Connecting cases to ground

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fedde

Member
Joined 2002
fedde.nu
Hi DIY fellows,

usually, I use a 220 ohm resistor parallel with a 220 nF capacitor to connect the case with the ground of my opamp circuits. I wonder if any theory exists why these (or similar) values should be used !?

The result on two variables can be considered:
- The amount of noise that is added to the ground
- The shielding properties of the case (Faraday cage)

Any information would be appreciated!

Fedde
 
Mad_K

Mad_K

Member
Joined 2002
breguetphile said:
A step further than just a low resistance would be what alephs use. If you use a thermistor which will only be resistive when current is flowing, otherwise its close to 0 ohms. This is described in one (if not more) of the aleph service manuals.
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Yup, I always do it like this. EXCEPT that you must use a NTC, not a PTC. This means that it should be about 5-10ohms when little or no current is flowing throug your ground, but it drops to a very low value when conducting any significant current. Another elegant solution would be to use a std rectifier bridge as in ZV4.
 
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fmak

Member
Joined 2001
Peter Daniel said:
It seems like 220ohm is a bit high. Usually 10ohm is being used.
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Theses things sre done in 110V areas. Be careful in 220V region. I don't believe one should connect reoung to chasis via any resistor!

Even the Japanese have abandoned this practice, but still seems to take place in Taiwan.

:(
 
pinkmouse

pinkmouse

Ex-Moderator
Joined 2002
Just a quick note, but using anything in line with the connection between chassis and the ground is bad practise, and, AFAIK, against IEE wiring regulations as being unsafe. It is also not really needed, if you think about your wiring and earth loops properly. ;)
 
M

mskeete

Member
Joined 2003
unsafe ?

Are you talking about putting a resistor in between the mains earth and the case

OR

connecting the case directly to mains earth and linking the circuit earth (e.g. transformer centre tap) to mains earth via a resistor

If you mean the second case (no pun intended), please explain why you think this is unsafe
 
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