Case capacitive coupled to Vcc and GND

[eTobey]

Hi,

i wonder, if anyone knows what this coupling is for:

I would have thought, that just connecting it to GND would be good enough? Its the controller of an ebike motor.

Cheers,

Toby

[Ian.M]

It provides RF grounding for EMI while avoiding any risk of current flowing via the mounting hole pads, possibly melting undersize ground tracks, if the negative feed to its power in terminal comes loose.

[magic]

This explains the AC coupling, but not the coupling to 50% of VCC rather than to GND.

Normally if GND is grounded, such connection would put 50% of VCC ripple on the case. Usually a cap is placed between the case and GND alone.

But this is a floating circuit, so maybe they figured that its average potential is constant and equal to the midpoint between BAT- and BAT+, hence such grounding.

[Ian.M]

... or they wanted it to work equally well (or at least equally poorly) for positive ground and negative ground vehicles.

[wraper]

I've seen plenty of bad engineering to first assume that engineers had no clue what they were doing rather than it actually serving some valid purpose.

[wraper]

On second thought this could potentially reduce EMI created by the circuit. However only if hardware is laid out in a quite specific way.

[magic]

... or they wanted it to work equally well (or at least equally poorly) for positive ground and negative ground vehicles.

Hmm, are e-bike batteries electrically connected to the frame like in cars? I assumed the whole thing would be floating.

[Ian.M]

Some cheap manufactures cant resist the temptation of using frame ground as return for low current accessories, even though its less than ideal from corrosion and reliability viewpoints.