So what is a resistor anyway?...the electrician asked! - Page 1
Forum section: electronics / general

author: MickeeYay
author_rank: Newbie
timestamp: August 19, 2024, 12:16:27 pm
content: We had a solar system installed recently. A consumption/energy meter was also installed.Becasue of cathedral ceilings we had to place the inverter on the opposite side of the house to the meter box. The 3 phase AC run through the basement was about 25 meters. All worked well.A few days later they came to install the consumption meter - the 3 phase Chint DTSU666. This meter communicates with the inverter using the RS485 interface. I had a read about the meter beforehand and found that for long RS485 runs, a dedicated RS485 sheilded cable was required. The installers came with what looked like SPEAKER WIRE.! I asked if that was for the consumption meter and they said yes, we use it all the time. I watched as the electrician wired up the meter with the CTs. He seemed to know what he was doing.Once the speaker wire was run and connected at each end....nothing worked. Lots of phone calls back and forth with the Sungrow technical guys resulted in nothing. Time caught up with them and they said they'd come back in a few days to finish.They arrived a few days later with a roll of 2 pair shielded RS485 cable...woohoo! The cable was run and terminated at each end and still no comms between the meter and the inverter. More frantic calls between Sungrow techs and the installers and it still didn't work. Then I overheard the installer on the phone say "yes, we put a resistor in", "no, you only gave us one resistor."With long distance runs, the RS485 needs a terminal resistor at each end in a duplex system to stop reflections and delays. They had only put a resisor at one end. The installer approached me and said they had a problem and needed to go get a component to make it all work. Having an electronics and radio background I asked the installer what value resistor he was after. Puzzled I would ask that, he said 120 ohm. I said I think I might have one. I found a small pack of 100 ohm 1/4 watt resistors and gave him one (RS485 terminal resistor can have a value between 100 and 130 ohms I think). He said to his fellow installer, "look at this bloke, he's got resistors".After terminating the other end with the 100 ohm resistor, low and behold, communications started happening. Hi five's all round. That's when the installer asked me, "what is a resistor anyway. What does it do?" I gave him a roundabout explanation but he seemed puzzled by what I said.But this all made me think, does an electrical installer need to ever deal with a 1/4 watt resistor and all the various resistances available.? What qualifications does a solar installer need to have? The installers did a great job and I was happy with the installation. Considering what happened, I did't want to be quick to judge but I thought they may know resistance only as shunts and rheostats.As they left, the installer approacehed me and said, "you couldn't spare a few of those resistors could you, we have a few more meters to install this week?" I gave him the whole pack and said you can get them form Jaycar if you need more.

author: EPAIII
author_rank: Super Contributor
timestamp: August 19, 2024, 12:23:43 pm
content: A resistor, anyway, is a device that turns electrical energy into HEAT energy, with 100% efficiency.Entropy at work, running the universe down one Watt at a time!

author: SlimEddie
author_rank: Newbie
timestamp: August 19, 2024, 12:24:54 pm
content: I don't think that most equipment installers are well versed in electronics at all, and they don't have to be. Usually there are comprehensive installation manuals that they have to follow, and the termination of RS485 would be described there. Seems like they didn't read the manual

author: Siwastaja
author_rank: Super Contributor
timestamp: August 19, 2024, 12:35:20 pm
content: Thanks for the story, this is something we have to deal all the time, being in business of producing a building automation smart controller, thing that connects to solar inverters, battery inverters, heatpumps, EVSEs - often by RS485.Even short RS485 links really should use proper termination on both ends. And yes, 100ohms works well in a pinch, especially if other end is 120 ohms it is unlikely to overstress the transceivers.A very small subset of electricians know RS485 and the requirement of termination resistors. But nearly no one knows aboutbiasing resistors, another thing RS485 needs; only true RS485 experts know about this; actual product therefore run with very poor noise margins and mostly by luck.Grounding is also misunderstood; RS485 requires a ground connection, explicit third wire is the best but at very least parasitic through mains earth.Add the mess of A/B naming mixup and it's PITA to get working. Electricians can't just wire it up, they need to test and try things like swapping A&B until it works. If they have other uncertainties, it adds combinations to test. For example some devices require enabling RS485 from 2-3 different configuration pages. Unless you have very good instructions, what you are going to do, click random things, test, swap A&B, test again, click random things again? Hours of expensive electrician time used.Most RS485 installations are against manufacturers instructions - and manufacturers often fail to give instructions, even when they should give those same terms that were used in their EMC qualification tests. As a result, RS485 based systems are equivalent to random hobbyist circuits on their table - work by luck, with no noise margins, and not EMC compliant (therefore illegal).And some products have integrated termination resistors; sometimes configurable (e.g. by shorting two terminals together), sometimes fixed so that it's always there want it or not. And sometimes the manufacturers forget to tell anyone there already is the resistor.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 19, 2024, 12:38:09 pm
content: Quote from: SlimEddie on August 19, 2024, 12:24:54 pmMy pet peeve: manuals that do not mentionanythingabout it. Is there going to be a fixed resistor inside the device "for making the install easy", and someone thought "let's not confuse people by mentioning it at all"? Or did the manufacturer forget to mention the need of the resistor because they thought "oh, this is not our responsibility, of course every installer knows that resistor is needed".That is why we write separate (one- or two-page) install instruction leaflets for every brand and model of inverter. We reverse-engineer (by taking a simple multimeter reading) if the resistor is there or not, then write the instructions accordingly.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 19, 2024, 02:38:33 pm
content: In my experience, "electricians" and installers are very good at following written instructions and a written code book (such as the NEC) butveryfew of them understand what they're doing, or more importantly, why.One of my early experiences I was asked to go help the plant "electrician" install a large 220VAC powered base board heater in manufacturing plant. He had already installed the heater and all of the wiring (very neatly I might add). But he was afraid to connect to two. He was absolutely convinced that I was going to short circuit the building's 220V power mains when I connected them and turned on the heater. As much as I tried to explain it to him, he just couldn't grasp the fact that a heater is acontrolledshort circuit and that's why it produces heat. But he did absolutely great work when it came to cook book type stuff like running wiring and bending conduit and putting together an electrical infrastructure.

author: tszaboo
author_rank: Super Contributor
timestamp: August 19, 2024, 03:22:08 pm
content: Good question, why the inverter doesn't come with either1 Termination resistors built in and switchable/jumper-able2 Instructions.I have to deal with RS485 and installers, it's all the time:- A/B swapped- GND missing- Termination resistor missing- And my favorite: We connected them together without checking with you first if it would even work, can you please change your firmware?Now I just build 120Ohm (and failsafe biasing) into the device by default. It's unlikely they would place 3 termination resistor, and then I think it would still work.

author: themadhippy
author_rank: Super Contributor
timestamp: August 19, 2024, 03:47:55 pm
content: Quotewe certainly used to on stuff like fire alarm systems. Maybe times have changed,but as part of my collage course we studied basic electronics ,and as part of the final practical exam to become qualified had to build a simple circuit from a circuit diagram.

author: AVGresponding
author_rank: Super Contributor
timestamp: August 19, 2024, 05:22:02 pm
content: Quote from: MickeeYay on August 19, 2024, 12:16:27 pmFuck my old boots... shit like this makes me embarrassed to admit to being a sparks. How they can get through their course without understanding that thewiresthey just installedareresistors, and the resistance ofthemis what you're measuring with an installation tester, is beyond me.As themadhippy mentions, resistors are common in (conventional radial zone, not so much in loop addressable) fire alarm systems as terminators, and some systems use RS485 to communicate between the main panel and repeaters.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 19, 2024, 06:51:48 pm
content: Quote from: tszaboo on August 19, 2024, 03:22:08 pmThis is so close to my experience I could have written the same word-to-word.As it happens, in our design I have this three-position switch: termination only, nothing, and termination+bias. And we set it to T+B in production, and remind to set to T+B in install instructions anyway. Our product is master 99.5% of the time, and at the end of the bus 99% of the time, so could have just hardwired termination + biasing, though, but it's good to avoid painting one into a corner, hence the switch.While blame game is sometimes fun, I don't truly know whom to blame for all the A/B confusion. Most Chinese manufacturers of actual products seem to follow the same naming as transceiver manufacturers mapping A->A, so if you try not to overthink it and design your product similarly, naming per transceiver pins, no extra signal inversions between the typical MCU UART peripheral and the transceiver, then it will work with most of the devices A->A, B->B. Except when some unique snowflake product pops up designed by someone who tried to read the standard, or the Wikipedia explanation thereof, and got it the opposite way (my head still hurts when I try to decipher if that is correctliterally by standard, or not).I also absolutely love those guys at the sales companies who sell our product. They never check the specifications or ask us about which devices are supported. So we get a call from a sparky (often very capable people) who is at a customer site, with a new whatever Growatt hybrid inverter this month, sold to a customer to be controlled by us. That's fine, sales is sales, and this forces me to add a driver which then simplifies future sales, but it's a very hectic and stressful way to deal with it.And the customer is calling all around and refusing to pay the bill (obviously) for a nonfunctional system. It would be so much easier if the sales folks gave us a quick call along these lines:"We have a new inverter we want to sell, how about we send you one for testing, we are expecting annual sales of x?"Writing the driver itself is anything from 2 hours to a week depending on the quirks of the product, but that is pretty damn hard to do if the only unit in existence is at some remote North-Eastern Finland 2-hour drive away from the folks who installed it, so that me calling "can you swap A and B" during firmware development becomes quite expensive pretty soon.And installers need to install these unsupported devices "blind". If only had they given meone freaking dayto prepare for it, I could have made a very simple piece of test code polling one known register and worked on the phone with the sparky swapping A&B for my heart's content while they are there, on-site.And to us, these customers are only worth a few hundreds of €. Luckily most of them require very little care and luckily out of the difficult cases come improvements in firmware, such as new supported devices. Necessity is a damn good motivator and customers like this pretty much make sure I don't need to think about what to prioritize, they took care of it for me.

author: MickeeYay
author_rank: Newbie
timestamp: August 20, 2024, 02:24:15 am
content: Quote from: SlimEddie on August 19, 2024, 12:24:54 pmI think the issue here is that these installers have only ever had short runs for the RS485 interface connections. That's how they've been getting away with using speaker wire. In Australia most solar inverters are installed only metres away from the home's main electical fuse and meter box. But they've learned something at my place - a 25 metre run needs more understanding of how RS485 transmission works.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: August 20, 2024, 04:38:53 am
content: Quote from: MickeeYay on August 20, 2024, 02:24:15 amThey're getting away with it because of a lack of nearby interference. I've had a 100m+ RS485 bus ruined by 30cm of Figure-8 wire. Replaced that short 30cm section with twisted pair, everything worked (the rest of the bus was twisted pair already, of course).

author: Brumby
author_rank: Unknown
timestamp: August 20, 2024, 06:31:08 am
content: I once supervised the installation of a sound system with 30m runs of cable between microphones and mixer. The installers rocked up with figure 8 shielded audio cable. I asked them if that was what they were going to use? They replied yes - they use it all the time. I expressed my doubts, but since the specific type of cable had not been specified in the quote, I had to let them run it.After the install, guess what..... Interference galore. I spent a couple of hours with them trying to make it work, but to no avail. I was not surprised and it was clearly unacceptable. They then brought in a roll of proper microphone cable and rolled it out completely, offering the greatest exposure to environmental EMI. I smiled, terminated it and - lo and behold - Silence!!To their credit, they did pull out and replace all 16 channels worth with the right stuff - at their cost - and the result was what it should have been from the outset. I did feel for them on one of the runs... There was a manufacturing fault in the cable that had a short somewhere along its length, so they had to replace that one once again.I daresay they made no profit on that job - but I give them full credit for making sure the job was good, without being difficult.

author: Haenk
author_rank: Super Contributor
timestamp: August 20, 2024, 10:28:29 am
content: Quote from: themadhippy on August 19, 2024, 03:47:55 pmI can certainly say, that's also used in our Siemens alarm system - called "Alarmschleife", which needs to be terminated with around 3k.