a TI INA260 power/current/voltage sensor: how much reverse voltage is too much? - Page 1
Forum section: electronics / beginners

author: wylbur
author_rank: Contributor
timestamp: July 17, 2020, 09:59:07 pm
content: I'm integrating a TI INA260 current/voltage sensor into a project. datasheet:https://www.ti.com/lit/ds/symlink/ina260.pdfbreakout board:https://www.adafruit.com/product/4226I've discovered (ahem) that these devices are very sensitive to reversed polarity on their IN+ and IN- pins. I'm curious exactly how much they can take before failing.I'd like to put a reverse polarity protection mechanism in front of the INA260 to reduce the chance that I'll fry another one. Obviously if I'm putting a voltage & current sensor in the system, it's because I'm observing something, so the reverse polarity mechanism cannot create a voltage drop.My idea is to put a Schottky diode that shorts when the polarity is reversed (then it's the power supply's problem what to do with the short). YouTuber Jeri Ellsworth calls this the "explody diode" approach, and while it's not ideal for a variety of reasons she and others explain, it might work for my application.I set up a version of this circut, described in the attached DaveCAD. The INA260 IN+ and IN- would go in parallel with the positive line if I were to add it.I tried this circuit connected to the 5v rail of an old ATX power supply, and I put scope leads after the diode. If I understand the scope's image correctly (image attached, and I'm not at all sure I'm interpreting it correctly), the voltage is negative for <1ms, at which point the diode shorts and the voltage goes positive. I'm guessing that's the power supply's reaction to the short? There's probably a big current surge and the power supply's short protection takes over, and 10-12 ms later, the supply shuts off after another little negative flutter.Without the diode, of course, the voltage goes negative and stays there, and I have ample evidence that will kill the INA260.My question is whether there's a way from the sensor's datasheet, or something else, to figure out whether the diode is fast enough to prevent damage to the INA260? I'm tempted just to try it, but, well, when the INA260 goes, it takes everything else on the I2C bus with it, and I can't just throw good arduinos after bad, can I?thanks for any thoughts & suggestions for (only partially destructive) experiments -- wylbur.

author: StillTrying
author_rank: Super Contributor
timestamp: July 17, 2020, 10:41:56 pm
content: "very sensitive to reversed polarity on their IN+ and IN- pins."No, they can measure + and - currents, but you can't just use those current sense inputs floating, they have to stay at or above the INA260's GND."The INA260 IN+ and IN- would go in parallel with the positive line"Inseries, so that the current flows through them.The device is a bidirectional, low-or high-side, current-shunt monitor that measures current flowing through the internal current-sensing resistor.

author: wylbur
author_rank: Contributor
timestamp: July 18, 2020, 12:38:14 am
content: thanks for the clarification on what the IN+ and IN- are measuring. I knew how to wire those pins with respect to ground, and you're right, it's series (which I kind-of know but often confuse in words). This is the beginners' forum, after allyour schematic is helpful. Question: if the diode goes on the load side of the INA260, what happens if the polarity of the supply being measured on the left is reversed? (in your schematic it's marked as 5V) That current will flow from the 5V positive (reversed) through ground and back through the INA260's sense, thus killing it? Or I misunderstand?thanks again -- wylbur.

author: StillTrying
author_rank: Super Contributor
timestamp: July 18, 2020, 01:44:40 am
content: "I knew how to wire those pins with respect to ground"If you're keeping IN+, IN- and VBUS, between the INA260's GND < 36V itshouldn'tblow up."That current will flow from the 5V positive (reversed)"Yes IN+ and IN- can only survive down to 0.3V below GND, even when the INA260 is un-powered, -5V on IN+ and IN- would kill it.Do you need to protect it from the 5V (to 30V) input being connected in reverse, of course a diode in series with the 5V would do but then there's a varying 0.4V - 1V5 voltage drop depending on the current drawn. How much current do you need.

author: wylbur
author_rank: Contributor
timestamp: July 18, 2020, 04:38:43 pm
content: I blew it up because IN+, IN-, and VBUS ere well below GND. My hope for the diode in parallel across IN+ and GND is to avoid blowing up another one if the IN+ and GND get switched. I'm connecting IN+ and GND to leads that are a few feet long, and in a recent test, the ends were mislabeled. The catastrophe was very quick, and now I need another INA260.So to your second question, yeah, I am trying to protect against the 5-30 input being reversed, but in a way that does not have a voltage drop (such as from the diode in series). You note that the INA260 cannot tolerate -0.3v. From my scope reading as I tested the "explody diode" approach, -0.3v is just about what gets through when the IN+/GND polarity is reversed, though only for a few milliseconds.Current need: the application is a constant current load:https://www.eevblog.com/forum/beginners/yet-another-constant-current-load-q/My idea is to draw a constant current across the INA260 in order to log the voltage. An arduino twiddles the DAC that's controlling the op-amp+mosfet to vary the current draw so I can measure the voltage at different currents. I'm trying to think about power supply stability (see graph--it worked before I blew up the INA260). Maybe I need to think about a more robust voltage sensor?thx again -- wylbur.

author: StillTrying
author_rank: Super Contributor
timestamp: July 18, 2020, 10:32:02 pm
content: "Maybe I need to think about a more robust voltage sensor?"The INA's VBUS input should be fine, if you don't need high speed V readings you could protect it from -0.3 to +40V spikes with a simple RC filter, and a reverse schottky diodeWith the input supply varying 2V - 14V I can't think of a simple way to protect it from reverse connection, other than the fuse + reverse diode above, even the fuse is going to cause a small volt drop with increasing current. Use a keyed or polarized power connector.The ground connections will have to be good and low inductance to reduce inductive voltage spikes getting to the INA during times the load is a bad or intermittent connection.

author: wylbur
author_rank: Contributor
timestamp: July 19, 2020, 02:38:03 am
content: Thanks, this all makes tons of sense to me.Last question: in your second schematic, there's a capacitor in parallel with the diode. I've checked the schematic on the breakout board I'm usinghttps://cdn-learn.adafruit.com/assets/assets/000/075/134/original/sensors_ina260_schematic.pngand while there's a small cap across Vcc-GND, there isn't one between IN+ and GND. Can you tell me the purpose of the cap you've drawn, and what size it should be?Thanks again, this is all really helpful -- wylbur.

author: StillTrying
author_rank: Super Contributor
timestamp: July 19, 2020, 01:28:48 pm
content: "there's a capacitor in parallel with the diode... Can you tell me the purpose of the cap you've drawn,"The cap on the 2-30V input is just a little bit of extra protection from + or - voltage spikes getting to the IN+, IN- and VBUS inputs from the 2-30V supply, or static while the 2-30V is being connected (hopefully the correct way around).In theory,the voltage spike would have to enough energy tocharge the cap firstbefore it could move the IN+ voltage outside of its -0.3 to +36V range.For a one IC schematic I don't think that Adafruit one is very clearly drawn.

author: wylbur
author_rank: Contributor
timestamp: July 19, 2020, 03:46:31 pm
content: Thanks for the explanation of the cap's function. Around how big should it be? Outside of RC timing circuits, I have no sense of what size caps should go where.

author: StillTrying
author_rank: Super Contributor
timestamp: July 19, 2020, 05:17:38 pm
content: "how big should it be"I don't know! I think around a 100uF would be too inductive to protect against fast voltage spikes, so I'd probably use 10uF to 22uF.The protection needed on inputs and outputs that are going to be connected and disconnected changes massively between a one off experiment and a commercial product, google TVS diodes for just some of the gory details.

author: wylbur
author_rank: Contributor
timestamp: July 21, 2020, 03:08:45 am
content: 10-22uF sounds good, I'll try it and hope I never learn if it worksthx again -- wylbur.