>This is heading off topic from the original thread, but...  How does
>dirt and humidity affect resistors?  I'm not challenging you, I've heard
>things like this before and would like to know more about it.  Does the
>humidty just cause the resistor to break down over time?  How does dirt
>come in to play?

Tag changed with topic change

Typically a collection of dirt on components will attract moisture when the
humidity rises, and in this case would produce a lowering of resistance in
the circuit . For this reason you will often see slots milled in PCB's where
tracking of voltage across dirt is likely to be a problem, such as in
switched mode power supplies. This is also the reason for the requirement
for isolation distances between primary and secondary in such devices.

A good starting point to get around the problem is to ensure that the PCB is
cleaned after testing and then a conformal coating of some sort applied.
This can be as simple as a spray on polyurethane finish, through to a full
two pack epoxy type mix, depending on the requirements of the product. This
keeps the moisture and dirt off the electrical circuits as a minimum making
it harder for the moisture to track the voltage across, and also possibly
making it easier to clean.

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