Carnhill inductors Earth Pin

[warpie]

I see that some Carnhill inductors (like the 9044) have an earth pin. However, it seems that it is only connected to the clips of the inductor (see attached photo). What's the point? Am I missing something?

 

[ruffrecords]

It is basically a shield of sorts. I just connect it to analogue 0V.

Cheers

Ian

 

[warpie]

It is basically a shield of sorts. I just connect it to analogue 0V.

Cheers

Ian

But does it shield anything at all? I very much doubt it.

Also, why to 0V and not to chassis?

 

[ruffrecords]

But does it shield anything at all? I very much doubt it.

Also, why to 0V and not to chassis?

I agree, which is why I called is a shield 'of sorts'. Its magnetic shielding properties will be close to nil. All it will do is help prevent finger hum reaching the coils if you touch them. It may be more mechanical than anything else, to help keep the two halves of the ferrite in place. Many of the smaller ones have no screen at all.

Cheers

Ian

 

[ccaudle]

why to 0V and not to chassis?

Shields should be connected to the location which allows the circulating currents to return to the source. Inside a piece of equipment there should not be external sources, those should have been terminated to the chassis at the entrance of external cables. For components inside the equipment the connections are about controlling parasitic currents between different parts of the circuit.

 

[warpie]

Shields should be connected to the location which allows the circulating currents to return to the source. Inside a piece of equipment there should not be external sources, those should have been terminated to the chassis at the entrance of external cables. For components inside the equipment the connections are about controlling parasitic currents between different parts of the circuit.

Excuse my ignorance and my broken English. So, in plain English this means that all the shields of TXs, inductors, etc... should be connected to 0V instead of chassis, right?

I remember reading a debate here about the cables inside a unit and whether their shields should be connected to 0V or to chassis, but I can't find it right now.

 

[ruffrecords]

Excuse my ignorance and my broken English. So, in plain English this means that all the shields of TXs, inductors, etc... should be connected to 0V instead of chassis, right?

I think generally this is correct. For balanced signals entering and leaving the equipment, the cable screen is not a signal carrying conductor. Its purpose is to route noise and interference to the chassis which is why pin one should be connected directly to chassis. Inside the equipment there should be almost no e-field interference but there might be some h-field which walks through your average aluminium chassis. This is why mic input transformers need a mu-metal shield to kill the h-field. Inside the equipment there will be e-fields set up by the active circuits, PCB tracks and cabling. What stops an e-field is an equipotential - a conductor at a fixed voltage. So internal unbalanced signals would also normally use screened cable with the screen connected to 0V (so it can act as an equipotential) and prevent the signal in the cable interfering with other local circuits and vice versa. In the case of inductors, they have exposed wires that could pick up e-field so some form of screen connected to 0V will again act as an equipotential. In all probability inductors will also be susceptible to h-field but as the signal level in them is line level rather than mic level magnetic screening is probably not necessary. Having said that, I have had custom inductors of several Henries made by both Sowter and Cinemag and they come in mu-metal shields.

Cheers

Ian