Understanding ground in HVDC motor controller at mains potential - Page 1
Forum section: electronics / beginners

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 13, 2025, 01:59:53 am
content: I learn a lot from looking at circuits in magazines like Practical Electronics and Elektor. In july 2025 issue of PE they present a 230V DC motor controller, and I have spent a few hours understanding the schematic. I am showing the part that relates to my question below (the skipped part to the left are just opamps)In the article they explain that the whole circuit operates at mains potential:I am trying to wrap my head around that the positive terminal of the BR1 is connected to the +12V net coming from "7812 REG1". Does that mean that +12V is "lifted" to mains potential, thus is it at 230+12=242V with reference to negativ terminal of BR1?Is it correct to say that the negativ terminal of BR1 forms a virtual ground together with ground pin(4) on IC5, which is an optoisolator, pin 4 being the emitter of the output transistor?Then, if I measured the voltage between that virtual ground and the circuit ground, i.e. between pin 4 and 3 on IC5, what would I get?

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2025, 07:14:35 am
content: That whole circuit is a mess of mis-applied 'ground' symbols.(earth ground) should be(signal ground) throughout that diagram.Seehttps://www.ni.com/en/support/documentation/supplemental/18/chassis--earth-and-signal-grounding--terminology-and-symbols.htmlWith that out of the way, signal ground is indeed not at earth ground potential, due to the link between '+12V' and the high-side DC bus from the mains voltage bridge rectifier feeding the motor. Loosely speaking its at mains potential from a safety viewpoint, i.e. the whole board must be treated as a mains live conductor, however if you were to scope it (assuming an appropriately CAT rated scope and probes were available - not usually the case) with respect to earth ground you wouldn't see a nice neat 230V level on '+12V', nor even a 230V RMS 50Hz sine wave (with or without the usual 'flat topping' distortion found on the mains supply). Assuming your supply's Neutral conductor is grounded *somewhere* upstream in the supply, the main bridge rectifier's positive terminal will have a positive going half-wave rectified mains waveform on it with respect to earth ground, with a complimentary negative going waveform on the bridge rectifier's negative terminal, giving full-wave rectified mains between them. Signal ground is 'floated' 12V more negative than this. See attached waveforms.N.B. Probing mains circuits with a scope and/or probes that do NOT have an appropriate CAT rating for the job is destructive of equipment and potentially suicidal. If you aren't 100% certain that your test equipment is appropriately rated and you have the skills and training to do so safely *DON'T* do it!For that reason, the waveforms are from a simulator, not a real scope screenshot.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 13, 2025, 08:56:03 pm
content: Quote from: Ian.M on July 13, 2025, 07:14:35 amI'll continue a bit...The whole schematic is a big mess. The first indication to me are the silly wire hops. These are generally cone by people with very limited electronics knowledge. But apparently this is printed in a magazine, which makes it quite weird. Maybe it's a magazine targeted at beginners? (But in that case, It's not the brightest light who publishes an article which is so intertwined with mains voltage. Next in line are the quite non standard diode symbols with those big circles and colored background around them. Why??? And why are the RefDes's removed for all resistors and capacitors? That's another non-standard weird practice. RefDes is quite important for correlating schematic symbols with PCB parts and the BOM.Next...The schematic is quite hard to read / decipher, and for a big part it's because of CON1. This sort of forces the wiring mess around the IGBT and the three relays.I tried to formulate a more decent answer a bunch of hours ago, but I could not get a good overview. Now I'm looking at it again, and I wonderwhy they tied the +12V to the positive output of BR1 in the first place. It forces them to use a power wasting voltage dropper and 15V power zener (ZD2, close to IC5, the TLP5701).You can go ask yourself why they did not just connect the "GND" of the three voltage regulators to pin 4 of the TLP5701. Also, the TLP5701 has an optocoupler to isolate the input from the output. And then they connect these together again with the wire bridge over CON5 / CON6?At some point I thought CON5/CON6 was just for testing, and open during normal operation, but it seems to be needed as the power supply for the TLP5701. Maybe it's clearer if you read the text, but I don't have it.As it is, I recommend you burn this magazine (or at least ignore this article) and go searching for a more "normally" built motor controller. It looks to be for a brushed DC motor, and this type of motor is very easy to control with PWM and some power stage. These motors are common in treadmills and in cheap chinese milling machines (but the motors in the milling machines at least tend to be of miserable quality, with a bit of bad luck they burn out after a few weeks of moderate use).

author: floobydust
author_rank: Super Contributor
timestamp: July 13, 2025, 09:20:40 pm
content: The entire board is hazardous live! Just say that 100 times. It says "12VDC" but that's on top of mains voltage.The original article 180-230V DC Motor Speed Controller is by Silicon Chip MagazineJuly,August, small errata September 2024 (it's a 12-0-12V transformer not 15V). D1 is RURG3060-F085.I forgot when they merged with Practical Electronics.

author: Whales
author_rank: Super Contributor
timestamp: July 13, 2025, 10:36:39 pm
content: Quote from: erikbrenn on July 13, 2025, 01:59:53 amBe wary of AC vs DC when adding voltages.230VAC + 12VDC = 230V of sinewave with a 12V offset (NOT 242V, peaks will actually be most of 400V)230VAC + 0VDC = 230V of sinewave with 0 offsetThe difference between these two voltages is a stable 12VDC, so electronics connected between the two will be happy.The difference between any of these voltages and earth ground (including your house's flooring and your feet) is 230VAC with an extra bit of DC on top. Any electronics (or people) connected between the two will be unhappy.

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 15, 2025, 02:09:18 am
content: Thanks a lot for your spice simulation, useful.> That whole circuit is a mess of mis-applied 'ground' symbols.Well, they have clearly differentiated between signal ground and earth ground, so I see no problem with that (If you look at mains plug the earth ground has a different symbol)> N.B. Probing mains circuits with a scopeyes, I have an isolation transformer AND Dave's high voltage differential probe thank you.Also, as you can see from the right-hand side of the circuit there is a big warning sign clearly stating that the whole circuit is at mains potential.> You can go ask yourself why they did not just connect the "GND" of the three voltage regulators to pin 4 of the TLP5701.Well, I did, and immediately saw that you can't do that because you would have mains voltage across the opamps (because BR1+ to +12V). So the real question is why do they choose to lift the whole circuit to mains potential? And the reason for that is that they are feeding back the negative side of the motor (collector of IGBT) into the opamp circuitry to measure back-EMF, and also the +/mains terminal of motor to measure current. Seems to be the main feature of this motor controller. (This is not a universal motor controller, which doesn't have back-EMF).> Now I'm looking at it again, and I wonderwhy they tied the +12V to the positive output of BR1 in the first place. It forces them to use a power wasting voltage dropper and 15V power zener (ZD2, close to IC5, the TLP5701).This is explained in the article. The resistors allows about 5mA to drive the zener providing 15v supply to the optocoupler, and this is required to fully turn on the IGBT or low resistance. Seems like a clever circuit to me, and the vasted energy is very small compared to what happens in the motor anyway.> As it is, I recommend you burn this magazineAbsolutely not. You haven't read the whole article so I guess you could be excused for saying that. I would encourage anyone that wants to learn electronics to buy this magazine. They have elaborate descriptions of their circuits. This magazine (SiliconChip) has been in print for ages and the authors seems very skilled, but what do I know... The article is 8 pages long and everything is explained in details (except perhaps for the grounding that was reason for my questionThe motor controller from this article is based on a well known lathe controller which has been extensively documented in a pdf here:https://pemag.au/link/abmnI am attaching a couple of other pages from the article so you have a bit more context, don't want to break copyright. The article will continue in the next issue where they will explain building, configuring and testing in detail, so it all seems very professional to me. Personally I find the color encoded schematics pleasent to look at, but I can understand that it breaks with traditionCheersPage1Page2

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 15, 2025, 07:20:50 pm
content: Quote from: erikbrenn on July 15, 2025, 02:09:18 amYes, they did that, but they also did that "wrongly". The convention is to use the triangular symbol with shorter bars on the bottom for PE.https://duckduckgo.com/?t=h_&q=protective+earth+symbol&ia=images&iax=imagesQuote from: erikbrenn on July 15, 2025, 02:09:18 amMaking it work that way would require a redesign of some other sections of the project.I see a potentiometer for the speed adjustment. There is probably mains voltage on that thing too, so make sure it has a plastic shaft (Should also be mentioned in the article).The whole design looks as a straightforward adaptation of an commercially available (probably chinese) circuit, or a circuit from the '80-ies. (I really did write that a few minutes before I saw the link to the Sieg motor controller PDF).It really would not cost much extra to create a solid galvanic barrier between the high power section and the whole control section.The TLP5701 already has a an isolation barrier, and instead of the current shunt a sensor like the ACS712 could be used.Compare the motor section from your sillycon magazine with the way it's drawn in the SIEG pdf. (See below). It also has wonky colors, but it is very easy to see all the motor connections, how the shunt is used in the circuit, and how most of the speed control works. The PWM generation is probably on another page. I still don't really understand why they connected +12V of the power to the rectified mains. To me it looks more logical to connect one of the sides of the power shunt resistor to the "GND" center tap of the opamps.This is a circuit that really begs for an isolation transformer during testing. Then you can connect the center tap of the opamp supply to the PE of the same extension cord as your oscilloscope. You can also simply completely remove the GND clip from your oscilloscope, and then only rely on the "GND" connection via the extension cord. Because of the big GND loop bandwidth is limited and you pick up a bunch of noise, but for relatively low frequency stuff such as this it's plenty good enough. You can also put some kind of plastic (freezer) box over the parts of the ciruit that are potentially dangerous to probe.