question

tom4x avatar image
tom4x asked

Cerbo connectors' shells and the negative DC potential

Measuring the voltage between Cerbo's Ethernet, USB or HDMI shells and the negative DC pole, I see a voltage difference on the order of 100 mV. I varies, but the difference is never 0.

Are the connectors' shells/ground on the same potential as DC negative?
Would it cause harm to bring the connector's GND to a common ground (e.g. earth or DC negative)?

With a standardized potential, connections problems based on different potential issues (e.g. https://community.victronenergy.com/questions/147472/probleme-mit-mehreren-usb-rs485-verbinungen-am-hos.html) would not exist and the need for costly galvanic isolation devices would also drop.

cerbo gxGroundingconnection issue
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2 Answers
wkirby avatar image
wkirby answered ·

The USB and HDMI connector shells are definitely referenced to the negative of the power input. If you measure from the connector shells to the negative terminal where the power goes in to the CerboGX you will not measure much of a Voltage difference. If you measure from the connector shell to the negative pole of the battery you will be able to measure a much higher Voltage because there is a piece of wire in between. The longer the wire, the higher the Voltage drop.
There will be Voltage drop over the negative wire since and electric current will be flowing through it if the CerboGX is on.

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tom4x avatar image tom4x commented ·
Thank you for confirming that the connector shells are on the same potential as the DC negative side, this is very helpful.

So in order minimize the potential differences on USB connections, additional grounding on the connectors shell, won't cause harm to the Cerbo, as the potential is already the same and the voltage difference comes only from the cables' resistance?

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johanndo avatar image
johanndo answered ·

Depends on the power supply used and how it is wired in relation to power input ground..

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