Hardware Assembly
I put together the following chart for you to fill out as you plan and assemble your power monitor. This will help prevent making any errors with resistor values and CT mappings.
| Input Source | ADC Input Channel # | CT Sensor Model # | Burden Resistor Value | PCB Resistor # |
|---|---|---|---|---|
| Cat5e: Orange | 0 | R0 | ||
| Cat5e: Green | 1 | R1 | ||
| Cat5e: Blue | 2 | R2 | ||
| Cat5e: Brown | 3 | R3 | ||
| 3.5mm jack (pcb label CT4) | 6 | R8 | ||
| 3.5mm jack (pcb label CT5) | 7 | R9 |
Make sure you have double checked your burden resistor values as the value has a direct connection to the accuracy and safety of the project. I covered burden resistors in more detail in the previous section, Hardware 1, which includes the resistor values for the two types of current transformers I used, along with a link to a page explaining how to manually calculate the value.
Here's another table containing all the resistor and capacitor values for the remaining positions on the PCB. These values are fixed.
| PCB Label | Value | Component Type |
|---|---|---|
| R4 | 10K Ω | n/a |
| R5 | 100K Ω | n/a |
| R6 | 12K Ω | n/a |
| R7 | 12K Ω | n/a |
| C0 | 10 nF | Plastic Film |
| C1 | 10 nF | Plastic Film |
| C2 | 10 nF | Plastic Film |
| C3 | 10 nF | Plastic Film |
| C4 | 100 nF | Electrolytic |
| C5 | 1 µF | Electrolytic |
| C6 | 220 µF | Electrolytic |
| C7 | 100 nF | Plastic Film |
| C8 | 10 nF | Plastic Film |
| C9 | 10 nF | Plastic Film |
Here is a picture of a finished board so you have it for reference:
Here are some general tips:
- First, do the hot air soldering of the op-amp, PCB component U1. This will prevent you from burning or melting other components on the board with your hot air gun.
- Next, solder all the capacitors, resistors, and RJ-45 jack, using the tables above for reference.
- Finally, solder the plastic components to the board (DIP socket, 40-pin header, and 3.5mm jacks). Note that the 40-pin header goes on the bottom of the PCB! (Waiting to solder the plastic components last prevents you from accidentally burning/melting them with your iron's tip.)
Here's an example of a bridge that happened when I soldered the op-amp. They can be fixed by simply dragging your iron's tip to pull the solder away.
Setting a camera up like this (or using wearable magnifying glasses) will really help you out in this stage. The op-amp chip is tiny!
The notches on the 16-pin socket and MCP should point in towards the center of the board!
When finished, give your PCB a nice scrub with flux cleaner/remover and an old toothbrush, with a final flux remover rinse. I used WD-40 brand Electrical Contact Cleaner Spray. Do one final check for bridges between pins and pads.
Let it dry for about 10 minutes, then slide it onto the Raspberry Pi's header. You can install the standoffs during this step now or wait to do it until later.
Feel free to continue on to the next page to test your PCB out! Or, continue reading below to wire up to four CTs to a cat5e cable.
The PCB has 6 inputs dedicated to current transformers:
- 2 via the 3.5mm jacks
- 4 via the RJ-45 jack
If you need to extend your CTs, or use more than 2 inputs, you'll likely end up using the RJ-45 jack and cat5e cable. If you want to extend your 3.5mm cables, the OpenEnergyMonitor project has discussed that here.
There are 8 wires in a cat5e cable, split up into colored pairs. Each CT will take a single pair of like-colored wires. One end of your cat5e cable will be terminated with a RJ-45, according to the TIA-568B standard (see pics below). On the other end, you'll attach your CTs to the pairs of colored wires that you intend to use.
Here is a diagram of the TIA-568B termination standard:
Take note that the green pair is 'separated' by the blue pair!
Following this standard, here is how the colors pair up to the input channel numbers:
| Cat5e Pair Color | Channel # |
|---|---|
| Orange | Ch. 0 |
| Green | Ch. 1 |
| Blue | Ch. 2 |
| Brown | Ch. 3 |
If you aren't using all four inputs, it doesn't matter which color you choose. Also, it does not matter which CT lead you connect to which wire in a single cat5e pair. The polarity can be reversed by simply changing the direction that the CT is installed around the conductor. I highly recommend labeling your CTs according to their channel number, though. This will help you later on when we go through the phase correction.
Attach your CTs to the cable any way you like... I personally twisted the cat5e and CT leads together and soldered them, followed by heat shrink tubing to protect the bare wire, followed by some PET braided sleeving to protect the cat5e from abrasions during install. Here's a picture of my finished cable, as an example:
