[SOLVED] DC buss diode to chassis ground

There is a Diode that is separate for each Channel A, B, C

Each channel A, B, C has it's own DC ground

There is a Diode that is connected from the DC ground to the chassis ground

What does this Diode do or protect?

A diode prevents its channel ground from going more negative than about -0.7V from the chassis ground, and it prevents the chassis ground from going more positive than about +0.7V from its channel ground.

A diode prevents its channel ground from going more negative than about -0.7V from the chassis ground,

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When would a ground have a voltage on it besides zero volts? when a component is shorted or open? the ground voltage goes to millivolts?

What is the problem if the DC ground has a potential different compared to the chassis ground?

it prevents the chassis ground from going more positive than about +0.7V from its channel ground.

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If the chassis ground goes to 0.9volts or higher, what is the problem?

Most circuits I have seen have potential differences from DC ground to chassis ground in millivolts or in milliohms 10ohms to 30ohms

Usually when used like that there is an additional low resistance ground connection as well. The diodes are, as Audioguru points out, there to limit the voltage differential that might appear if the ground connecton is removed. You see it sometimes where units are plugged in to something else and when unpluged the ground conection would be broken, the diodes (often Zeners or two diodes in reverse parallel) then ensure the potential is limited to their Vf. When the connection is made, they are shorted out and the direct ground connection is used instead.

Brian.

then ensure the potential is limited to their Vf. When the connection is made, they are shorted out and the direct ground connection is used instead.

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What is the problem of having the AC and DC grounds at a different potential compared with the chassis ground?

If the AC ground, DC ground and Chassis ground all have different potential differences what is the problem? or when does it become a problem

There is no particular problem unless you try to measure something on one while using the other as the reference.

In most instances they are linked together somewhere but residual currents through wiring will drop voltages along the wiring resistance and the resulting different potentials can upset small signals and voltage measurements.

Try this to prove it: take a mains AC powered stereo system and connect one of the inputs through a wire to real ground (under your feet). In theory, the chassis of the stereo system is also at ground potential so you should hear nothing at all. In practice, you will hear loud humming noises, crackles and possibly radio stations. Now with the wire still connected, link it also to the chassis of the stereo (outside of the socket if it's an RCA 'phono' type) and you will note the noises disappear. The chassis and real ground are connected but you could hear the voltage difference between them. If that had been a sensitive DVM instead of audio the readings would be completely wrong.

Brian.