What are the benefits of a resistance decade box having a ground connection?

[Rene]

Hello,

I was researching resistance decade boxes and notice that one of the boxes (Extech 380400 Resistance Decade Box) had a ground connection (see Picture1). Could someone tell me what benefits a resistance decade box offers by having a ground connection versus one that does not offer that feature?

Thanks.

[rdl]

I have one similar to that, it's Elenco brand that I got years ago. The manual says the ground is connected to the front panel. I guess this could be good if you were using the box with a mains powered device.

Unfortunately their implementation was flawed because the connection was from the circuit board through one of the mounting posts to the front panel The mounting screws are painted and the front panel is anodized and because of that there's absolutely no continuity between the ground terminal to the front panel.

[Michaela Joy]

If it's internally connected to ground, you could ground it to minimize noise being coupled into the circuit that it's hooked to.

[Richard Crowley]

Yes, as @Michaela Joy said, it provides a Faraday shield around the "resistor" so that it can be shielded from picking up EMI from the environment when used in a sensitive part of a circuit.  A conventional resistor would be very small and not provide much of an "antenna" to pick up noise and interference.  But when running the signal path several inches/cm away from the circuit, through unshielded leads, that gadget can potentially be a big antenna if not properly shielded.

[Rene]

I provides a Faraday shield around the "resistor" so that it can be shielded from picking up EMI from the environment when used in a sensitive part of a circuit.

Thanks Richard.

Out of curiosity, would it be possible for you to describe how does the internal implementation of the "Faraday Shield" looks inside the resistance decade box?

In other words, if I was to open the box an look inside, what would I see that would let me know there is a "Faraday Shield" implemented?

Thanks.

[Richard Crowley]

A Faraday Shield (or cage or screen, etc.) is simply an "envelope" around the protected circuit. It may be solid metal, or it may be woven or perforated, etc. But in any case the purpose is to simply block outside interference from reaching the protected circuit by "intercepting" the interference and conducting it away to ground/earth so that it can't reach any sensitive part of the circuit.

Examples of Faraday Shields include your microwave oven (the Faraday Shield keeps the microwaves INSIDE the box so they don't escape).  Many devices we use every day have metallic (or conductive) shields around them including cables, connectors, even storage devices like tubes that ICs are shipped in, or those metalic-looking plastic bags that protect against static damage. Even your car is a "Faraday Cage" against a lightning strike (and also somewhat reduces the efficiency of your cell phone unless close to a window, etc.)

http://en.wikipedia.org/wiki/Faraday_cage

Inside your resistor substitution box, you might find one of several different materials. If the box is metal, then it forms the Faraday Shield intrinsically. If it is plastic, then there may be sheets of metal inside, or the inside may be painted/coated with a conductive substance.  I sometimes glue common aluminum foil inside a plastic case when I need a Faraday Shield.