Okay, a quick overview about the things that you need to know about ESD with regard to the choice of the resistor. (if you want to know more about ESD, how environmental factors such as humidity and other things influence ESD voltage levels, how a resistor impacts the discharge in detail, and so on.... google is never far away
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Static electricity charges are often in the region of close to and beyond 10,000V. Sometimes even reaching 30,000V and more. The overall energy of an static electric charge as you usually encounter in your household is not much (just from a few microjoules to a couple hundred millijoules).
If this charge is being discharged (the ESD event) through a conducting path with low resistance, the discharge will happen in a very short period of time with a comparatively high current. It could hurt electronic components if they find themselves being a part of this conducting path. To protect such a component, the static electric charge has to be discharged before the component is becoming part of a potential conducting path to earth, and build up of more/another static charge has to be made impossible. The function of the resistor in the discharge path is two-fold. First, it will protect you (see further below) and secondly, it will limit the current flow (i.e. discharge rate) which can help protecting an electronic component which finds itself being an unwitting part of the discharge path.
When the static electric charge is dumped into the resistor it will be confronted with the voltage pulse in all its mighty glory. And the resistor needs to be able to withstand this high voltage (hopefully) without taking damage, albeit only for a short time until the static electric charge fizzles out. Resistors which are not able to handle those voltage pulses take damage. Such a damage can lead to significant change of resistance, it can lead to a shortcut or it can burn the resistor open. Depending on the specific resistor model, it might just take a few ESD events or it might take more ESD events until the damage becomes noticeable.
So, what can you do and how should you select a resistor? In the following i use the datasheet of the ceramic resistor i mentioned in one of my previous posts as example (
http://www.ohmite.com/cat/res_ox_oy.pdf).
First, the resistor should be able to handle mains voltage. This has nothing to do with the ESD capabilities but is rather to your own safety. As others already explained in this thread, the resistor can protect you against an electric shock in case something goes totally haywire in your modified mains plug. It is important to know that in a 220V mains network the actual mains voltage is higher than 220V. 220V is only the RMS of the AC voltage. In a 220V network, the voltage swings from about +312V to -312V and back (240V network would have about +-340V). Thus, the resistor should be rated (i include some margin) for at least 400V.
Second, the resistor should have a max pulse voltage as high as possible and a resistance drift caused by ESD events as low as possible (while remaining financially feasible
) The OY series resistor i mentioned earlier has a max pulse voltage of 20KV (which means that it might someday have to face ESD events with higher voltages >20KV which it will not like and perhaps take damage). Depending on the voltage of the ESD event, the resistor will drift a certain amount (i.e. change its resistance). The diagram on the datasheet tells you how the resistance drift rate changes over a number of ESD events (bad: it shows only for up to 10KOhm, grr... which requires you to extrapolate with the tongue at the right angle...). Both the max pulse voltage rating as well as the drift rate are important parameters that tell you how well a resistor is suited for a particular ESD application. (FYI: Foil resistors usually have higher max pulse voltage and lower drift rate; but yeah, that comes with a price to match...)
Note that the resistance of a resistor does not only drift due to ESD. It also drifts due to changes in temperature, humidity or simply due to age. Ceramics can be really bad at this. However, that the resistor is able to maintain more or less exactly 1MOhm is not really that important in your application. Whether it is 850K or 1.2MOhm does not matter that much. It would become a health risk for you if the resistor falls below 800K and your ESD box is exposed to mains voltage (due to a defect in the plug, for example).
In summary, that's what you should looking for:
- voltage rating of 400V or more
- max pulse voltage as high as possible, but at least around 20KV
- ESD event-induced resistance drift as low as possible
If you look into the datasheet you have given in your last post, you will notice that it does not tell anything with regard to max pulse voltage. That's a clear sign that you should not use it for ESD applications.
Hope that helps you when selecting a resistor from the catalog of your preferred vendor.
P.S.: You might have noticed that the resistor in my example has 2W power rating. Does the power rating matter for your application? No. ESD events you will encounter are rather low-energy events which even an 1/4W resistor could handle, well, if he meets the requirements as outlined above. The reason why i did choose this particular resistor is simply because it was the cheapest resistor i could quickly find on Digikey which fullfills the requirements (and is available in single quantities and in stock).
P.P.S.: Sorry for that long post. I couldn't help myself.
EDIT: It would be a good idea to measure the resistance of your ESD resistors from time to time (perhaps one to four times per year, depending how often you use your ESD box), so that you will be aware when the resistors "lose" their resistance and start drifting towards 800 KOhm ...