Power Jack Grid Tie Inverter RF noise suppression mod

PowerJack EMI Filter 

The Power Jack 600w Grid Tie inverter produces a high level of broadband RF noise when operating.  I would be very surprised if the smaller 300w or larger 1200w+ models didn't suffer the same shortcomings. The noise is caused by the high frequency switching circuits used to convert the DC supply to high voltage AC. In more expensive inverters, the input and output circuits are filtered to prevent this noise from being radiated by cabling. Unfortunately, the PowerJack inverters provide no such filtering and so much of this noise escapes the inverter cabinet and is then radiated by the solar panel and mains wiring. As the solar panel wiring in particular can act as an efficient antenna, the noise can cause significant interference to AM and shortwave band radio reception for quite some distance around your house. The attached circuit provides individual low pass filters for both +ve and –ve solar array lines. This circuit will prevent most of tthe broadband noise from entering the solar array wiring. In addition, 100-150v 10kA varistors have been added on the filters input terminals to offer a degree of protection from high voltage spikes that may occur from nearby lightning strikes. It is important to note that the varistors will not protect the inverter from a direct lightning strike and they will offer no protection unless the filter case and inverter are earthed via a dedicated earth stake (the mains earth wire in the IEC plug is not directly connected to the inverter chassis or earth post - ref text in original article). The varistors are optional and do not affect the filter characteristics. Not shown in the circuit diagram, is a clip-on/hinged , 30mm square, ferrite core (see below)  that is attached to the Positive and Negative leads that run between the filter and inverter input. This core reduces the level of radiated interference further still.

Inside layout of Solar/DC side noise filter. Cable on right goes to inverter input (note clip-on ferrite core). Solar panels connect to the red & black terminals on the left. The green Earth wire connects to the inverters earth stud. Capacitors and varistors (if fitted) are soldered to brass screws near the input & output terminals.

Critical aspects of this circuit:

Ensure that the filter grounds (capacitors and varistors) are well bonded to its enclosure and the enclosure is then connected to the earth post on the inverter. Without this connection, the filter will do nothing! Keep the DC lines between the filter and the inverters input as short as possible! Preferably, no more than 150mm. Longer interconnect leads will result in higher levels of RF noise being radiated.

Inductors: 25-30mm OD powdered-iron core with 7-8 turns of insulated 3-4mm wire. 3-4mm wire is required to reduce DC resistance losses or voltage drop at high current levels.

Capacitors: 0.1uF @ 100v. 

Varistors: 100-150V @ 10kA varistors (optional). 

Enclosure: This circuit is enclosed in a suitable diecast aluminium enclosure (120x95x50mm works well). The filters are ‘grounded’ to the enclosure and enclosure is earthed to the inverters ‘Earth’ post and to an external earth stake (earth stake required only if varistors are installed).

After construction, ensure all terminal posts are tight and check for minimal voltage drop between input and output terminals under load. With 36v @ 3 Amps of current flow, my unit induced a voltage drop of only 30mV on each power rail (60mV total).