I am testing it with 9 ohms.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 07, 2023, 09:01:19 pm
content: From the TI LM358-N datasheet:QuoteIt 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 aschottkygermanium diode would be better?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 09:30:03 pm
content: Quote from: Mark on November 07, 2023, 12:51:53 pmCheck the input voltages of IC1B.Here is a nice scope shot of the inversion effect in vanilla opampshttps://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/447545/opa-output-inversion-during-input-overdrive

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 07, 2023, 10:18:55 pm
content: Quote from: Mark on November 07, 2023, 07:57:39 pmThen what is "-Vout" pin?

author: Mark
author_rank: Frequent Contributor
timestamp: November 08, 2023, 09:47:38 am
content: Quote from: Vovk_Z on November 07, 2023, 10:18:55 pm+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.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 08, 2023, 10:14:12 am
content: 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.