A kind of power distribution unit for low voltage DC, design idea, guidance needed

Thread Starter

SaleB

Joined Jul 3, 2017
7
Like so many of us I have many device that are connected through a wall charger/adapter in the range of 3,3-20v using 0.15-3A current. I intend to combine all of them in a single box unit using one AC voltage source (220-230V@50Hz). I have already brought a high current (25amps) smps power supply which I have used with a multiple step down voltage regulators for a kind of test phase.

Now I would like to extend to safety and usability of that setup. There is a nice idea somewhere on the web, someone has made a pcb that precisely mounts on the screw terminals on the metal enclosed smps power supplies. I would like to extend on that idea, to make a box for the pcb that has a C14 socket double pole switch and a power meter on the ac side. On the DC side I would like to make some type of inexpensive medium current connection (5A) which could be freely obtained. First I tough of using some of these connectors used for RC car batteries, but they are all cable to cable, so it will most likely be some Molex Mini-Fit Jr or similar, pitch 4.2mm is rated at 13A which is enough even if the cable is not properly crimped. Seems to me that the connector is to big, so if anyone has an other idea, it is most welcome. It needs to be that the connector must have the locking clip and some orientation marker, and there should be readily available horizontal pcb connector male connector.

On the DC side I also intend to have a current measurement for each channel (probably 12 channels) and a relay for each channel. All the control signals would go over a i2c capable multiplexer to an arduino (probably some circuit that already has a library made by someone), and all the current sensors should be INA219 (according to datasheet there are 16 i2c channels available).

On each cable to the end device there will be a small box with a step-up (XL60009 based boards) or step-down (LM2596 and MP1854 based boards) board with appropriate voltage selected for that device.
The device should also have a rtc with a battery for time tracking, some memory for local log file, probably 1-2 temperature sensors, but I have not yet thought in that direction, because these elements are easy to add at the end.

Now, the questions:

What ic should I use for ac current measurement? I have taken a look at TI MSP430AFE2x3, ST STPM01, AD ADE7753, Atmel 90E24, but I cannot really see what makes one better then another for my use case. What I need is a device with some kind of digital output, prefer a device which works with arduino with available library (AD/Atmel), and it should not be BGA case ( have successfully hand soldered 0.65mm tsop smd-s, but bga is beyond me).

For the relays there are a few options available, one is a regular relay. The other is a regular relay with a selectable output between NO and NC, so the devices which are almost always on can use the NO output, and the devices which are sometimes on, to use the NC output. The third option is to use the double latching relays, which are expensive and need appropriate driving circuits. My first idea was the latching relay, but then, it is not a battery powered circuit, and the current consumption is not on the first place, the idea is more to minimize the heat sources, and avoid a relay being powered 24/7. What are your toughts?

Any ideas, suggestions, thoughts that have not been mentioned, are also welcome.

Thank you
 

Reloadron

Joined Jan 15, 2015
5,668
Were it not for the 20 volts I would suggest a simple off the shelf ATX form factor home computer PSU. Your 220 VAC line side current draw is not going to be very high. Depending on what you plan to spend any number of AC Current Transducers would give you what you want. All you need is to get from AC Current to a DC low voltage level proportional to the AC mains current. You could use a CT (Current Transformer) and do all your signal conditioning or simply buy an AC Current Transducer. On the DC side Ebay is flooded with DC voltage & current meter combinations or again use a DC current transducer and DC Voltage transducer, there is no limit to possible solutions. You do not mention any budget? Budget can determine what is roll your own verse buy a turn key solution. Finally when we say for example DC Power Distribution System we need to consider wire length runs and wire gauge so I * R loss can be taken into consideration. If all you want is a bench DC power supply and only plan to use a single supply at a time you can likely buy much cheaper than build.

I could suggest for example an AC Current Transducer but haven't a clue what your budget might be. There are several good projects out there on measuring AC Mains Voltage using an Arduino, here is but a single example. You need to create a list on a nice clean sheet of white paper and list what units (volts/amps/etc) you wish to measure and what ranges of those units to what uncertainty. Then start with getting what you have to what you want so it can be measured.

Ron
 

Thread Starter

SaleB

Joined Jul 3, 2017
7
Thank you for the answer.

I have already bought one of these power supplies with 12V@29A amps output, with all the protections already implemented. I have also tested it for about a month with step-up (20V) and step-down (5V) converters to reach both borderline values that I need. There was no heating up in the step converters. The wire lengths are 1-3m, so the losses are in mV range at 0.5-0.75mm wires. I have managed the loses by inserting the step converter 50cm before the consumer. and setting voltage (measured at the cable endpoint) to 1% above the targeted voltage.

I have also found the solution, the add-on board (shield) for the power supply, that I liked so much, people from X-Carve used it for their smps supply. It's an elegant solution. For the connectors I have found that JST VH or Molex UltraFit may me a good solution. JST solution is probably better, as it's much more affordable.

Budget is an interesting question, i do not have a limit. That said, I do want to take time and to learn, I do want to go as cheap as possible, but I do not want to sacrifice safety. I do want to design a board(s) and use some of these cheap prototype pcb makers to make a functional board. At the end I would like it to be something affordable that someone other can also use by downloading the necessary pdf files and source codes. I have not yet made an estimate, but I hope that 100eur would cover all the parts, without the pcbs.

I would divide the pcb in two, one small, a shied for the power supply with ac measurement and all AC with a pass-trough for dc, so it can be used with a dc circuit with fewer outputs.

Currents and voltages, for the input I need current measurement, for 350w unit at 90% efficiency the current should not go over 2A, but the device should be precise enough to catch the 2nd decimal place changes, and I would like it to be electrically isolated form the logic. I wanted it to be a digital circuit, but that is just because I am afraid of the analog outputs, and filters, and opamps, thermal compensation and all that that is involved in conditioning an analog signal (I have learned some of it in theory, but never used). There is a nice Allegro ACS circuit series, but I was afraid to use a circuit with analog output. For the low voltage section, it could be practical, but not needed to control the PSU output at one place, to check if it's 12v and how much current is used, maybe some header for a display (I would not use it, but someone in future may find it practical). For each endpoint on the other board, there should be a current sensor. I liked the idea of a all digital solution like TI INA series, but I will revisit the datasheet for the Allegro ACS series. If I would use analog current sensors, I would also need a multiplexer for the input, because there is not enough inputs either on the Arduino Nano or ESP8266 that I like to use.

I hoped that someone would point me to a current measurement circuit that he used in some project, because I lack the experience to distinguish advantages of one circuit over another.

I will update this post with my findings, conclusions, block diagram in a few days, and the first version of the schematic hopefully in a week or two. I hope that I have made my intentions more clear and not less clear.
 
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