How much resistor wattage to use?

Thread Starter

suryagoel

Joined Dec 21, 2017
1
Hi

I am making a grounding mat and I live in an apartment with all PVC pipes. So I am left with only 1 option to plug my grounding mat to the Ground of the wall outlet. There are suggestions that I should use a 1 M Ohm resistor in series to prevent myself from unwanted spikes or currents.

Now, my question is for a 110V-120V environment in Canada, what kind of wattage should be of the 1 M Ohm resistor that I will use.

Also, if you have any other advise in this regard that would be useful.

Thanks
 

MrChips

Joined Oct 2, 2009
22,090
The purpose of the resistor is to dissipate static electric charge which has extremely low amperage. Any resistor value and wattage will do. At the same time, we want the resistor to be large in resistance to avoid large currents should the grounding strap come into contact with any electrical source. The 110V-120V mains parameter has little bearing on the choice of resistors.

1MΩ ¼W or ½W will do.

You want to make certain that the wall outlet is properly wired and that the ground connection is truly earth ground. You may want to consider installing a GFCI at that outlet.
 

neonstrobe

Joined May 15, 2009
83
I suggest that the voltage rating of the resistor should be considered. A resistor failure could lead to a nasty shock or worse, so I tend to use higher power ratings simply because of the voltage capability. For 110-120V peak voltage might be ~200V, I would not use a resistor less than 350V for this.
I don't know if there is a standard for ground strap earthing resistors but there probably should be.
I am not aware, however, of any safety considerations that went into the old neon screwdriver for mains testing, which work on a similar principle.
 

MrChips

Joined Oct 2, 2009
22,090
In this application, the resistor will never be subjected to anything near that voltage. The purpose of the resistor and grounding strap is to bleed off any static charge that should ever attempt to build up in normal usage.
 

AnalogKid

Joined Aug 1, 2013
8,687
While the voltages should never approach those of an offline circuit, those guys do have safety protocols that can't hurt. The body of a 1/4 W axial lead resistor is more than long enough to meet reinforced insulation requirements, but the safe thing to do is put two 470 K or 1 M resistors in series. This increases the creepage distances, eliminates a single-point-of-failure, and reduces accidental current to below 3.5 mA (the limit in UL1950/60950).

ak
 

AnalogKid

Joined Aug 1, 2013
8,687
The stray capacitance across a long thin conductor is very small. My guess is single-digit pF. At 60 Hz the impedance is greater than that of the resistor:
1 pF > 2.6 Gohm at 60 Hz

If it is 1000 times larger:
1 nF > 2.6 Mohm

ak
 
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