digital logic - Recommended voltage/current for dry contact input?

What would be a reasonable value for voltage and max current to used in conjunction with a remote dry contact output so that it can return to my MCU (through isolator)? I plan on using this input for both dry contact and open collector (in other words, I plan on having a pull up on my board). I don't want it too high so that a standard open collector output (30V/50mA-100mA) can work on it as well.

Answer

A dry contact can sometimes require what is known as a wetting current. This means that when the contact closes, a current is available to flow through the contact of a certain amount (usually specified by the vendor). This is usually achieved by the load and, in the case of a relay, it might be a few milliamps to several amps. The current has the effect of cleaning the contact.

When the contact is used solely for signalling, oxide layers can form and although the contact may appear to be closed, it registers an open or partially-closed circuit. Normally, vendors (like in the case of relays) suggest a wetting current and this wetting current is typically supplied by a pull-up resistor to a local DC supply (maybe 5V). The current will go some way towards ensuring the contact remains clean but, the supply voltage is also important - if too low, no matter what potential current may be available the contact oxides that are built up remain unpenetratable.

Here are a few words from wiki on wetting current. Below is a quote from a link on that page: -

Wetting current is the minimum current needing to flow through a mechanical switch while it is operated to break through any film of oxidation that may have been deposited on the switch contacts.[12] The film of oxidation occurs often in areas with high humidity. Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" due to contact oxidation.

Here is a thread from a control.com site giving user's experience of the problem. In short - you need to do your homework on the contact if it's exposed to humidity.

If you read thru this discussion 6mA is mentioned BUT there is no excuse for doing homework on the dry contact and finding-out what the manufacturer says.

I'd say some dry contacts will be OK at well under 0.1mA but some may not be OK at 10mA. A sealed contact such as one in a reed relay will nearly always be good for low micro amps.