Ground Loops and Galvanic Isolation

Discussion and support for the N2K0183 gateway between the Nmea2000 bus and Nmea0183 devices. This is a multiplexer with WiFi, 2 input and 1 output Nmea 0183 ports and bidirectional Nmea2000 communications.
Post Reply
Luis Sa
Site Admin
Posts: 834
Joined: Thu May 04, 2017 4:12 am

Ground Loops and Galvanic Isolation

Post by Luis Sa » Sat Dec 16, 2023 11:35 pm

Hello,

All Vela-Navega multiplexers have Nmea0183 opto isolated inputs ports and a Nmeaa0183 differential output port. If we need to connect the Nmea0183 output port to a Nmea0183 listener, we need to know if the listener has an opto isolated input. If the listener has a single ended input we should not connect the B terminal of the output port to the GND terminal of the (listener) input port. This is the reason why the Nmea2Wifi, the Nmea3Wifi and the Nmea4Wifi multiplexers use a jumper whose default position is 1. By setting the jumper to 1 we transform the differential output into a single ended output but we do not risk to short circuit the "B side" of the driver. We loose the extra benefit of noise robustness and we create a ground loop. The situation is illustrated in the following picture:

GROUND-LOOP.jpg

The loop is indicated in the picture and the currents Ix and Iy which, ideally should be equal, are undetermined. For example, if the power GND cable from the battery to the talker is cut at the shown point, the circuit could fail as the Iy current could create a noise voltage in the GND connection between the talker and the listener. If drastic situations like the one just described are avoided, the shown connection in small boats and short distances, is tolerable as, if a problem occurs, it will be restricted to the communication between the talker and the listener. On the contrary, when there is a bus supporting several devices, as in the case of the N2K bus, ground loops should be completely avoided. A ground loop introduced by one device could add noise in the system and compromise the communications between all the remaining devices. Considering the connection between the N2K0183 multiplexer and a Nmea0183 listener, we note that if the listener as an opto isolated input we do not introduce a ground loop. As shown in the following figure,

ix-iy.jpg

the two currents, Ix and Iy, have the same value which is determined by the driving voltage of the talker, the value of the resistor R and the characteristics of opto and protection diodes. We also emphasize that, in opposition to the other Vela-Navega multiplexers, the N2K0183 multiplexer has not the jumper referred to above and so, its Nmea0183 output port is always a differential port and can not be "transformed" into a single ended port by changing the position of the jumper. Therefore, if we want to connect the P2 port to a single ended input listener, we should use the following scheme:

N2K-P2-TALKER.jpg

which introduces a ground loop as referred to above. In order to avoid this and in cases where the user needs to connect the Nmea0183 P2 port to a single ended listener, the N2K0183 multiplexer is supplied with a small (30 x 12 mm) PCB that makes the listener to appear to the N2K0183 multiplexer as an opto isolated device. This is illustrated in the following picture:

N2K-P2-TALKER-OPTO.jpg

The small device, called "Opto Isolated Nmea0183 to RX Adapter" should be mounted near the listener and should receive power from the listener. For the best possible noise robustness, a twisted pair cable should be used as shown. The adapter circuit diagram is attached below.

Finally, we address the connection of the N2K0183 multiplexer to a PC or laptop through the USB connector. If the PC (or the laptop) is not directly powered by the boat batteries that supply power to the N2K backbone, we could use the USB port without creating a ground loop. For other cases, we developed an adapter called "Bidirectional USB to Nmea 0183 Adapter" that guarantees opto isolation between the multiplexer and the PC or laptop. The circuit diagram of this adapter is attached below and the following video shows one of its possible applications


Regards, Luis
Attachments
Opto-Nmea0183-RX-Adapter.jpg
Bidirectional-USB-Nmea0183-Adapter.jpg

Post Reply