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Chassis capacitors to power and ground

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Rusttree

Member
I'm looking at a circuit that has a capacitor between the metal chassis and power, as well as a capacitor between the metal chassis and ground. See the attached image.

I'm having trouble figuring out the theory for why that's a beneficial circuit. I would have just put a large resistor between the chassis and ground and been done with it. What exactly is happening with these two capacitors?
 

duffy

Well-Known Member
It's so the chassis can exhibit RF shielding without being connected to the line. Shielding requires surprisingly high currents, but can still work with a DC offset. The caps provide low-impedance AC paths to ground, yet isolate the DC potentials.
 

Rusttree

Member
Ah, that makes sense. You don't want the AC noise to get into your ground or power plane, so you wouldn't ground the chassis directly. Thanks for the explanation.
 

kubeek

Well-Known Member
Most Helpful Member
Ah, that makes sense. You don't want the AC noise to get into your ground or power plane, so you wouldn't ground the chassis directly. Thanks for the explanation.
I assume that the power is mains phase, right? Then the circuit is missing one important thing - grounded chassis. Without that you have a capacitive divider, which on 240V mains will present the user with warm and fuzzy 120VAC on the chassis.
If it is on the DC side, then it is solely for AC coupling the chassis to block RF radiation.
 
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