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| Wayward DC currents |
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| ebastler:
--- Quote from: paulca on May 02, 2023, 01:24:09 pm ---I suppose the only way to stop this is use isolated DC/DC converters? --- End quote --- Hmm, I would have thought that you want a common ground after the DC/DC converters. Are their respective negative outputs connected? (The polarity of the left DC/DC output seems wrong, but that's most likely just a glitch in the drawing, right?) |
| paulca:
Yes, drawing glitch. The voltages at all points check normal. 13.00V out of the PSU with the battery, 12.99V across it, 19.50V going out to the laptop, just un-matched but mostly symmetrical current. Voltage between them checks out too. 0V between their negative posts. 6.50V between their positive. Checks. I suppose it is just alt paths. The only concern is the route they end up taking involving monitor cables and USB hubs. Is it any wonder my audio sounds like I'm in a room with a bunch of dot matrix printers in the background? The only concern of course is that neither current limit function is fully aware of what is really going on. I also have concerns about one of the PSUs suddenly disconnecting and the 19.50V managing to also migrate back up stream. I have a diode across one of the PSUs, the other fell off ages ago, not sure they would help with the later concern though. |
| paulca:
Also, it points a finger at one set of cables as having a high enough "-" impedence that the current has felt it necessary and easier to travel so far. As I also got a resistance tester today I might be able to check that. |
| paulca:
So I am re-doing the whole lot to use an inverter to bring 240V up to the office where I can convert to DC and power the same DC loads at about half the efficiency. Granted it means I can far more easily power those devices which aren't easy to surrogate a power supply for. In the process of doing this, the only direct DC loads on the system where reduced to the "Admin" rail on the solar power system itself. A Raspberry PI, two ESP32s and a 2.4Ghz travel wifi router. 4 watts. What does the charge controller who load output I am using see? 0W. 0.0A. Why? The DC current is returning via the RaspberryPI, the USB interface to the inverter and back to the battery and not via the "Load shunt" in the charge controller. Accepting this is just part of the parcel with DC it does raise concerns with making "DC power buses" in the first place. Consider a use case I was considering. Battery -> Inverter -> DC Brick -> 48VDC -> Fan-out DC power with appropriate boost/buck to power 5V, 12V, 20V DC power outlets around the office. The 48VDC 10A power brick won't even be impressed with the 100W load or so. It will barely notice the 2A of current and it's fan might hit 20% duty. Let's say I know turn on the DC/DC bench PSU to work on a micro-controller project. When I connect the MCU to the USB stuff a DC current path is formed and some much larger current decides to source itself through that USB connector and via my little weedy 100W bench supply. It might appear my MCU is pulling 2 Amp! Not least will the current limiter go nuts and be completely unable to stop the 2.0A, but in some cases even switching the PSU off by it's single pole switch results in it remaining on and displaying voltage AND current. It would suggest that putting the AC/DC conversions close to the devices might have advantages as those DC sources are usually "decoupled" or at least partially isolated. Device to device currents probably can still exist, but there is a much more limited set of pathways it can take. Have I just stepped down a rabbit hole? Is it worth anything other than frustration going further? EDIT: "Load shunts". Low side "load shunts". Basically this comes down to who has the smallest load shunt R value or better yet, as a DC current I would be looking for the device which has NO shunts at all! |
| tautech:
--- Quote from: paulca on May 02, 2023, 03:36:11 pm ---I suppose it is just alt paths. --- End quote --- Certainly but at any time did you measure with a LoZ DMM ? |
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