What's this resistor for?

[john12ax7]

In fact I had a rack with this problem

That is quite unusual. Which rack?

 

[emrr]

That is quite unusual. Which rack?

OSA

https://groupdiy.com/threads/old-sc...ences-from-api-vpr-500-series-standard.48324/

 

[rock soderstrom]

Using back to back diodes works nicely. A beefy bridge rectifier also works, with AC sides to chassis ground and to circuit ground. + - terminals shorted with a fat copper wire to each other.

like that?


I still haven't quite understood why I want to do this and what problem it can help me with? I think with transformer balanced inputs and outputs you might not need it?

 

[ccaudle]

with the circuit common, what would happen if for some reason is exposed to mains voltage? If the transformer goes bad and passes 120/ 240 AC onto the secondaries for example

That does seem to be a valid concern if you are using a true class I transformer. A lot of transformers have reinforced insulation even if they are going into a class I device, so I guess I should not have been quite so absolute about the idea you can do "anything" with the secondary 0V connection.

 

[ccaudle]

I still haven't quite understood why I want to do this and what problem it can help me with?

I think the idea is that relatively low common mode differences between connected equipment and the protective earth will not result in current flow through your circuitry, but a fault condition would generate high voltage and so would be higher than the diode drops.

If your circuit reference node is only connected to the chassis in one point then it should not matter, because there is no second entry or exit point for the current to take (which would cause a voltage drop across your circuit reference).
It seems more like an attempt to work around an improper design, but there are admittedly cases where it is more difficult to always keep the design "proper" (I think usually involving cases where you must implement non-differential connections, or where for whatever reason you do not have an isolated phantom power supply, for example).

 

[warpie]

What wattage should the resistor be? I've seen anything from 0.6W to 5W resistors there.

 

[Cqwet Dbdfte]

like that?
View attachment 141411

I still haven't quite understood why I want to do this and what problem it can help me with? I think with transformer balanced inputs and outputs you might not need it?

This way the signal ground is ~ 2V away from chassis ground.
Every mains transformer has a parasitic leakage current, medical class has less.
You can check this with a DMM AC volts (or micro amps) between and "isolated" point to chassis ground.
Transformer leakage is reduced by a primary to secondary shield, and split bobbin types, leakage can be resistive, but most likely capacitive. Easy to mesure with a DMM.
True differential circuits will have som common mode rejection. Some Phantom power microphone circuits may be DC balanced, but signal only carried between pin1 (0V) and one of tte other.
If it is not a problem best to ignore it.