For a long time there have been multiple signals on this remote base that appear to be digital hash and not legitimate radio signals. On the water fall they look like noise from switching power supplies. Considerable work has been done trying to get these signals chased down. Over the last year each switching power supply has been replaced with a linear supply or the switching power supply has been mounted in a metal box with ferrite chokes on the leads. Since the noise continued, looking elsewhere was necessary. The next suspects are the solar controllers considering they switch power on and off rapidly just like a switching power supply and considering they are about the only devices that haven’t been investigated. Searching the web turned up numerous reports that solar controllers are major contributors of rfi. The controllers used at the remote site* are specifically selected because of their FCC Class B certifications. They aren’t supposed to be generating rfi. That’s why they haven’t been investigated earlier. Today’s testing was very revealing. The controllers are generating tremendous rfi. Later it was discovered the interference occurred only in the mode where the batteries are fully charged. The controllers are in a state of “high voltage disconnect” to avoid overcharging the LifePO4 batteries. When the system is in a charge state there is no interference. Below is a picture of a water fall on 17 meters on a sunny day when the solar system is generating full capacity in a “high voltage disconnect” status.
Obviously those big wide bands of yellow-green are not supposed to be there. They are digital hash caused by something. Their huge signal strength indicates the source is probably local. Next picture is with one of the four controllers turned off. Observe the band on the right and the band in the center have disappeared as the waterfall continues to scroll down. Two bands on the left are still present.
Next, another of the controllers is turned off revealing an amazingly rfi free band. What a stunning difference. Apparently the other two controllers are not generating hash, for some reason yet to be determined.
Toroid chokes on the controller wires should be an easy fix. A hand full of Mix 31 ferrite toroid chokes was placed on the wires that come in and out of the controllers and no noticeable change occurred. Paraphrasing the captain of the boat in the movie Jaws, “We’re going to need a bigger choke”. Upon more Web scouring back home, an article was found that discussed a rarely mentioned bit of information about ferrite chokes.
“Ferrite material choking performance degrades in the presence of strong DC current. For this reason, it is better to pass both DC wires from the solar panels through the same snap on ferrite as this will eliminate the DC bias in the core.”
The chokes had been placed on individual wires in the initial test. About 15 amps of DC was present on those wires. Is this DC current enough to degrade the performance of the chokes? On the next trip to the site, both wires will be placed through the cores and the results will be reported here.
*The controllers used at the remote site are Morningstar PWM ProStar PS-30 and Morningstar MPPT ProStar PS-MPPT-25M.
Chokes On Both Wires Together
Next site visit and the first thing noticed is that different controllers are causing interference than the ones that caused it last time. Here is the first picture upon walking in the door without any testing.
Two lines of digital hash coming down the waterfall are from two of the four controllers, but not the same ones as last time. Next picture is after turning off three controllers and at the 7 second mark placing a choke on both wires of the 4th controller.
The choke clears up a good amount of noise but not nearly all of it. More chokes were added and there was almost no more improvement. Chokes don’t seem to be the answer.
Next topic is why only two controllers at a time cause interference. What is the difference? PWM and MPPT controllers are both contributing equally. It was soon noticed that the interference is coming from the controllers where the batteries are fully charged. When a battery is not fully charged and the controller is working hard there is no interference. When a battery reaches it charged state and the controller stops charging, it starts generating the digital hash. Solutions come to mind both elegant and crude. An elegant solution would be to monitor the modbus data output and watch for the fully charged messages. Use a microcontroller like an Arduino to turnoff the controller. That sounds like a lot of coding and debugging and time spent. Turning to the crude solution, that would be a relay on the solar input cables driven by a voltage sensor on the battery. When the battery reaches full voltage the relay would open and effectively turn off the controller. Call this solution the Rube Goldberg, band-aid, patchwork-quilt solution but voltage sensors and relays are now on order from China. The interference will have to be lived with for a month until the parts arrive.
While waiting for the parts from China an article surfaced that suggested trying 4 turns of both wires through one toroid of mix 31. That was tried and it did not reduce the noise noticeably. In an act of desperation bypass relays were inserted in the solar panel input leads so the panels could be cut off completely if they were causing interference. This is the method referred to above as “Rube Goldberg”. The difference is the relays are controlled remotely from home over the Internet instead of by an Arduino monitoring the modbus or instead of a voltage detector. So far it works perfectly. Case closed. For now.